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Please answer these questions : (No plagiarism )

1. Explain the layers of the OSI, TCP/IP, and the Internet models in detail, and elaborate on their similarities and differences.
2. Explain the concept of Capsulation/De-capsulation.
3. Explain TCP and UDP, their differences, and their applications. Which one is better and why?
4. Explain in detail the hub, active hub, switch, and routers, and their purposes, similarities, or differences.
5. List and explain guided and unguided mediums and give a couple of examples of each.
6. Explain the difference between wire and fiber optic and list the advantages and disadvantages of each one in detail. 
7. Assume that you have a wired network. Explain how you would covert this system to a fiber optic system.
8. A RF amplifier has a power gain of 30 dB. How many times does this amplifier amplify the incoming signal? Show your calculations step-by-step.
9. Explain how analog signals are converted into digital.
10. Explain how wireless networks work.

Network Design
Analyzing Business Goals and Constraints

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Top-Down Network Design
Network design should be a complete process that matches business needs to available technology to deliver a system that will maximize an organization’s success.
In the LAN area it is more than just buying a few devices.
In the WAN area it is more than just calling the phone company.

Start at the Top
Don’t just start connecting the dots.
Analyze business and technical goals first.
Explore divisional and group structures to find out who the network serves and where they reside.
Determine what applications will run on the network and how those applications behave on a network.
Focus on Layer 7 and above first.

Layer 8 of the OSI model encompasses office politics, budgets, training, and other human factors.

Layers of the OSI Model

Application
Presentation
Session
Transport
Network
Data Link
Physical
Layer 1
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2

Structured Design
A focus is placed on understanding data flow, data types, and processes that access or change the data.
A focus is placed on understanding the location and needs of user communities that access or change data and processes.
Several techniques and models can be used to characterize the existing system, new user requirements, and a structure for the future system.
A logical model is developed before the physical model.
The logical model represents the basic building blocks, divided by function, and the structure of the system.
The physical model represents devices and specific technologies and implementations.

Systems Development Life Cycles
SDLC: Does it mean Synchronous Data Link Control or Systems Development Life Cycle?
The latter for the purposes of this class!
Typical systems are developed and continue to exist over a period of time, often called a systems development life cycle (SDLC).

Analyze requirements

Develop logical design
Develop physical design
Test, optimize, and document design
Monitor and optimize network performance
Implement and test network
Top-Down Network Design Steps

Network Design Steps
Phase 1 – Analyze Requirements
Analyze business goals and constraints
Analyze technical goals and tradeoffs
Characterize the existing network
Characterize network traffic

Network Design Steps
Phase 2 – Logical Network Design
Design a network topology
Design models for addressing and naming
Select switching and routing protocols
Develop network security strategies
Develop network management strategies

Network Design Steps
Phase 3 – Physical Network Design
Select technologies and devices for campus networks
Select technologies and devices for enterprise networks

Network Design Steps
Phase 4 – Testing, Optimizing, and Documenting the Network Design
Test the network design
Optimize the network design
Document the network design

The PDIOO Network Life Cycle
Plan

Design
Implement
Operate
Optimize
Retire

Business Goals
Increase revenue
Reduce operating costs
Improve communications
Shorten product development cycle
Expand into worldwide markets
Build partnerships with other companies
Offer better customer support or new customer services

Recent Business Priorities
Mobility
Security
Resiliency (fault tolerance)
Business continuity after a disaster
Network projects must be prioritized based on fiscal goals
Networks must offer the low delay required for real-time applications such as VoIP

Resiliency means how much stress a network can handle and how quickly the network can rebound from problems, including security breaches, natural and unnatural disasters, human error, and catastrophic software or hardware failures.
Some experts, including Howard Berkowitz, have a mild dislike of the word “resiliency” as it sounds too much like a stretched rubber band or a trampoline. As Berkowitz says in his excellent book, WAN Survival Guide (Wiley 2001), “I avoid designing networks that stretch too far, bounce up and down, or oscillate between normal and backup states.”
So he likes “fault tolerance,” but he points out that it does not mean “immune to any conceivable threat.” Berkowitz states that, “A sufficient quantity of explosives can overcome the tolerance of any network.” 🙂

Business Constraints
Budget
Staffing
Schedule
Politics and policies

Collect Information Before the First Meeting
Before meeting with the client, whether internal or external, collect some basic business-related information
Such as
Products produced/Services supplied
Financial viability
Customers, suppliers, competitors
Competitive advantage

Meet With the Customer
Try to get
A concise statement of the goals of the project
What problem are they trying to solve?
How will new technology help them be more successful in their business?
What must happen for the project to succeed?

Meet With the Customer
What will happen if the project is a failure?
Is this a critical business function?
Is this project visible to upper management?
Who’s on your side?

Meet With the Customer
Discover any biases
For example
Will they only use certain company’s products?
Do they avoid certain technologies?
Do the data people look down on the voice people or vice versa?
Talk to the technical and management staff

Meet With the Customer
Get a copy of the organization chart
This will show the general structure of the organization
It will suggest users to account for
It will suggest geographical locations to account for

Meet With the Customer
Get a copy of the security policy
How does the policy affect the new design?
How does the new design affect the policy?
Is the policy so strict that you (the network designer) won’t be able to do your job?
Start cataloging network assets that security should protect
Hardware, software, applications, and data
Less obvious, but still important, intellectual property, trade secrets, and a company’s reputation

The Scope of the Design Project
Small in scope?
Allow sales people to access network via a VPN
Large in scope?
An entire redesign of an enterprise network
Use the OSI model to clarify the scope
New financial reporting application versus new routing protocol versus new data link (wireless, for example)
Does the scope fit the budget, capabilities of staff and consultants, schedule?

Gather More Detailed Information
Applications
Now and after the project is completed
Include both productivity applications and system management applications
User communities
Data stores
Protocols
Current logical and physical architecture
Current performance

User communities, data stores, protocols, and the current architecture and performance will be discussed in the next few chapters. This chapter focuses on business needs and applications, which should be the first area of research in a top-down network design project. Network design is iterative, however, so many topics are addressed more than once as the designer gathers more detailed information and conducts more precise planning. So, gaining a general understanding of the size and location of user communities, for example, might be appropriate at this stage of the design project, but user communities should be investigated again when characterizing network traffic.

Network Applications

Name of Application
Type of Application
New Application?
Criticality

Comments

Summary
Systematic approach
Focus first on business requirements and constraints, and applications
Gain an understanding of the customer’s corporate structure
Gain an understanding of the customer’s business style

Review Questions
What are the main phases of network design per the top-down network design approach?
What are the main phases of network design per the PDIOO approach?
Why is it important to understand your customer’s business style?
What are some typical business goals for organizations today?

Chapter Four
Making Connections

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After reading this chapter,
you should be able to:
List the four components of all interface standards
Discuss the basic operations of the USB and EIA-232F interface standards
Cite the advantages of FireWire, Lightning, SCSI, iSCSI, InfiniBand, and Fibre Channel interface standards
Outline the characteristics of asynchronous, synchronous, and isochronous data link interfaces

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After reading this chapter,
you should be able to (continued):
Recognize the difference between half-duplex and full-duplex connections
Identify the operating characteristics of terminal-to-mainframe connections and why they are unique compared to other types of computer connections

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Introduction
Connecting peripheral devices to a computer has, in the past, been a fairly challenging task
Newer interfaces have made this task much easier
Let’s examine the interface between a computer and a device
This interface occurs primarily at the physical layer

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Interfacing a Computer to
Peripheral Devices
The connection to a peripheral is often called the interface
The process of providing all the proper interconnections between a computer and a peripheral is called interfacing

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Characteristics of Interface Standards
There are essentially two types of standards
Official standards
Created by standards-making organizations such as ITU (International Telecommunications Union), IEEE (Institute for Electrical and Electronics Engineers), (now defunct) EIA (Electronic Industries Association), ISO (International Organization for Standardization), and ANSI (American National Standards Institute)
De facto standards
Created by other groups that are not official standards but because of their widespread use, become “almost” standards

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Characteristics of Interface
Standards (continued)
There are four possible components to an interface standard:
Electrical component: deals with voltages, line capacitance, and other electrical characteristics
Mechanical component: deals with items such as the connector or plug description
Functional component: describes the function of each pin or circuit that is used in a particular interface
Procedural component: describes how the particular circuits are used to perform an operation

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Two Important Interface Standards
In order to better understand the four components of an interface, let’s examine two interface standards
EIA-232F – an older standard originally designed to connect a modem to a computer
USB (Universal Serial Bus) – a newer standard that is much more powerful than EIA-232F

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An Early Standard: EIA-232F
Originally named RS-232 but has gone through many revisions
All four components are defined in the EIA-232F standard:
Electrical
Mechanical (DB-25 connector and DB-9 connector)
Functional
Procedural

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An Early Standard: EIA-232F
EIA-232F also used the definitions DTE and DCE
An example of a DTE, or data terminating equipment, is a computer
An example of a DCE, or data circuit-terminating equipment, is some form of modem

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What is meant by duplexity?
EIA-232F defines a full-duplex connection. What does this mean?
A full-duplex connection transmits data in both directions and at the same time
A half-duplex connection transmits data in both directions but in only one direction at a time
A simplex connection can transmit data in only one direction
Can you think of a modern example of each?

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Universal Serial Bus (USB)
The USB interface is a modern standard for interconnecting a wide range of peripheral devices to computers
Supports plug and play
Can daisy-chain multiple devices
USB 2.0 can support 480 Mbps (USB 1.0 is only 12 Mbps)
USB 3.0 can support 4.8 Gbps

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Universal Serial Bus (USB) (continued)
The USB interface defines all four components
The electrical component defines two wires VBUS and Ground to carry a 5-volt signal, while the D+ and D- wires carry the data and signaling information
The mechanical component precisely defines the size of four different connectors and uses only four wires (the metal shell counts as one more connector)

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Universal Serial Bus (USB) (continued)
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Universal Serial Bus (USB) (continued)
The functional and procedural components are fairly complex but are based on the polled bus
The computer takes turns asking each peripheral if it has anything to send
More on polling near the end of this chapter

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FireWire
Low-cost digital interface
Capable of supporting transfer speeds of up to 800 Mbps
Hot pluggable
Supports two types of data connections:
Asynchronous connection
Isochronous connection

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Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
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The term asynchronous is usually used to describe communications in which data can be transmitted intermittently rather than in a steady stream. For example, a telephone conversation is asynchronous because both parties can talk whenever they like.

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Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
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An isochronous data transfer system combines the features of an asynchronous and synchronous data transfer system. An isochronous data transfer system sends blocks of data asynchronously, in other words the data stream can be transferred at random intervals. Each transmission begins with a start packet.

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Thunderbolt
Digital interface currently found on Apple products
Capable of supporting transfer speeds of up to 10 Gbps
Uses same connector as existing Mini DisplayPort and similar protocol as PCI Express
Can daisy-chain devices and may get even faster with later versions

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Lightning
Newer digital interface currently found on Apple products
Replaces the older 30-pin connector found on devices such as iPhones with a new 8-pin connector
Cannot be plugged in backwards

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SCSI and iSCSI
SCSI (Small Computer System Interface)
A technique for interfacing a computer to high-speed devices such as hard disk drives, tape drives, CDs, and DVDs
Designed to support devices of a more permanent nature
SCSI is a systems interface
Need SCSI adapter
iSCSI (Internet SCSI)
A technique for interfacing disk storage to a computer via the Internet

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InfiniBand and Fibre Channel
InfiniBand – a serial connection or bus that can carry multiple channels of data at the same time
Can support data transfer speeds of 2.5 billion bits (2.5 gigabits) per second and address thousands of devices, using both copper wire and fiber-optic cables
A network of high-speed links and switches
Fibre Channel – also a serial, high-speed network that connects a computer to multiple input/output devices
Supports data transfer rates up to billions of bits per second, but can support the interconnection of up to 126 devices only

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Asynchronous Connections
A type of connection defined at the data link layer
To transmit data from sender to receiver, an asynchronous connection creates a one-character package called a frame
Added to the front of the frame is a start bit, while a stop bit is added to the end of the frame
An optional parity bit can be added which can be used to detect errors

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Asynchronous Connections (continued)
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Asynchronous Connections (continued)
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Asynchronous Connections (continued)
The term asynchronous is misleading here because you must always maintain synchronization between the incoming data stream and the receiver
Asynchronous connections maintain synchronization by using small frames with a leading start bit

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Synchronous Connections
A second type of connection defined at the data link layer
A synchronous connection creates a large frame that consists of header and trailer flags, control information, optional address information, error detection code, and data
A synchronous connection is more elaborate but transfers data in a more efficient manner

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Synchronous Connections (continued)
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Isochronous Connections
A third type of connection defined at the data link layer used to support real-time applications
Data must be delivered at just the right speed (real-time) – not too fast and not too slow
Typically an isochronous connection must allocate resources on both ends to maintain real-time
USB and Firewire can both support isochronous

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Terminal-to-Mainframe
Computer Connections
Point-to-point connection – a direct, unshared connection between a terminal and a mainframe computer
Multipoint connection – a shared connection between multiple terminals and a mainframe computer
The mainframe is the primary and the terminals are the secondaries

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Terminal-to-Mainframe
Computer Connections (continued)
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Terminal-to-Mainframe
Computer Connections (continued)
To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal
Two basic forms of polling: roll-call polling and hub polling
In roll-call polling, the mainframe polls each terminal in a round-robin fashion
In hub polling, the mainframe polls the first terminal, and this terminal passes the poll onto the next terminal

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Round-robin (RR) is one of the algorithms employed by process and network schedulers in computing. As the term is generally used, time slices (also known as time quanta) are assigned to each process in equal portions and in circular order, handling all processes without priority (also known as cyclic executive).

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Terminal-to-Mainframe
Computer Connections (continued)
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Making Computer Connections
In Action (continued)
Power cord connection (why does the power cord have a big “brick” on it?)
USB connectors (one or more)
RJ-11 (telephone jack)
RJ-45 (LAN jack)
PC Card / SmartCard
DisplayPort (to connect your laptop to a video device)
Media card slot (SD, SDHC, xD, etc)
DB-15 (to connect to an external monitor or video projector)

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Making Computer Connections
In Action (continued)
A company wants to transfer files that are typically 700K chars in size
If an asynchronous connection is used, each character will have a start bit, a stop bit, and maybe a parity bit
700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits

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Making Computer Connections
In Action (continued)
If a synchronous connection is used, assume maximum payload size – 1500 bytes
To transfer a 700K char file requires 467 1500-character (byte) frames
Each frame will also contain 1-byte header, 1-byte address, 1-byte control, and 2-byte checksum, thus 5 bytes overhead

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Making Computer Connections
In Action (continued)
1500 bytes payload + 5 byte overhead = 1505 byte frames
467 frames * 1505 bytes/frame = 716,380 bytes, or 5,731,040 bits
Significantly less data using synchronous connection

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Summary
Connection between a computer and a peripheral is often called the interface
Process of providing all the proper interconnections between a computer and a peripheral is called interfacing
The interface between computer and peripheral is composed of one to four components: electrical, mechanical, functional, and procedural
A DTE is a data terminating device
Computer
A DCE is a data circuit-terminating device
Modem

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Summary (continued)
Two interface standards worthy of additional study: Universal Serial Bus, and EIA-232F
EIA-232F was one of the first highly popular standards
Universal Serial Bus is currently the most popular interface standard
Half-duplex systems can transmit data in both directions, but in only one direction at a time
Full-duplex systems can transmit data in both directions at the same time
Other peripheral interfacing standards that provide power, flexibility, and ease-of-installation include FireWire, Lightning, SCSI, iSCSI, InfiniBand, and Fibre Channel

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Summary (continued)
While much of an interface standard resides at the physical layer, a data link connection is also required when data is transmitted between two points on a network
Three common data link connections include asynchronous connections, synchronous connections, and isochronous connections
Asynchronous connections use single-character frames and start and stop bits to establish the beginning and ending points of the frame
Synchronous connections use multiple-character frames, sometimes consisting of thousands of characters
Isochronous connections provide real-time connections between computers and peripherals and require a fairly involved dialog to support the connection

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Summary (continued)

A point-to-point connection is one between a computer terminal and a mainframe computer that is dedicated to one terminal
A multipoint connection is a shared connection between more than one computer terminal and a mainframe computer

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Chapter Six
Errors, Error Detection, and Error Control

Data Communications and Computer Networks: A Business User’s Approach
Eighth Edition
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After reading this chapter,
you should be able to:
Identify the different types of noise commonly found in computer networks
Specify the different error-prevention techniques, and be able to apply an error-prevention technique to a type of noise
Compare the different error-detection techniques in terms of efficiency and efficacy
Perform simple parity and longitudinal parity calculations, and enumerate their strengths and weaknesses

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After reading this chapter,
you should be able to (continued):
Cite the advantages of arithmetic checksum
Cite the advantages of cyclic redundancy checksum, and specify what types of errors cyclic redundancy checksum will detect
Differentiate between the basic forms of error control, and describe the circumstances under which each may be used
Follow an example of a Hamming self-correcting code

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Introduction
Noise is always present
If a communications line experiences too much noise, the signal will be lost or corrupted
Communication systems should check for transmission errors
Once an error is detected, a system may perform some action
Some systems perform no error control, but simply let the data in error be discarded

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White Noise
Also known as thermal or Gaussian noise
Relatively constant and can be reduced
If white noise gets too strong, it can completely disrupt the signal

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White Noise (continued)
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Impulse Noise
One of the most disruptive forms of noise
Random spikes of power that can destroy one or more bits of information
Difficult to remove from an analog signal because it may be hard to distinguish from the original signal
Impulse noise can damage more bits if the bits are closer together (transmitted at a faster rate)

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Impulse Noise (continued)
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Impulse Noise (continued)
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The bottom figure should show much more distortion, completely blowing out one or two bits of information.

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Crosstalk
Unwanted coupling between two different signal paths
For example, hearing another conversation while talking on the telephone
Relatively constant and can be reduced with proper measures

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Crosstalk (continued)
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Echo
The reflective feedback of a transmitted signal as the signal moves through a medium
Most often occurs on coaxial cable
If echo bad enough, it could interfere with original signal
Relatively constant, and can be significantly reduced

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Echo (continued)
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Jitter
The result of small timing irregularities during the transmission of digital signals
Occurs when a digital signal is repeated over and over
If serious enough, jitter forces systems to slow down their transmission
Steps can be taken to reduce jitter

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Jitter (continued)
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Delay Distortion
Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal
Can be reduced

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Attenuation
The continuous loss of a signal’s strength as it travels through a medium

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Error Prevention
To prevent errors from happening, several techniques may be applied:
Proper shielding of cables to reduce interference
Telephone line conditioning or equalization
Replacing older media and equipment with new, possibly digital components
Proper use of digital repeaters and analog amplifiers
Observe the stated capacities of the media

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Error Prevention (continued)
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Error Detection
Despite the best prevention techniques, errors may still happen
To detect an error, something extra has to be added to the data/signal
This extra is an error detection code
Three basic techniques for detecting errors: parity checking, arithmetic checksum, and cyclic redundancy checksum

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Parity Checks
Simple parity
If performing even parity, add a parity bit such that an even number of 1s are maintained
If performing odd parity, add a parity bit such that an odd number of 1s are maintained
For example, send 1001010 using even parity
For example, send 1001011 using even parity

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Parity Checks (continued)
Simple parity (continued)
What happens if the character 10010101 is sent and the first two 0s accidentally become two 1s?
Thus, the following character is received: 11110101
Will there be a parity error?
Problem: Simple parity only detects odd numbers of bits in error

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Parity Checks (continued)
Longitudinal parity
Adds a parity bit to each character then adds a row of parity bits after a block of characters
The row of parity bits is actually a parity bit for each “column” of characters
The row of parity bits plus the column parity bits add a great amount of redundancy to a block of characters

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Parity Checks (continued)
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Parity Checks (continued)
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Parity Checks (continued)
Both simple parity and longitudinal parity do not catch all errors
Simple parity only catches odd numbers of bit errors
Longitudinal parity is better at catching errors but requires too many check bits added to a block of data
We need a better error detection method
What about arithmetic checksum?

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Arithmetic Checksum
Used in TCP and IP on the Internet
Characters to be transmitted are converted to numeric form and summed
Sum is placed in some form at the end of the transmission

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Arithmetic Checksum
Simplified example:

56
72
34
48
210
Then bring 2 down and add to right-most position
10
2
12
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Arithmetic Checksum
Receiver performs same conversion and summing and compares new sum with sent sum
TCP and IP processes a little more complex but idea is the same
But even arithmetic checksum can let errors slip through. Is there something more powerful yet?

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Cyclic Redundancy Checksum
CRC error detection method treats the packet of data to be transmitted as a large polynomial
Transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given generating polynomial
Quotient is discarded but the remainder is “attached” to the end of the message

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Cyclic Redundancy Checksum (continued)
The message (with the remainder) is transmitted to the receiver
The receiver divides the message and remainder by the same generating polynomial
If a remainder not equal to zero results, there was an error during transmission
If a remainder of zero results, there was no error during transmission

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Cyclic Redundancy Checksum (continued)
Some standard generating polynomials:
CRC-12: x12 + x11 + x3 + x2 + x + 1 
CRC-16: x16 + x15 + x2 + 1
CRC-CCITT: x16 + x15 + x5 + 1 
CRC-32: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
ATM CRC: x8 + x2 + x + 1

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Cyclic Redundancy Checksum (continued)
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Error Control
Once an error is detected, what is the receiver going to do?
Do nothing (simply toss the frame or packet)
Return an error message to the transmitter
Fix the error with no further help from the transmitter

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Do Nothing (Toss the Frame/Packet)
Seems like a strange way to control errors but some lower-layer protocols such as frame relay perform this type of error control
For example, if frame relay detects an error, it simply tosses the frame
No message is returned
Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission

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Return A Message
Once an error is detected, an error message is returned to the transmitter
Two basic forms:
Stop-and-wait error control
Sliding window error control

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Stop-and-Wait Error Control
Stop-and-wait is the simplest of the error control protocols
A transmitter sends a frame then stops and waits for an acknowledgment
If a positive acknowledgment (ACK) is received, the next frame is sent
If a negative acknowledgment (NAK) is received, the same frame is transmitted again

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Stop-and-Wait Error Control (continued)
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Sliding Window Error Control
These techniques assume that multiple frames are in transmission at one time
A sliding window protocol allows the transmitter to send a number of data packets at one time before receiving any acknowledgments
Depends on window size
When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next

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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
Older sliding window protocols numbered each frame or packet that was transmitted
More modern sliding window protocols number each byte within a frame
An example in which the packets are numbered, followed by an example in which the bytes are numbered:

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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
Notice that an ACK is not always sent after each frame is received
It is more efficient to wait for a few received frames before returning an ACK
How long should you wait until you return an ACK?

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Sliding Window Error Control (continued)
Using TCP/IP, there are some basic rules concerning ACKs:
Rule 1: If a receiver just received data and wants to send its own data, piggyback an ACK along with that data
Rule 2: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms for another packet
If while waiting, another packet arrives, send the ACK immediately
Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms
No packet, send ACK

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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
What happens when a packet is lost?
As shown in the next slide, if a frame is lost, the following frame will be “out of sequence”
The receiver will hold the out of sequence bytes in a buffer and request the sender to retransmit the missing frame

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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
What happens when an ACK is lost?
As shown in the next slide, if an ACK is lost, the sender will wait for the ACK to arrive and eventually time out
When the time-out occurs, the sender will resend the last frame

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Sliding Window Error Control (continued)
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Correct the Error
For a receiver to correct the error with no further help from the transmitter requires a large amount of redundant information to accompany the original data
This redundant information allows the receiver to determine the error and make corrections
This type of error control is often called forward error correction and involves codes called Hamming codes

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Correct the Error (continued)
Hamming codes add additional check bits to a character
These check bits perform parity checks on various bits
Example: One could create a Hamming code in which 4 check bits are added to an 8-bit character
We can number the check bits c8, c4, c2 and c1
We will number the data bits b12, b11, b10, b9, b7, b6, b5, and b3
Place the bits in the following order: b12, b11, b10, b9, c8, b7, b6, b5, c4, b3, c2, c1

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Correct the Error (continued)
Example (continued):
c8 will perform a parity check on bits b12, b11, b10, and b9
c4 will perform a parity check on bits b12, b7, b6 and b5
c2 will perform a parity check on bits b11, b10, b7, b6 and b3
c1 will perform a parity check on bits b11, b9, b7, b5, and b3
The next slide shows the check bits and their values

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Correct the Error (continued)
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Correct the Error (continued)
The sender will take the 8-bit character and generate the 4 check bits as described
The 4 check bits are then added to the 8 data bits in the sequence as shown and then transmitted
The receiver will perform the 4 parity checks using the 4 check bits
If no bits flipped during transmission, then there should be no parity errors
What happens if one of the bits flipped during transmission?

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Correct the Error (continued)
For example, what if bit b9 flips?
The c8 check bit checks bits b12, b11, b10, b9 and c8 (01000)
This would cause a parity error
The c4 check bit checks bits b12, b7, b6, b5 and c4 (00101)
This would not cause a parity error (even number of 1s)
The c2 check bit checks bits b11, b10, b7, b6, b3 and c2 (100111)
This would not cause a parity error

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Correct the Error (continued)
For example, what if bit b9 flips? (continued)
The c1 check bit checks b11, b9, b7, b5, b3 and c1 (100011)
This would cause a parity error
Writing the parity errors in sequence gives us 1001, which is binary for the value 9
Thus, the bit error occurred in the 9th position

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Error Detection In Action
FEC is used in transmission of radio signals, such as those used in transmission of digital television (Reed-Solomon and Trellis encoding) and 4D-PAM5 (Viterbi and Trellis encoding)

Some FEC is based on Hamming Codes

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Summary
Noise is always present in computer networks, and if the noise level is too high, errors will be introduced during the transmission of data
Types of noise include white noise, impulse noise, crosstalk, echo, jitter, and attenuation
Among the techniques for reducing noise are proper shielding of cables, telephone line conditioning or equalization, using modern digital equipment, using digital repeaters and analog amplifiers, and observing the stated capacities of media

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Summary (continued)
Three basic forms of error detection are parity, arithmetic checksum, and cyclic redundancy checksum
Cyclic redundancy checksum is a superior error-detection scheme with almost 100 percent capability of recognizing corrupted data packets
Once an error has been detected, there are three possible options: do nothing, return an error message, and correct the error

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Summary (continued)
Stop-and-wait protocol allows only one packet to be sent at a time
Sliding window protocol allows multiple packets to be sent at one time
Error correction is a possibility if the transmitted data contains enough redundant information so that the receiver can properly correct the error without asking the transmitter for additional information

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Chapter Five
Making Connections Efficient: Multiplexing and Compression

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After reading this chapter,
you should be able to:
Describe frequency division multiplexing and list its applications, advantages, and disadvantages
Describe synchronous time division multiplexing and list its applications, advantages, and disadvantages
Outline the basic multiplexing characteristics of T-1 and SONET/SDH telephone systems
Describe statistical time division multiplexing and list its applications, advantages, and disadvantages

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After reading this chapter,
you should be able to (continued):
Cite the main characteristics of wavelength division multiplexing and its advantages and disadvantages
Describe the basic characteristics of discrete multitone
Cite the main characteristics of code division multiplexing and its advantages and disadvantages
Apply a multiplexing technique to a typical business situation

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After reading this chapter,
you should be able to (continued):
Describe the difference between lossy and lossless compression
Describe the basic operation of run-length, JPEG, and MP3 compression

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Introduction
Under simplest conditions, medium can carry only one signal at any moment in time
For multiple signals to share a medium, medium must somehow be divided, giving each signal a portion of the total bandwidth
Current techniques include:
Frequency division multiplexing
Time division multiplexing
Code division multiplexing

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Frequency Division Multiplexing
Assignment of nonoverlapping frequency ranges to each “user” or signal on a medium
Thus, all signals are transmitted at the same time, each using different frequencies
A multiplexor accepts inputs and assigns frequencies to each device

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Frequency Division Multiplexing (continued)
Each channel is assigned a set of frequencies and is transmitted over the medium
A corresponding multiplexor, or demultiplexor, is on the receiving end of the medium and separates the multiplexed signals
A common example is broadcast radio

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Frequency Division Multiplexing (continued)
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Frequency Division Multiplexing (continued)
Analog signaling is used in older systems; discrete analog signals in more recent systems
Broadcast radio and television, cable television, and cellular telephone systems use frequency division multiplexing
This technique is the oldest multiplexing technique
Since it involves a certain level of analog signaling, it may be susceptible to noise

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Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users
Digital signaling is used exclusively
Time division multiplexing comes in two basic forms:
Synchronous time division multiplexing
Statistical time division multiplexing

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Synchronous Time Division Multiplexing
The original time division multiplexing
The multiplexor accepts input from attached devices in a round-robin fashion and transmits the data in a never -ending pattern
T-1 and SONET telephone systems are common examples of synchronous time division multiplexing

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Synchronous Time Division Multiplexing (continued)
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Synchronous Time Division Multiplexing (continued)
If one device generates data at faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream
If a device has nothing to transmit, the multiplexor must still insert something into the multiplexed stream

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Synchronous Time Division Multiplexing (continued)
Figure 5-3
Multiplexor transmission stream with only one input device transmitting data
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Synchronous Time Division Multiplexing (continued)
So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream

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Synchronous Time Division Multiplexing (continued)
Figure 5-4
Transmitted frame with added synchroni-zation bits
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T-1 Multiplexing
The T-1 multiplexor stream is a continuous series of frames
Note how each frame contains the data (one byte) for potentially 24 voice-grade telephone lines, plus one sync bit
It is possible to combine all 24 channels into one channel for a total of 1.544 Mbps

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T-1 Multiplexing (continued)
Figure 5-4
T-1 multiplexed data stream
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SONET/SDH Multiplexing
Similar to T-1, SONET incorporates a continuous series of frames
SONET is used for high-speed data transmission
Telephone companies have traditionally used a lot of SONET but this may be giving way to other high-speed transmission services
SDH is the European equivalent to SONET

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SONET/SDH Multiplexing (continued)
Figure 5-6
SONET STS-1 frame layout
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Statistical Time Division Multiplexing
A statistical multiplexor transmits the data from active workstations only
If a workstation is not active, no space is wasted in the multiplexed stream

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Statistical Time Division Multiplexing (continued)
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Statistical Time Division Multiplexing (continued)
A statistical multiplexor accepts the incoming data streams and creates a frame containing the data to be transmitted
To identify each piece of data, an address is included

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Statistical Time Division Multiplexing (continued)
Figure 5-8
Sample address and data in a statistical multiplexor output stream
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Statistical Time Division Multiplexing (continued)
If the data is of variable size, a length is also included

Figure 5-9
Packets of address, length, and data fields in a statistical multiplexor output stream
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Statistical Time Division Multiplexing (continued)
More precisely, the transmitted frame contains a collection of data groups

Figure 5-10
Frame layout for the information packet transferred between statistical multiplexors
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Wavelength Division Multiplexing
Wavelength division multiplexing multiplexes multiple data streams onto a single fiber-optic line
Different wavelength lasers (called lambdas) transmit the multiple signals

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Wavelength Division Multiplexing (continued)
Each signal carried on the fiber can be transmitted at a different rate from the other signals
Dense wavelength division multiplexing combines many (30, 40, 50 or more) onto one fiber
Coarse wavelength division multiplexing combines only a few lambdas

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Wavelength Division Multiplexing (continued)
Figure 5-11
Fiber optic line using wavelength division multiplexing and supporting multiple- speed transmissions
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Discrete Multitone
Discrete Multitone (DMT) – a multiplexing technique commonly found in digital subscriber line (DSL) systems
DMT combines hundreds of different signals, or subchannels, into one stream
Interestingly, all of these subchannels belong to a single user, unlike the previous multiplexing techniques

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Discrete Multitone (continued)
Each subchannel is quadrature amplitude modulated (recall eight phase angles, four with double amplitudes)
Theoretically, 256 subchannels, each transmitting 60 kbps, yields 15.36 Mbps
Unfortunately, there is noise, so the subchannels back down to slower speeds

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Discrete Multitone (continued)
Figure 5-12
256 quadrature amplitude modulated streams combined into one DMT signal for DSL
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Code Division Multiplexing
Also known as code division multiple access
An advanced technique that allows multiple devices to transmit on the same frequencies at the same time
Each mobile device is assigned a unique 64-bit code

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Code Division Multiplexing (continued)
To send a binary 1, a mobile device transmits the unique code
To send a binary 0, a mobile device transmits the inverse of the code
To send nothing, a mobile device transmits zeros

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Code Division Multiplexing (continued)
Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values
Interprets as a binary 1 if sum is near +64
Interprets as a binary 0 if sum is near -64

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Code Division Multiplexing (continued)
For simplicity, assume 8-bit code
Example
Three different mobile devices use the following codes:
Mobile A: 11110000
Mobile B: 10101010
Mobile C: 00110011
Assume Mobile A sends a 1, B sends a 0, and C sends a 1
Signal code: 1-chip = +N volt; 0-chip = -N volt

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Code Division Multiplexing (continued)
Example (continued)
Three signals transmitted:
Mobile A sends a 1, or 11110000, or ++++—-
Mobile B sends a 0, or 01010101, or -+-+-+-+
Mobile C sends a 1, or 00110011, or –++–++
Summed signal received by base station: -1, +1, +1, +3, -3, -1, -1, +1

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Code Division Multiplexing (continued)
Example (continued)
Base station decode for Mobile A:
Signal received: -1, +1, +1, +3, -3, -1, -1, +1
Mobile A’s code: +1, +1, +1, +1, -1, -1, -1, -1
Product result: -1, +1, +1, +3, +3, +1, +1, -1
Sum of Products: +8
Decode rule: For result near +8, data is binary 1

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Code Division Multiplexing (continued)
Example (continued)
Base station decode for Mobile B:
Signal received: -1, +1, +1, +3, -3, -1, -1, +1
Mobile B’s code: +1, -1, +1, -1, +1, -1, +1, -1
Product result: -1, -1, +1, -3, -3, +1, -1, -1
Sum of Products: -8
Decode rule: For result near -8, data is binary 0

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Comparison of Multiplexing Techniques
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Compression–Lossless versus Lossy
Compression is another technique used to squeeze more data over a communications line or into a storage space
If you can compress a data file down to one half of its original size, the file will obviously transfer in less time
Two basic groups of compression:
Lossless – when data is uncompressed, original data returns
Lossy – when data is uncompressed, you do not have the original data

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Compression–Lossless versus Lossy (continued)
Compress a financial file?
You want lossless
Compress a video image, movie, or audio file?
Lossy is OK
Examples of lossless compression include:
Huffman codes, run-length compression, Lempel-Ziv compression, and FLAC
Examples of lossy compression include:
MPEG, JPEG, and MP3

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Lossless Compression
Run-length encoding
Replaces runs of 0s with a count of how many 0s.

00000000000000100000000011000000000000000000001…11000000000001
^
(30 0s)

14 9 0 20 30 0 11

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Lossless Compression (continued)
Run-length encoding (continued)
Now replace each decimal value with a 4-bit binary value (nibble)
Note: If you need to code a value larger than 15, you need to use two consecutive 4-bit nibbles
The first is decimal 15, or binary 1111, and the second nibble is the remainder
For example, if the decimal value is 20, you would code 1111 0101 which is equivalent to 15 + 5

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Lossless Compression (continued)
Run-length encoding (continued)
If you want to code the value 15, you still need two nibbles: 1111 0000
The rule is that if you ever have a nibble of 1111, you must follow it with another nibble

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Lossy Compression
Audio and video files do not compress well using lossless techniques
And we can take advantage of the fact that the human ear and eye can be tricked into hearing and seeing things that aren’t really there
So let’s use lossy compression techniques on audio and video (just as long as we don’t lose too much of the audio or video!)

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Audio Compression
Much audio is now compressed – MP3 players found in cell phones and iPod-like devices store and play compressed music
Audio compression is tricky and hard to describe. For example, a louder sound may mask a softer sound when both played together (so drop the softer sound)
Some people don’t like compressed audio and prefer to store their music in uncompressed form (such as FLAC), but this takes more storage

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Video Compression
Video (both still images and moving video) does not compress well using run-length encoding
When examining the pixel values in an image, not many are alike
But what about from frame to frame within a moving video?
The difference between video frames is usually very small
So what if we just sent the difference between frames?

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5 7 6 2 8 6 6 3 5 6
6 5 7 5 5 6 3 2 4 7
8 4 6 8 5 6 4 8 8 5
5 1 2 9 8 6 5 5 6 6
First Frame
5 7 6 2 8 6 6 3 5 6
6 5 7 6 5 6 3 2 3 7
8 4 6 8 5 6 4 8 8 5
5 1 3 9 8 6 5 5 7 6
Second Frame
0 0 0 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 -1 0
0 0 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0 1 0
Difference
Video Compression (continued)
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MPEG
MPEG (Motion Picture Experts Group) is a group of people that have created a set of standards that can use these small differences between frames to compress a moving video (and audio) to a fraction of its original size

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Image Compression
What about individual images?
For example, a color image can be defined by red/green/blue, or luminance/chrominance/ chrominance, which are based on RGB values (Red, Green, Blue)
If you have three color values and each is 8 bits, you have 24 bits total (or 224 colors!)

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Image Compression (continued)
Consider a VGA screen is 640 x 480 pixels
24 bits x 640 x 480 = 7,372,800 bits – Ouch!
We need compression!

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Image Compression (continued)
JPEG (Joint Photographic Experts Group)
Compresses still images
Lossy
JPEG compression consists of 3 phases:
Discrete cosine transformations (DCT)
Quantization
Run-length encoding

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JPEG (continued)
JPEG Step 1 – DCT
Divide image into a series of 8×8 pixel blocks
If the original image was 640×480 pixels, the new picture would be 80 blocks x 60 blocks (next slide)
If B&W, each pixel in 8×8 block is an 8-bit value (0-255)
If color, each pixel is a 24-bit value (8 bits for red, 8 bits for blue, and 8 bits for green)

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80 blocks
60 blocks
640 x 480 VGA Screen Image
Divided into 8 x 8 Pixel Blocks
JPEG (continued)
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JPEG (continued)
JPEG Step 1 – DCT (continued)
So what does DCT do?
Takes an 8×8 matrix (P) and produces a new 8×8 matrix (T) using cosines
T matrix contains a collection of values called spatial frequencies
These spatial frequencies relate directly to how much the pixel values change as a function of their positions in the block

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Data Communications and Computer Networks: A Business User’s Approach, Seventh Edition
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JPEG (continued)
JPEG Step 1 – DCT (continued)
An image with uniform color changes (little fine detail) has a P matrix with closely similar values and a corresponding T matrix with many zero values
An image with large color changes over a small area (lots of fine detail) has a P matrix with widely changing values, and thus a T matrix with many non-zero values (as shown on next slide)

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Original pixel
values (P matrix)
Values (T matrix)
after the cosine
transformation
has been
applied (note
larger values in
upper-left)

652 32 -40 54 -18 129 -33 84

111 -33 53 9 123 -43 65 100

-22 101 94 -32 23 104 76 101

88 33 211 2 -32 143 43 14

132 -32 43 0 122 -48 54 110

54 11 133 27 56 154 13 -94

-54 -69 10 109 65 0 17 -33

199 -18 99 98 22 -43 8 32

120 80 110 65 90 142 56 100

40 136 93 188 90 210 220 56

95 89 134 74 170 180 45 100

9 110 145 93 221 194 83 110

65 202 90 18 164 90 155 43

93 111 39 221 33 37 40 129

55 122 52 166 93 54 13 100

29 92 153 197 84 197 83 83

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JPEG (continued)
JPEG Step 2 -Quantization
The human eye can’t see small differences in color
So take T matrix and divide all values by 10
Will give us more zero entries
More 0s means more compression!
But this is too lossy
And dividing all values by 10 doesn’t take into account that upper left of matrix has more weight
So divide T matrix by a matrix like the following:

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JPEG (continued)
If we divide the T matrix by the above matrix,
we might get something like the next slide:
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1 4 7 10 13 16 19 22

4 7 10 13 16 19 22 25

7 10 13 16 19 22 25 28

10 13 16 19 22 25 28 31

13 16 19 22 25 28 31 33

16 19 22 25 28 31 33 36

19 22 25 28 31 34 37 40

22 25 28 31 34 37 40 43

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JPEG (continued)
The resulting matrix Q after cosine
transformation and quantization
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652 8 -5 5 -1 8 0 3

27 -4 5 0 7 -2 2 4

-3 10 7 2 1 4 3 3

8 2 13 0 -1 5 1 0

10 -2 2 0 4 -1 1 3

3 0 6 1 2 4 0 -2

-2 -3 0 3 2 0 0 0

9 0 3 3 0 -1 0 0

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JPEG (continued)
JPEG Step 3 – Run-length encoding
Now take the quantized matrix Q and perform run-length encoding on it
But don’t just go across the rows
Longer runs of zeros if you perform the run-length encoding in a diagonal fashion

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JPEG (continued)
Figure 5-13
Run-length encoding of a JPEG image
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JPEG (continued)
How do you get the image back?
Undo run-length encoding
Multiply matrix Q by matrix U yielding matrix T
Apply similar cosine calculations to get original P matrix back

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Business Multiplexing In Action
Bill’s Market has 10 cash registers at the front of their store
Bill wants to connect all cash registers together to collect data transactions
List some efficient techniques to link the cash registers

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Business Multiplexing In Action (continued)
Possible solutions
Connect each cash register to a server using point-to-point lines
Transmit the signal of each cash register to a server using wireless transmissions
Combine all the cash register outputs using multiplexing, and send the multiplexed signal over a conducted-medium line

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Summary
For multiple signals to share a single medium, the medium must be divided into multiple channels
Frequency division multiplexing involves assigning nonoverlapping frequency ranges to different signals
Uses analog signals
Time division multiplexing of a medium involves dividing the available transmission time on a medium among the users
Uses digital signals

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Summary (continued)
Synchronous time division multiplexing accepts input from a fixed number of devices and transmits their data in an unending repetitious pattern
Statistical time division multiplexing accepts input from a set of devices that have data to transmit, creates a frame with data and control information, and transmits that frame
Wavelength division multiplexing involves fiber-optic systems and the transfer of multiple streams of data over a single fiber using multiple, colored laser transmitters
Discrete multitone is a technology used in DSL systems

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Summary (continued)
Code division multiplexing allows multiple users to share the same set of frequencies by assigning a unique digital code to each user
Compression is a process that compacts data into a smaller package
Two basic forms of compression exist: lossless and lossy
Two popular forms of lossless compression include run-length encoding and the Lempel-Ziv compression technique
Lossy compression is the basis of a number of compression techniques

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WIDE AREA NETWORK ESSENTIALS
GUIDE TO NETWORKING ESSENTIALS

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OBJECTIVES
Describe the fundamentals of WAN operation and devices
Discuss the methods used to connect to WANs
Configure and describe remote access protocols
Describe the three major areas of cloud computing

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WIDE AREA NETWORK FUNDAMENTALS
Internetworks and WANs can be described as two or more LANs connected together
Most obvious difference between internetworks and WANs is the distance between the LANs being connected.
They also differ in two other areas:
WANs use the services of carriers or service providers (phone companies and ISPs) for network connection
WANs use serial communication that can span miles compared to LAN technologies that span distances measured in hundreds of meters

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WAN DEVICES
WANs operate at the Data Link and Physical layers (Layers 2 and 1) of the OSI model
Several types of devices are likely to be used in WANS for media access, signal transmission, and reception and to connect a WAN to a LAN:
Modems
Channel service units/data service units
Routers

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MODEMS
A modem is a device that allows a computer (which works with digital signals) to communicate over lines that analog signals
A digital signal is a series of binary 1s and 0s represented by some type of signal that has two possible states (0v or 5v)
An analog signal varies over time continually and smoothly (transitions from 0v to 5v)

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CSU/DSUS
A channel service unit/data service unit (CSU/DSU) is a device that creates a digital connection between a LAN device (router) and the WAN link from the service provider
The WAN link is usually a T-carrier technology, such as a T1 or T3 (discussed later)
Similar to a modem, only all signals are digital
Converts one type of digital signal to another type of digital signal

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ROUTERS
A router is responsible for getting packets from one network to another
In a WAN, it is usually the device connecting a LAN to the WAN service provider via a modem or CSU/DSU

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WAN CONNECTION METHODS
Many WAN technologies are available and differ in speed, level of security and reliability, and cost
Four most common connection methods:
Circuit-switched
Leased line
Packet-switched
VPN over the Internet

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CIRCUIT-SWITCHED WANS
A circuit-switched WAN creates a temporary dedicated connection between sender and receiver on demand
Analog example: a phone line connection from the PSTN, also known as plain old telephone service (POTS)
Digital example: Integrated Services Digital Network (ISDN)
Not as common today due to faster technologies but still in use in some areas

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CIRCUIT-SWITCHED WANS
Plain Old Telephone Service (POTS) – limited in bandwidth due to the digital-to-analog conversion that is performed, usually by modems
The conversion process degrades signal quality and limits data transfer speeds to about 56 Kbps
The most common modem standard for connecting to the Internet is V.92
V.92 modems use a technique called pulse code modulation (PCM) that digitizes analog signals and introduces less noise into the signal
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CIRCUIT-SWITCHED WANS

Modem communication using the V.92 standard

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CIRCUIT-SWITCHED WANS
Integrated Services Digital Network (ISDN) – a digital communication technology developed in 1984 to replace the analog phone system
Was not as popular as expected but can still be found in many US metropolitan areas and Western Europe
Defines communication channels of 64 Kbps
Two formats or rates:
BRI – Basic Rate Interface: consists of two B-channels (64 Kbps) and a D-channel (16 Kbps). B-channels are used for data transfer so BRI can operate at up to 128 Kbps
PRI – Primary Rate Interface (PRI): consists of 23 B-channels and a D-channel. Can provide up to 1.544 Mbps

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LEASED LINES
A leased line provides a dedicated point-to-point connection from the customer’s LAN through the provider’s network and the destination network
Provides permanent, secure, and dedicated bandwidth limited only by the provider’s technology and how much the customer is willing to spend
Most expensive WAN connectivity because it is dedicated
Should be considered:
When high quality, 24/7 access is needed
For mission-critical applications
When fast upstream as well as downstream communication is required

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LEASED LINES
Leased lines are based on one of two types of digital technology: T-carriers and SONET
T-carriers
Typical lines are T1 and T3 that operate at 1.544 Mbps and
44 Mbps, respectively
Derived from multiple 64 Kbps channels, making a T1 connection a grouping of 24 channels, and a T3 connection a grouping of 672 channels
Uses a signaling method called time division multiplexing (TDM): Allocates a time slot for each channel
If a portion of a T-carrier line is used for one purpose and a different portion for another purpose, the line has been fractionalized

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LEASED LINES
T-Carriers (cont.)
Multiplexing:enables several communication streams to travel simultaneously over the same cable segment

Require a CSU/DSU at each end of the link to convert the signals used by the T-carrier line into signals used by the LAN
T1 lines can use twisted-pair, coaxial or fiber-optic cabling
T3 lines can use coaxial or fiber-optic cabling
T1 lines are the most common WAN connection method in the US

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LEASED LINES
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LEASED LINES
Synchronous Optical Network (SONET) – flexible, highly fault-tolerant technology that can operate at different capacities over fiber cabling
Typical SONET rates are OC-3 (155 Mbps), OC-12 (622 Mbps), OC-48 (almost 2.5 Gbps), OC-192, and OC-768 (used by large ISPs)
SONET networks can carry traffic from a variety of other network types, such as T-carrier and ATM
SONET uses a dual-ring topology (like FDDI), making it very fault-tolerant

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PACKET-SWITCHED WANS
A packet-switched WAN does not create a dedicated connection between sender and receiver
Each packet is transmitted through the provider’s network independently (similar to LAN traffic)
Data shares bandwidth with your provider’s other customers
Most common packet-switched networks are:
X.25
Frame relay
ATM
MPLS

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PACKET-SWITCHED WANS
Virtual Circuits – a logical connection created between two devices in a shared network
No single cable exists between the two endpoints
Maps a path through the network of switches between two points
The pathway is created after sender and receiver agree on bandwidth requirements and request a pathway
Switched virtual circuit (SVCs): established when needed and then terminated when the transmission is completed
Permanent virtual circuit (PVCs): pathway between two communication points is established as a permanent logical connection (more expensive than SVCs)

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X.25 NETWORKS
Packet-switching technology developed in the
mid-1970s running over older copper phone lines
Offer both SVCs and PVCs – although not all X.25 providers offer PVCs
Earlier X.25 could only operate at 64 Kbps
A 1992 specification revision improved the maximum throughput of X.25 to 2 Mbps, but the new version was not widely deployed
Even though X.25 offers reliable and error-free communications, this technology has been largely replaced by other higher-speed technologies

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FRAME RELAY NETWORKS
A PVC packet-switching technology that offers WAN communication over a fast, reliable digital link
Can maintain transmission rates from 64 Kbps to 44 Mbps (T3 speed)
Allows customers to specify the bandwidth needed
Charges depend on the PVC’s bandwidth allocation (known as Committed Information Rate [CIR])
CIR is the guaranteed minimum transmission rate
Connection is established by using a pair of CSU/DSUs with a router or bridge at each end to direct traffic on and off the WAN link

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FRAME RELAY NETWORKS
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ATM NETWORKS
Asynchronous Transfer Mode (ATM) – high-speed network technology designed for both LAN and WAN use
ATM bandwidth can be as low as a few Mbps up to 622 Mbps, but the most common speed is 155 Mbps
Cell-based packet switching technology
Cells are of a fixed length rather than typical packet-based systems that use variable length packets
Fixed length cells can be switched more efficiently than variable length packets
ATM is used quite heavily for the backbone and infrastructure in large communications companies

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MULTIPROTOCOL LABEL SWITCHING (MPLS)
MPLS runs over ATM, frame relay, SONET, and even Ethernet
Creates a connection-oriented virtual circuit using labels assigned to each packet
The label is used to make packet-forwarding decisions within the MPLS network, making it unnecessary to view the contents of the packet
Capable of supporting different Layer 3 protocols, it is currently used exclusively in IP networks
Supports both IPv4 and IPv6

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WANS OVER THE INTERNET
Using VPN connections over inexpensive Internet connections is becoming a popular WAN alternative
VPNs offer the following advantages over other WAN methods discussed:
Inexpensive: Cost of Internet access is much lower than leased lines or packet-switched WAN connections
Convenience: A VPN can be configured as soon as Internet access is established
Security: Advanced authentication and encryption protocols protect the integrity and privacy of VPN traffic
Flexibility: After a corporate VPN infrastructure is in place, it is available for WAN connections from branch offices as well as mobile users and telecommuters

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WAN CONNECTIONS METHODS
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WAN EQUIPMENT
Customer Equipment
Customer Premises Equipment (CPE): The equipment at the customer site that’s usually the responsibility of the customer
Customer might own or lease the equipment from the provider
Usually includes routers, modems and CSU/DSUs
The demarcation point is the point at which the CPE ends and the provider’s responsibility begins (where the WAN connection is made)

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WAN EQUIPMENT
Provider Equipment
The provider location nearest the customer site is usually referred to as the central office (CO)
Media (usually coax or fiber) runs from the customer site demarcation point to the CO of the WAN service provider
The connection between the demarcation point and the CO is called the local loop or last mile

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WAN EQUIPMENT
Going the Last Mile
The device that sends data to the local loop is called the data circuit-terminating equipment (DCE): The CSU/DSU or modem
The device that passes data from the customer LAN to the DCE is called the data terminal equipment (DTE): Router or bridge that has one connection to the customer LAN and another connection to the DCE that makes the WAN connection

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WAN EQUIPMENT

A WAN connection showing the CPE, demarcation point, and local loop
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REMOTE ACCESS NETWORKING
Windows server OSs include the Routing and Remote Access Service (RRAS) that supports both dial-up remote access and VPN remote access
Users can dial in over POTS or use a VPN from any type of Internet connection

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MAKING A VPN CONNECTION IN WINDOWS
In Windows 7, you create a new connection from the Network and Sharing Center by selecting “Set up a new connection or network”
This will start the “Set Up a Connection or Network Wizard”

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MAKING A DIAL-UP CONNECTION
All versions of Windows, starting with Windows 95, include Dial-Up Networking (DUN) software to make an RRAS connection
The protocol used is Point-to-Point Protocol (PPP) and is used to carry a variety of protocols over different types of network connections
Two protocols that are integral to PPP:
Link Control Protocol (LCP): Sets up the PPP connection and defines communications parameters and authentication protocols
Network Control Protocol (NCP): Encapsulates higher layer protocols such as IP and provides services such as dynamic IP addressing

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REMOTE ACCESS NETWORKING
VIA THE WEB
Another remote access model is remote control of the desktop of your office computer using a Web browser
Several online services connect your Web browser to your desktop, including LogMeIn and GoToMyPC
A client component is installed on your computer and then log on to the online service which connects you
Uses authentication and encryption to maintain a secure connection
Third party software can also be used
Microsoft’s Terminal Services Gateway (TSG) allows remote connections by using SSL, the protocol that secures communication between Web browsers and Web servers

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CLOUD COMPUTING
Cloud computing is a computer networking model in which data, applications, and processing power are managed by servers on the Internet, and users of resources pay for what they use rather than for the equipment and software needed to provide the resources
Benefits:
Reduced physical plant costs
Reduced upfront costs
Reduced personnel costs

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CLOUD COMPUTING
There are three primary categories of cloud computing:
Hosted applications
Hosted platforms
Hosted infrastructure

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HOSTED APPLICATIONS
Hosted applications are also referred to as on-demand applications or software as a service (SaaS)
Usually offered as a subscription based on the number of users
Customers can take advantage of new software editions much faster
Available anywhere the customer has a connection to the Internet
Most well-known example is Google Apps

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HOSTED PLATFORMS
Hosted platform or platform as a service (PAAS)
A customer develops applications using the service providers development tools and infrastructure
Once developed, the applications can be delivered to the customer’s users from the provider’s servers
Most common hosted platforms available are Force.com’s Apex, Azure for Windows, Google’s AppEngine for Phython and Java, WaveMaker for Ajax, and Engine Yard for Ruby on Rails

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HOSTED INFRASTRUCTURE
Hosted infrastructure or infrastructure as a service (IaaS) allows a company to use storage or entire virtual servers
If a customer needs another 100 GB of space, they can pay for the space without worrying about how that space is actually provided
If a customer needs another server they pay for the amount of processing and storage the additional server actually requires
Customers rent the resources they are using

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CHAPTER SUMMARY
The most obvious difference between internetworks and WANs is the distance between the LANs being connected
Several types of devices are likely to be used in WANs for media access, signal transmission, and reception and to connect a WAN to a LAN: Modems, CSU/DSU, and Routers
The methods used to make a WAN connection often dictate the technologies that can be used and the connection’s properties. The four most common are circuit-switched, leased line, packet-switched, and VPN over the Internet

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CHAPTER SUMMARY
WAN equipment can be categorized into customer equipment, provider equipment, and the circuit that makes the connections between the demarcation point and the central office; called the last mile or local loop
Large and small businesses alike are leveraging fast, affordable remote access technologies that allow employees to access their office desktops and corporate resources from home and on the road

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CHAPTER SUMMARY
Cloud computing is a computer networking model in which data, applications, and processing power are managed by servers on the Internet, and users pay for what they use rather than for the equipment and software needed to provide the resources
There are three primary categories of cloud computing: hosted applications, hosted platforms, and hosted infrastructure

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Chapter Two
Fundamentals of Data and Signals

Data Communications and Computer Networks: A Business User’s Approach
Eighth Edition
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Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
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After reading this chapter,
you should be able to:
Distinguish between data and signals, and cite the advantages of digital data and signals over analog data and signals
Identify the three basic components of a signal
Discuss the bandwidth of a signal and how it relates to data transfer speed
Identify signal strength and attenuation, and how they are related

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Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
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After reading this chapter,
you should be able to (continued):
Outline the basic characteristics of transmitting analog data with analog signals, digital data with digital signals, digital data with analog signals, and analog data with digital signals
List and draw diagrams of the basic digital encoding techniques, and explain the advantages and disadvantages of each
Identify the different shift keying (modulation) techniques, and describe their advantages, disadvantages, and uses

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After reading this chapter,
you should be able to (continued):

Identify the two most common digitization techniques, and describe their advantages and disadvantages
Identify the different data codes and how they are used in communication systems

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Introduction
Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine)
Signals are the electric or electromagnetic encoding of data (telephone conversation, web page download)
Computer networks and data/voice communication systems transmit signals
Data and signals can be analog or digital

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Introduction (continued)
Table 2-1 Four combinations of data and signals

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Data and Signals
Data are entities that convey meaning within a computer or computer system
Signals are the electric or electromagnetic impulses used to encode and transmit data

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Analog vs. Digital
Data and signals can be either analog or digital

Analog is a continuous waveform, with examples such as (naturally occurring) music and voice
It is harder to separate noise from an analog signal than it is to separate noise from a digital signal (see the following two slides)

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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
Digital is a discrete or non-continuous waveform
Something about the signal makes it obvious that the signal can only appear in a fixed number of forms (see next slide)
Noise in digital signal
You can still discern a high voltage from a low voltage
Too much noise – you cannot discern a high voltage from a low voltage

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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)

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Fundamentals of Signals
All signals have three components:
Amplitude
Frequency
Phase

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Fundamentals of Signals – Amplitude
Amplitude
The height of the wave above or below a given reference point
Amplitude is usually measured in volts

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Fundamentals of Signals – Amplitude
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Fundamentals of Signals – Frequency
Frequency
The number of times a signal makes a complete cycle within a given time frame; frequency is measured in Hertz (Hz), or cycles per second (period = 1 / frequency)
Spectrum – Range of frequencies that a signal spans from minimum to maximum
Bandwidth – Absolute value of the difference between the lowest and highest frequencies of a signal
For example, consider an average voice
The average voice has a frequency range of roughly 300 Hz to 3100 Hz
The spectrum would be 300 – 3100 Hz
The bandwidth would be 2800 Hz

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Fundamentals of Signals – Frequency
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Fundamentals of Signals – Phase
Phase
The position of the waveform relative to a given moment of time or relative to time zero
A change in phase can be any number of angles between 0 and 360 degrees
Phase changes often occur on common angles, such as 45, 90, 135, etc.

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Fundamentals of Signals – Phase
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Fundamentals of Signals
Phase
If a signal can experience two different phase angles, then 1 bit can be transmitted with each signal change (each baud)
If a signal can experience four different phase angles, then 2 bits can be transmitted with each signal change (each baud)
Note: number of bits transmitted with each signal change = log2 (number of different phase angles)
(You can replace “phase angles” with “amplitude levels” or “frequency levels”)

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Loss of Signal Strength
All signals experience loss (attenuation)
Attenuation is denoted as a decibel (dB) loss
Decibel losses (and gains) are additive

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Loss of Signal Strength (continued)
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Loss of Signal Strength
Formula for decibel (dB):

dB = 10 x log10 (P2 / P1)

where P1 is the beginning power level and P2 is the ending power level

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Loss of Signal Strength (continued)
So if a signal loses 3 dB, is that a lot?
What if a signal starts at 100 watts and ends at 50 watts? What is dB loss?

dB = 10 x log10 (P2 / P1)
dB = 10 x log10 (50 / 100)
dB = 10 x log10 (0.5)
dB = 10 x -0.3
dB = -3.0
So a 3.0 decibel loss losses half of its power

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Converting Data into Signals
There are four main combinations of data and signals:
Analog data transmitted using analog signals
Digital data transmitted using digital signals
Digital data transmitted using discrete analog signals
Analog data transmitted using digital signals
Let’s look at each these

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1. Transmitting Analog Data with
Analog Signals
In order to transmit analog data, you can modulate the data onto a set of analog signals
Broadcast radio and the older broadcast television are two very common examples of this
We modulate the data onto another set of frequencies so that all the different channels can coexist at different frequencies

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1. Transmitting Analog Data with
Analog Signals (continued)
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2. Transmitting Digital Data with Digital Signals: Digital Encoding Schemes
There are numerous techniques available to convert digital data into digital signals. Let’s examine five:
NRZ-L
NRZI
Manchester
Differential Manchester
Bipolar AMI
These are used in LANs and some telephone systems

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2. Transmitting Digital Data with Digital Signals: Digital Encoding Schemes (continued)
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Nonreturn to Zero Digital Encoding Schemes
Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages
Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0
Fundamental difference exists between NRZ-L and NRZI
With NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1,
With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1

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Manchester Digital Encoding Schemes
Note how with a Differential Manchester code, every bit has at least one significant change. Some bits have two signal changes per bit (baud rate = twice bps)

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Manchester Digital Encoding Schemes (continued)
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Bipolar-AMI Encoding Scheme
The bipolar-AMI encoding scheme is unique among all the encoding schemes because it uses three voltage levels
When a device transmits a binary 0, a zero voltage is transmitted
When the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted
Which of these is transmitted depends on the binary 1 value that was last transmitted

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4B/5B Digital Encoding Scheme
Yet another encoding technique; this one converts four bits of data into five-bit quantities
The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes
The five-bit code is then transmitted using an NRZI encoded signal

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4B/5B Digital Encoding Scheme (continued)
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3. Transmitting Digital Data with
Discrete Analog Signals
Three basic techniques:
Amplitude shift keying
Frequency shift keying
Phase shift keying
One can then combine two or more of these basic techniques to form more complex modulation techniques (such as quadrature amplitude modulation)

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Amplitude Shift Keying
One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)

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Amplitude Shift Keying (continued)
Note: here we have four different amplitudes, so we can encode 2 bits
in each signal change (bits per signal change = log2 (amplitude levels)).
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Frequency Shift Keying
One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)

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Phase Shift Keying
One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)

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Phase Shift Keying (continued)
Quadrature Phase Shift Keying
Four different phase angles used
45 degrees
135 degrees
225 degrees
315 degrees

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Phase Shift Keying (continued)
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Phase Shift Keying (continued)
Quadrature amplitude modulation
As an example of QAM, 12 different phases are combined with two different amplitudes
Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations
With 16 signal combinations, each baud equals 4 bits of information (log2(16) = 4, or inversely, 2 ^ 4 = 16)

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Phase Shift Keying (continued)
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4. Transmitting Analog Data with
Digital Signals
To convert analog data into a digital signal, there are two techniques:
Pulse code modulation (the more common)
Delta modulation

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Pulse Code Modulation
The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values

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Pulse Code Modulation (continued)
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Pulse Code Modulation (continued)
When the binary values are later converted to an analog signal, a waveform similar to the original results

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Pulse Code Modulation (continued)
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Pulse Code Modulation (continued)
The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution

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Pulse Code Modulation (continued)
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Pulse Code Modulation (continued)
Since telephone systems digitize human voice, and since the human voice has a fairly narrow bandwidth, telephone systems can digitize voice into either 128 or 256 levels
These are called quantization levels
If 128 levels, then each sample is 7 bits (2 ^ 7 = 128)
If 256 levels, then each sample is 8 bits (2 ^ 8 = 256)

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Pulse Code Modulation (continued)
How fast do you have to sample an input source to get a fairly accurate representation?
Nyquist says 2 times the highest frequency
Thus, if you want to digitize voice (4000 Hz), you need to sample at 8000 samples per second

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Delta Modulation
An analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop

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Delta Modulation (continued)
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The Relationship Between Frequency and Bits Per Second
Higher Data Transfer Rates
How do you send data faster?
Use a higher frequency signal (make sure the medium can handle the higher frequency
Use a higher number of signal levels
In both cases, noise can be a problem

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The Relationship Between Frequency and Bits Per Second (continued)
Maximum Data Transfer Rates
How do you calculate a maximum data rate?
Use Shannon’s equation
S(f) = f x log2 (1 + S/N)
Where f = signal frequency (bandwidth), S is the signal power in watts, and N is the noise power in watts
For example, what is the data rate of a 3400 Hz signal with 0.2 watts of power and 0.0002 watts of noise?
S(f) = 3400 x log2 (1 + 0.2/0.0002)
= 3400 x log2 (1001)
= 3400 x 9.97
= 33898 bps

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Data Codes
The set of all textual characters or symbols and their corresponding binary patterns is called a data code
There are three common data code sets:
EBCDIC
ASCII
Unicode

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EBCDIC
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ASCII
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Chapter One
Introduction to Computer Networks and Data Communications

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After reading this chapter,
you should be able to:
Define the basic terms of computer networks
Recognize the individual components of the big picture of computer networks
Outline the common examples of communications networks
Define the term “convergence” and describe how it applies to computer networks
Cite the reasons for using a network architecture and explain how they apply to current network systems

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After reading this chapter,
you should be able to (continued):
List the layers of the TCP/IP protocol suite and describe the duties of each layer
List the layers of the OSI model and describe the duties of each layer
Compare the TCP/IP protocol suite and the OSI model and list their differences and similarities

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Introduction
Who today has not used a computer network?
Mass transit, interstate highways, 24-hour bankers, grocery stores, cable television, cell phones, businesses and schools, and retail outlets support some form of computer network

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The Language of Computer Networks
Computer network – an interconnection of computers and computing equipment using either wires or radio waves over small or large geographic areas
Local area network – networks that are small in geographic size spanning a room, floor, building, or campus
Metropolitan area network – networks that serve an area of 1 to 30 miles, approximately the size of a typical city

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The Language of Computer Networks (continued)
Wide area network – a large network that encompasses parts of states, multiple states, countries, and the world
Personal area network – a network of a few meters, between wireless devices such as PDAs, laptops, and similar devices
Campus area network – a network that spans multiple buildings on a business or school campus

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The Language of Computer Networks (continued)
Voice network – a network that transmits only telephone signals (essentially xtinct)
Data network – a network that transmits voice and computer data (replacing voice networks)

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The Language of Computer Networks (continued)
Data communications – the transfer of digital or analog data using digital or analog signals
Telecommunications – the study of telephones and the systems that transmit telephone signals (becoming simply data communications)
Network management – the design, installation, and support of a network, including its hardware and software
Network cloud – a network (local or remote) that contains software, applications, and/or data

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The Big Picture of Networks
Networks are composed of many devices, including:
Workstations (computers, tablets, wireless phones, etc)
Servers
Network switches
Routers (LAN to WAN and WAN to WAN)
Network nodes and subnetworks

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The Big Picture of Networks (continued)
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Common Examples of Communications Networks

The desktop computer and the Internet
A laptop computer and a wireless connection
Cell phone networks
Industrial sensor-based systems
Mainframe systems
Satellite and microwave networks

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Common Examples of Communications Networks
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The Desktop Computer and the Internet
Common throughout business, academic environments, and homes
Typically a medium- to high-speed connection
Computer (device) requires a NIC (network interface card)
NIC connects to a hub-like device (switch)
Often considered a client/server system

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The Desktop Computer and the Internet
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The Desktop Computer and the Internet
At work or at school – connection is typically some form of Ethernet
At home, for some, a dial-up modem is used to connect user’s microcomputer to an Internet service provider
Technologies such as DSL and cable modems are replacing dial-up modems

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The Desktop Computer and the Internet
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A Laptop Computer and a Wireless Connection
At work or at school – connection is typically some form of wireless Ethernet
Laptop wirelessly communicates with a wireless router or wireless access point
Wireless router is typically connected to a wired-network

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A Laptop Computer and a Wireless Connection
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Cell Phone Networks
Constantly expanding market across the U.S. and world
Third generation services available in many areas and under many types of plans with fourth generation services starting to appear
Latest generation includes higher speed data transfers (100s to 1000s of kilobits per second)

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Cell Phone Networks (continued)
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Industrial Sensor-based Systems
Not all local area networks deal with microcomputer workstations
Often found in industrial and laboratory environments
Assembly lines and robotic controls depend heavily on sensor-based local area networks

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Industrial Sensor-based Systems
(continued)
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Mainframe Systems
Predominant form in the 1960s and 1970s
Still used in many types of businesses for data entry and data retrieval
Few dumb terminals left today – most are microcomputers with terminal emulation card, a web browser and web interface, Telnet software, or a thin client

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Mainframe Systems (continued)
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Satellite and Microwave Networks
Typically long distance wireless connections
Many types of applications including long distance telephone, television, radio, long-haul data transfers, and wireless data services
Typically expensive services but many companies offer competitive services and rates
Newer shorter-distance services such as Wi-Max

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Satellite and Microwave Networks (continued)
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Network Architectures
A reference model that describes the layers of hardware and software necessary to transmit data between two points or for multiple devices / applications to interoperate
Reference models are necessary to increase likelihood that different components from different manufacturers will converse
Two models to learn: TCP/IP protocol suite and OSI model

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The TCP/IP Protocol Suite
Note: Some authors show only four layers, combining the two
bottom layers.
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The TCP/IP Protocol Suite (continued)
Application layer
Where the application using the network resides
Common network applications include web browsing, e-mail, file transfers, and remote logins
Transport layer
Performs a series of miscellaneous functions (at the end-points of the connection) necessary for presenting the data package properly to the sender or receiver

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The TCP/IP Protocol Suite (continued)
Network (Internet or internetwork or IP) layer
Responsible for creating, maintaining and ending network connections
Transfers data packet from node to node (e.g. router to router) within network
Network access (data link) layer
Responsible for taking the data and transforming it into a frame with header, control and address information, and error detection code, then transmitting it between the workstation and the network
Physical layer
Handles the transmission of bits over a communications channel
Includes voltage levels, connectors, media choice, modulation techniques

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The TCP/IP Protocol Suite (continued)

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The Open Systems Interconnection (OSI) Model
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The Open Systems Interconnection (OSI) Model (continued)
Application layer
Equivalent to TCP/IP’s application layer
Presentation layer
Responsible for “final presentation” of data (code conversions, compression, encryption)
Session layer
Responsible for establishing “sessions” between users

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The Open Systems Interconnection (OSI) Model (continued)
Transport layer
Equivalent to TCP/IP’s transport layer
Network layer
Equivalent to TCP/IP’s network layer
Data link layer
Responsible for taking the data and transforming it into a frame with header, control and address information, and error detection code

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The Open Systems Interconnection (OSI) Model (continued)
Physical layer
Handles the transmission of bits over a communications channel
Includes voltage levels, connectors, media choice, modulation techniques

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Logical and Physical Connections
A logical connection is one that exists only in the software, while a physical connection is one that exists in the hardware
Note that in a network architecture, only the lowest layer contains the physical connection, while all higher layers contain logical connections

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Logical and Physical Connections (continued)
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Logical and Physical Connections (continued)
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The TCP/IP Protocol Suite in Action
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Summary
Many services and products that we use every day employ computer networks and data communications in some way
Field of data communications and computer networks includes data networks, voice networks, wireless networks, local area networks, metropolitan area networks, wide area networks, and personal area networks

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Summary (continued)
Common examples of communications networks:
The desktop computer and the Internet
A laptop computer and a wireless connection
Cell phone networks
Industrial sensor-based systems
Mainframe systems
Satellite and microwave networks

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Summary (continued)
Key concept in networking is convergence
A network architecture, or communications model, places network pieces in layers
Layers define model for functions or services that need to be performed
The TCP/IP protocol suite is also known as the Internet model and is composed of five layers (some show four):
Application layer
Transport layer
Network layer
Network access layer
Physical layer

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Summary (continued)
The International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) model
OSI model is based on seven layers: application layer, presentation layer, session layer, transport layer, network layer, data link layer, physical layer
A logical connection is a flow of ideas that occurs, without a direct physical connection, between the sender and receiver at a particular layer

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INTRODUCTION TO COMPUTER NETWORKS
GUIDE TO NETWORKING ESSENTIALS

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OBJECTIVES
Describe basic computer components and operations
Explain the fundamentals of network communication
Define common networking terms
Compare different network models
Identify the functions of various network server types
Describe specialized networks

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AN OVERVIEW OF COMPUTER CONCEPTS
Most of the devices you encounter when working with a network involve a computer
Most obvious devices are workstations and network servers
These run operating systems such as Windows, Linux, UNIX, and Mac OS
Also includes routers and switches
These are specialized computers used to move data from computer to computer and network to network
You will learn more about them in later chapters

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BASIC FUNCTIONS OF A COMPUTER
Computer’s functions can be broken down into three basic tasks:
Input: A user running a word-processing program types the letter “A” on the keyboard, which results in sending a code representing the letter “A” to the computer
Processing: The computer’s central processing unit (CPU) determines what letter was typed by looking up the keyboard code in a table.
Output: The CPU sends instructions to the graphics cards to display the letter “A”, which is then sent to the computer monitor.

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INPUT COMPONENTS
Common user-controlled devices such as keyboards, mice, microphones, Web cameras, and scanners
External interfaces, such as serial, FireWire, and USB ports can also be used to get input from peripheral devices
Storage devices such as hard disks and CDs/DVDs

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PROCESSING COMPONENTS
A computer’s main processing component is the CPU
Executes instructions from computer programs such as word processors and from the computer’s operating system
Current CPUs are composed of two or more processors called cores
A graphics processing unit (GPU) takes a high-level graphics instruction and performs the calculation needed for the instruction to be displayed on the monitor
Other devices, such as network interface cards and disk controller cards, might also include onboard processors

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OUTPUT COMPONENTS
Most obvious are monitors and printers
Also includes storage devices, network cards, and speakers
External interfaces
For example, a disk drive connected to a USB port allows reading files from the disk (input) and writing files to the disk (output).

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STORAGE COMPONENTS
The more storage a computer has, the better
Most storage components are both input and output devices
Most people think of storage as disk drives, CD/DVD drives, and USB flash drives
However, there are two main categories of storage
Short-term storage
Long-term storage

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RAM: SHORT-TERM STORAGE
Random Access Memory (RAM) – when power to the computer is turned off, RAM’s contents are gone
The amount of RAM in a computer is crucial to the computer’s capability to operate efficiently
RAM is also referred to as working storage
If there’s not enough RAM to run a program, the computer will use the disk drive to supplement

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LONG-TERM STORAGE
Maintains its data even when there’s no power
Examples:
Hard disks
CDs/DVDs
USB flash drives

Used to store document and multimedia files
Amount of storage a computer needs depends on the type and quantity of files to be stored

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DATA IS STORED IN BITS
Data on a computer is stored as binary digits (“bits” for short)
A bit holds a 1 or 0 value
A pulse of 5 volts of electricity can represent a 1 bit and a pulse of 0 volts can represent a 0 bit
With fiber-optic cable, a 1 bit is represented by the presence of light and a 0 bit by the absence of light
A “byte” is a collection of 8 bits

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PERSONAL COMPUTER HARDWARE
Four major PC components:
Motherboard
Hard drive
RAM
BIOS/CMOS

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MOTHERBOARD
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COMPUTER BUS FUNDAMENTALS
Bus: a collection of wires carrying data from one place to another on the computer
All data that goes into or comes out of a computer goes through the motherboard
There are buses between:
CPU and RAM
CPU and disk drives
CPU and expansion slots

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HARD DRIVE FUNDAMENTALS
Hard drive: primary long-term storage component on your computer
Consists of magnetic disks called platters that store data in the form of magnetic pulses
Stores the OS your computer loads when it boots
Stores the documents you use as well as the applications that open those documents

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RAM FUNDAMENTALS
RAM is the main short-term storage component on your computer
RAM has no moving parts so accessing data in RAM is much faster than accessing data on a hard drive
In general, the more RAM your system has, the faster it will run

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BIOS/CMOS FUNDAMENTALS
BIOS: basic input/output system
Set of instructions located in a chip on the motherboard
Those instructions tell the CPU to perform certain tasks when power is first applied to the computer
One of those instructions is to perform a power-on self test (POST)
When a computer boots, the BIOS program offers a chance to run the Setup program in order to configure hardware components
This configuration is stored in a type of memory called complementary metal oxide semiconductor (CMOS)

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COMPUTER BOOT PROCEDURE
Power is applied to the motherboard.
The CPU starts.
The CPU carries out the BIOS startup routines, including the POST.
Boot devices, as specified in the BIOS configuration, are searched for an OS.
The OS is loaded into RAM.
OS services are started.
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HOW THE OPERATING SYSTEM AND HARDWARE WORK TOGETHER
A computer’s OS provides many critical services:
a user interface
memory management
a file system
multitasking
the interface to a computer’s hardware devices
Without an OS, each application would have to provide the above services

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USER INTERFACE
Enables people to interact with computers
Graphical user interfaces (GUIs) allow users to point and click to run applications and access services
Without a user interface, a computer could process only information that has been programmed into memory

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MEMORY MANAGEMENT
When the OS loads an application, memory must be allocated for the application to run in
When the application exits, the memory it was using must be marked as available
This memory management is performed by the OS

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FILE SYSTEM
File system is used to organize space on storage devices
Objectives of contemporary file systems:
Provide a convenient interface for users and applications to open and save files
Provide an efficient method to organize space on a drive
Provide a hierarchical filing method to store files
Provide an indexing system for fast retrieval of files
Provide secure access to files by authorized users

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MULTITASKING
Multitasking is an OS’s capability to run more than one application or process at the same time
The OS is designed to look for applications that have work to do and then schedule CPU time so that the work gets done

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INTERFACE TO HARDWARE DEVICES
When an application needs to communicate with computer hardware, it calls on the OS, which then calls on a device driver
A device driver is software that provides the interface between the OS and computer hardware
Every device performing an input or output function requires a device driver

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FUNDAMENTALS OF NETWORK COMMUNICATION
A computer network consists of two or more computers connected by some kind of transmission medium, such as a cable or air waves.
In order to access the Internet, a computer has to be able to connect to a network
The next few slides will cover what is required to turn a standalone computer into a networked computer

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NETWORK COMPONENTS
Hardware components
Network interface card—A NIC is an add-on card that’s plugged into a motherboard expansion slot and provides a connection between the computer and the network.
Network medium—A cable that plugs into the NIC and makes the connection between a computer and the rest of the network. Network media can also be the air waves, as in wireless networks.
Interconnecting—Interconnecting devices allow two or more computers to communicate on the network without having to be connected directly to one another.

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A typical network
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NETWORK COMPONENTS
Software Components
Network clients and servers—Network client software requests information that’s stored on another network computer or device. Network server software allows a computer to share its resources by fielding resource requests generated by network clients.
Protocols—Network protocols define the rules and formats a computer must use when sending information across the network. Think of it as a language that all devices on a network understand.
NIC drivers—NIC drivers receive data from protocols and then forward this data to the physical NIC, which transmits data onto the medium.

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LAYERS OF THE NETWORK COMMUNICATION PROCESS
Each step required for a client to access network resources is referred to as a “layer”
Each layer has a task and all layers work together

Simulation 1 – Layers of the network communication process

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LAYERS OF THE NETWORK COMMUNICATION PROCESS
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of the network communication process

Step
Description
Layer

1
An application tries to access a network resource.

User application

2
Client software detects the attempt to access the network and passes the message on to the network protocol.

Network software

3
The protocol packages the message in a format suitable for the network and sends it to the NIC driver.

Network protocol

4
The NIC driver sends the data in the request to the NIC card, which converts it into the necessary signals to be transmitted across the network medium.

Network interface

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HOW TWO COMPUTERS COMMUNICATE
TCP/IP is the most common protocol (language) used on networks
TCP/IP uses 2 addresses to identify devices on a network
Logical address (called IP address)
Physical address (called MAC address)
Just as a mail carrier needs an address to deliver mail, TCP/IP needs an address in order to deliver data to the correct device on a network
Think of the Logical address as a zip code and the Physical address as a street address

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COMMUNICATION BETWEEN TWO COMPUTERS
A user at Comp A types ping 10.1.1.2 at a command prompt
The network software creates a ping message
The network protocol packages the message by adding IP address of sending and destination computers and acquires the destination computer’s MAC address
The network interface software adds MAC addresses of sending and destination computers and sends the message
Comp B receives message, verifies that the addresses are correct and then sends a reply to Comp A using Steps 2 – 4
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Simulation 2 – Communication between two computers

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NETWORK TERMS EXPLAINED
Every profession has its own language and acronyms
Need to know the language of networks to be able to study them

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LANS, INTERNETWORKS, WANS, AND MANS
Local area network (LAN) – small network, limited to a single collection of machines and connected by one or more interconnecting devices in a small geographic area

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LANS, INTERNETWORKS, WANS, AND MANS
An internetwork is a networked collection of LANs tied together by devices such as routers
Reasons for creation:
Two or more groups of users and their computers need to be logically separated but still need to communicate
Number of computers in a single LAN has grown and is no longer efficient
The distance between two groups of computers exceeds the capabilities of most LAN devices

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LANS, INTERNETWORKS, WANS, AND MANS
Wide area networks (WANs) use the services of third-party communication providers to carry network traffic from one location to another
Metropolitan area networks (MANs) use WAN technologies to interconnect LANs in a specific geographic region, such as a county or city

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PACKETS AND FRAMES
Computers transfer information across networks in shorts bursts of about 1500 bytes of data
Data is transferred in this way for a number of reasons:
The pause between bursts might be necessary to allow other computers to transfer data during pauses
The pause allows the receiving computer to process received data, such as writing it to disk
The pause allows the receiving computer to receive data from other computers at the same time
The pause gives the sending computer an opportunity to receive data from other computers and to perform other processing tasks
If an error occurs during transmission of a large file, only the chunks of data involved in the error have to be sent again, not the entire file

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PACKETS
Chunks of data sent across the network are usually called packets or frames, with packets being the more well-known term
Packet is a chunk of data with source and destination IP address added to it
Using the U.S. mail analogy, you can look at a packet as an envelope that has had the zip code added to the address but not the street address

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FRAMES
A frame is a packet with the source and destination MAC addresses added to it
The packet is “framed” by the MAC addresses on one end and an error-checking code on the other
A frame is like a letter that has been addressed and stamped and is ready to go
The process of adding IP addresses and MAC addresses to chunks of data is called encapsulation
Information added to the front of the data is called a header and information added to the end is called a trailer

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CLIENTS AND SERVERS
A client can be a workstation running a client OS or it can also refer to the network software on a computer that requests network resources from a server
The word “client” is usually used in these three contexts:
Client operating system: The OS installed on a computer
Client computer: Primary role is to run user applications and access network resources
Client software: The software that requests network resources from server software running on another computer

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CLIENTS AND SERVERS
A computer becomes a server when software is installed on it that provides a network service to client computers
The term “server” is also used in three contexts:
Server operating system: When the OS installed on a computer is designed mainly to share network resources and provide other network services
Server computer: When a computer’s primary role in the network is to give client computers access to network resources and services
Server software: Responds to requests for network resources from client software running on another computer

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NETWORK MODELS
A network model defines how and where resources are shared and how access to these resources is regulated
Fall into two major types
Peer-to-peer network: Most computers function as clients or servers (no centralized control over who has access to network resources)
Server-based network: Certain computers take on specialized roles and function mainly as servers, and ordinary users’ machines tend to function mainly as clients

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PEER-TO-PEER/WORKGROUP MODEL
Computers on a peer-to-peer network can take both a client and a server role
Any user can share resources on his/her computer with any other user’s computer
Every user must act as the administrator of his/her computer
Can give everyone else unlimited access to their resources or grant restricted access to other users
Usernames and passwords (credentials) are used to control that access

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PEER-TO-PEER/WORKGROUP MODEL
Problems with Peer-to-peer networks:
Must remember multiple sets of credentials to access resources spread out over several computers
Desktop PCs and the OSs installed on them aren’t made to provide network services as efficiently as dedicated network servers
Data organization: If every machine can be a server, how can users keep track of what information is stored on which machine?
Peer-to-peer networks are well suited for small organizations that have small networks and small operating budgets

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SERVER/DOMAIN-BASED MODEL
Server-based networks provide centralized control over network resources
Users log on to the network with a single set of credentials maintained by one or more servers running a server OS
In most cases, servers are dedicated to running network services and should not be used to run user applications

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SERVER/DOMAIN-BASED MODEL
A domain is a collection of users and computers whose accounts are managed by Windows servers called domain controllers
Users and computers in a domain are subject to network access and security policies defined by a network administrator
The software that manages this security is referred to as a directory service
On Windows servers, the directory service software is Active Directory

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SERVER/DOMAIN-BASED MODEL (CONT.)
Other network services usually found on network servers:
Naming services: Translate computer names to their address
E-mail services: Manage incoming and outgoing email
Application services: Grant client computers access to complex applications that run on the server
Communication services: Give remote users access to a network
Web services: Provide comprehensive Web-based application services

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SERVER/DOMAIN-BASED MODEL (CONT.)
Server-based networks are easier to expand than peer-to-peer
Peer-to-peer should be limited to 10 or fewer users, but
server-based networks can handle up to thousands of users
Multiple servers can be configured to work together, which can be used to run a more efficient network or can provide fault tolerance
Peer-to-peer and server-based networks both have advantages so using a combination of the two models isn’t uncommon

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STRENGTHS AND WEAKNESSES OF THE TWO NETWORK MODELS
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peer versus server-based networks

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NETWORK SERVERS
A server is at the heart of any network that is too large for a peer-to-peer configuration
A single server can be configured to fill a single role or several roles at once
Most common server roles found on networks:
Domain controller/directory servers
File and print servers
Application servers
Communication servers
E-mail/fax servers
Web servers

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NETWORK SERVERS (CONT.)
Domain Controller/Directory Servers
Directory services make it possible for users to locate, store, and secure information about a network and its resources.
Windows servers permit combining computers, users, groups, and resources into domains. The server handling the computers and users in a domain is called a domain controller.
File and Print Servers
Provide secure centralized file storage and sharing and access to networked printers.
Any Windows or Linux computer can act as a file and print server, however the Server version of Windows provides advanced sharing features.

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NETWORK SERVERS (CONT.)
Application Servers
Supply the server side of client/server applications to network clients
Differ from basic file and print servers by providing processing services as well as handling requests for file or print services
Communication Servers
Provide a mechanism for users to access a network’s resources remotely
Enable users who are traveling or working at home to dial in to the network via a modem or their existing Internet connection
E-mail/Fax Servers

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NETWORK SERVERS (CONT.)
Web Servers
Windows Server includes a complete Web server called Internet Information Services (IIS) as well as File Transfer Protocol (FTP)
Apache Web Server is available as a part of most Linux distributions and remains the most widely used Web server in the world
Other Network Services
Most networks require additional support services to function efficiently. The most common are Domain Name System (DNS) and Dynamic Host Configuration Protocol (DHCP)
DNS allows users to access both local and Internet servers by name rather than by address
DHCP provides automatic addressing for network clients so that network administrators do not have to assign addresses manually
You will learn more about these services in Chapter 5

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SPECIALIZED NETWORKS
Storage area network (SAN) – uses high-speed networking technologies to provide servers with fast access to large amounts of disk storage
Wireless personal area network (WPAN) – short-range networking technology designed to connect personal devices to exchange information
These devices include cell phones, pagers, personal digital assistants (PDAs), global positioning system (GPS) devices, MP3 players, and even watches

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SAN
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CHAPTER SUMMARY
All computers perform three basic tasks: input, processing, and output
Storage is a major part of a computer’s configuration
Personal computer hardware consists of four major components: motherboard, hard drive, RAM, and BIOS/CMOS
The operating system and device drivers control access to computer hardware and provide a user interface, memory management and multitasking
The components needed to make a stand-alone computer a networked computer include a NIC, network medium, and usually a device to interconnect with other computers
The layers of the network communication process can be summarized as user application, network software, network protocol, and network interface

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CHAPTER SUMMARY
The four terms used to describe networks of different scope are LAN, Internetwork, WAN, and MAN
Packets and frames are the units of data handled by different network components. Packets have the source and destination IP address added and are processed by the network protocol. Frames have the MAC addresses and an error code added and are processed by the network interface.
A client is the computer or network software that requests network data and a server is the computer or network software that makes the network data available to requesting clients
A peer-to-peer network model has no centralized authority over resources while a server-based network usually uses a directory service to provide centralized resource management
Network servers can perform a number of specialized roles
Specialized networks can include storage area networks (SANs) and wireless personal area networks (WPANs)

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Chapter Three
Conducted and Wireless Media

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After reading this chapter,
you should be able to:
Outline the characteristics of twisted pair wire, including the advantages and disadvantages
Outline the differences among Category 1, 2, 3, 4, 5, 5e, 6, and 7 twisted pair wire
Explain when shielded twisted pair wire works better than unshielded twisted pair wire
Outline the characteristics, advantages, and disadvantages of coaxial cable and fiber-optic cable
Outline the characteristics of terrestrial microwave systems, including the advantages and disadvantages

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After reading this chapter,
you should be able to (continued):
Outline the characteristics of satellite microwave systems, including the advantages and disadvantages as well as the differences among low-Earth-orbit, middle-Earth-orbit, geosynchronous orbit, and highly elliptical Earth orbit satellites
Describe the basics of cellular telephones, including all the current generations of cellular systems
Outline the characteristics of short-range transmissions, including Bluetooth

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After reading this chapter,
you should be able to (continued):
Describe the characteristics, advantages, and disadvantages of Wireless Application Protocol (WAP), broadband wireless systems, and various wireless local area network transmission techniques
Apply the media selection criteria of cost, speed, right-of-way, expandability and distance, environment, and security to various media in a particular application

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Introduction
The world of computer networks would not exist if there were no medium by which to transfer data
The two major categories of media include:
Conducted media
Wireless media

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Twisted Pair Wire
One or more pairs of single conductor wires that have been twisted around each other
Twisted pair wire is classified by category. Twisted pair is currently Category 1 through Category 7, although Categories 1, 2 and 4 are nearly obsolete
Twisting the wires helps to eliminate electromagnetic interference between the two wires
Shielding can further help to eliminate interference

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Twisted Pair Wire (continued)
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Twisted Pair Wire (continued)
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Twisted Pair Wire (continued)
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Twisted Pair Wire (continued)
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Twisted Pair Summary
Most common form of wire
Relatively inexpensive
Easy to install
Carries high data rates (but not the highest)
Can suffer from electromagnetic noise
Can be easily wire-tapped
Comes in shielded and unshielded forms

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Coaxial Cable
A single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable comes in various thicknesses
Baseband coaxial technology uses digital signaling in which the cable carries only one channel of digital data
Broadband coaxial technology transmits analog signals and is capable of supporting multiple channels

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Coaxial Cable (continued)
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Coaxial Cable (continued)
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Coaxial Cable Summary
A single wire surrounded by a braided shield
Because of shielding, can carry a wide bandwidth of frequencies
Thus is good with applications such as cable television
Not as easy to install as twisted pair
More expensive than twisted pair

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Fiber-Optic Cable
A thin glass cable approximately a little thicker than a human hair surrounded by a plastic coating and packaged into an insulated cable
A photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor

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Fiber-Optic Cable (continued)
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Fiber-optic cable is capable of supporting millions of bits per second for 1000s of meters
Thick cable (62.5/125 microns) causes more ray collisions, so you have to transmit slower. This is step index multimode fiber. Typically use LED for light source, shorter distance transmissions
Thin cable (8.3/125 microns) – very little reflection, fast transmission, typically uses a laser, longer transmission distances; known as single mode fiber

Fiber-Optic Cable (continued)
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Fiber-Optic Cable (continued)
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Fiber-Optic Cable (continued)
Fiber-optic cable is susceptible to reflection (where the light source bounces around inside the cable) and refraction (where the light source passes out of the core and into the surrounding cladding)
Thus, fiber-optic cable is not perfect either. Noise is still a potential problem

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Fiber-Optic Cable (continued)
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Fiber-Optic Cable (continued)
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Fiber-Optic Cable Summary
Fiber optic cable can carry the highest data rate for the longest distances
Initial cost-wise, more expensive than twisted pair, but less than coaxial cable
But when you consider the superiority of fiber, initial costs outweighed by capacities
Need to fibers for a round-trip connection
Not affected by electromagnetic noise and cannot be easily wiretapped, but still noise

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Conducted Media
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Wireless Media
Radio, satellite transmissions, and infrared light are all different forms of electromagnetic waves that are used to transmit data
Technically speaking – in wireless transmissions, space is the medium
Note in the following figure how each source occupies a different set of frequencies

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Wireless Media (continued)
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Terrestrial Microwave Transmission
Land-based, line-of-sight transmission
Approximately 20-30 miles between towers
Transmits data at hundreds of millions of bits per second
Signals will not pass through solid objects
Popular with telephone companies and business to business transmissions

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Terrestrial Microwave Transmission (continued)
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Terrestrial Microwave Transmission (continued)
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Satellite Microwave Transmission
Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station
Can also transmit signals from one satellite to another
Satellites can be classified by how far out into orbit each one is (LEO, MEO, GEO, and HEO)

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Satellite Microwave Transmission (continued)
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Satellite Microwave Transmission (continued)
LEO (Low-Earth-Orbit) – 100 to 1000 miles out
Used for wireless e-mail, special mobile telephones, pagers, spying, videoconferencing
MEO (Middle-Earth-Orbit) – 1000 to 22,300 miles
Used for GPS (global positioning systems) and government
GEO (Geosynchronous-Earth-Orbit) – 22,300 miles
Always over the same position on earth (and always over the equator)
Used for weather, television, government operations

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Satellite Microwave Transmission (continued)
HEO (Highly Elliptical Earth orbit) – satellite follows an elliptical orbit
Used by the military for spying and by scientific organizations for photographing celestial bodies

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Satellite Microwave Transmission (continued)
Satellite microwave can also be classified by its configuration (see next figure):
Bulk carrier configuration
Multiplexed configuration
Single-user earth station configuration (e.g., VSAT)

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Satellite Microwave Transmission (continued)
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Satellite Microwave Transmission (continued)
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Satellite Microwave Transmission (continued)
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Satellite Microwave Transmission (continued)
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Cellular Telephones
Wireless telephone service, also called mobile telephone, cell phone, and PCS
To support multiple users in a metropolitan area (market), the market is broken into cells
Each cell has its own transmission tower and set of assignable channels

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Cellular Telephones (continued)
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The CTSO is responsible for switching cell phone signals from to other and maintaining the best connection possible

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Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
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Optional Reading
Rest of this Lecture slides

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Cellular Telephones (continued)
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Cellular Telephones (continued)
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Cellular Telephones (continued)
Placing a call on a cell phone
You enter a phone number on your cell phone and press Send. Your cell phone contacts the nearest cell tower and grabs a set-up channel. Your mobile identification information is exchanged to make sure you are a current subscriber.
If you are current, you are dynamically assigned two channels: one for talking, and one for listening. The telephone call is placed. You talk.

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Cellular Telephones (continued)
Receiving a call on a cell phone
Whenever a cell phone is on, it “pings” the nearest cell tower every several seconds, exchanging mobile ID information. This way, the cell phone system knows where each cell phone is.
When someone calls your cell phone number, since the cell phone system knows what cell you are in, the tower “calls” your cell phone.

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Cellular Telephones (continued)
1st Generation
AMPS (Advanced Mobile Phone Service) – first popular cell phone service; used analog signals and dynamically assigned channels
D-AMPS (Digital AMPS) – applied digital multiplexing techniques on top of AMPS analog channels

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Cellular Telephones (continued)
2nd Generation
PCS (Personal Communication Systems) – essentially all-digital cell phone service
PCS phones came in three technologies:
TDMA – Time Division Multiple Access
CDMA – Code Division Multiple Access
GSM – Global System for Mobile Communications

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Cellular Telephones (continued)
2.5 Generation
AT&T Wireless, Cingular Wireless, and T-Mobile now using GPRS (General Packet Radio Service) in their GSM networks (can transmit data at 30 kbps to 40 kbps)
Verizon Wireless, Alltel, U.S.Cellular, and Sprint PCS are using CDMA2000 1xRTT (one carrier radio- transmission technology) (50 kbps to 75 kbps)
Nextel uses IDEN technology

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Cellular Telephones (continued)
3rd Generation
UMTS (Universal Mobile Telecommunications System) – also called Wideband CDMA
The 3G version of GPRS
UMTS not backward compatible with GSM (thus requires phones with multiple decoders)
1XEV (1 x Enhanced Version) –3G replacement for 1xRTT
two forms:
1xEV-DO for data only
1xEV-DV for data and voice

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Cellular Telephones (continued)
4th Generation
LTE (Long Term Evolution) – theoretical speeds of 100 Mbps or more, actual download speeds 10-15 Mbps
WiMax – introduced in a couple slides – theoretical speeds of 128 Mbps; actual download speeds 4 Mbps (didn’t make it for cellular)
HSPA (High Speed Packet Access) – 14 Mbps downlink, 5.8 Mbps uplink; (didn’t make it)
HSPA+ – theoretical downlink of 84 Mbps, 22 Mbps uplink (T-Mobile) (didn’t make it)

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WiMax – Broadband Wireless Systems
Delivers Internet services into homes, businesses and mobile devices
Designed to bypass the local loop telephone line
Transmits voice, data, and video over high frequency radio signals
Maximum range of 20-30 miles and transmission speeds in Mbps
IEEE 802.16 set of standards

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WiMax (continued)
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Bluetooth
Bluetooth is a specification for short-range, point-to-point or point-to-multipoint voice and data transfer
Bluetooth can transmit through solid, non-metal objects
Its typical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the power

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Bluetooth (continued)
Bluetooth will enable users to connect to a wide range of computing and telecommunication devices without the need of connecting cables
Typical uses include phones, pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs

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Wireless Local Area Networks
(IEEE 802.11)
This technology transmits data between workstations and local area networks using high-speed radio frequencies
Current technologies allow up to 100 Mbps (theoretical) data transfer at distances up to hundreds of feet
Three popular standards: IEEE 802.11b, a, g, n
More on this in Chapter Seven (LANs)

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Free Space Optics
Free space optics
Uses lasers, or more economically, infrared transmitting devices
Line of sight between buildings
Typically short distances, such as across the street
Newer auto-tracking systems keep lasers aligned when buildings shake from wind and traffic

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Free Space Optics (continued)
Free space optics (continued)
Current speeds go from T-3 (45 Mbps) to OC-48 (2.5 Gbps) with faster systems in development
Major weakness is transmission thru fog
A typical FSO has a link margin of about 20 dB
Under perfect conditions, air reduces a system’s power by approximately 1 dB/km
Scintillation is also a problem (especially in hot weather)

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Ultra-Wideband
Ultra-wideband
UWB not limited to a fixed bandwidth but broadcasts over a wide range of frequencies simultaneously
Many of these frequencies are used by other sources, but UWB uses such low power that it “should not” interfere with these other sources
Can achieve speeds up to 100 Mbps but for small distances such as wireless LANs

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Ultra-Wideband (continued)
Ultra-wideband (continued)
Proponents for UWB say it gets something for nothing, since it shares frequencies with other sources. Opponents disagree
Cell phone industry against UWB because CDMA most susceptible to interference of UWB
GPS may also be affected
One solution may be to have two types of systems – one for indoors (stronger) and one for outdoors (1/10 the power)

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Infrared Transmissions
Transmissions that use a focused ray of light in the infrared frequency range
Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer

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Near-Field Communications
Very close distances or devices touching
Magnetic induction (such as radio frequency ID) used for transmission of data
Commonly used for data transmission between cellphones (non-Apple devices)

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ZigBee
Based upon IEEE 802.15.4 standard
Used for low data transfer rates (20-250 Kbps)
Also uses low power consumption
Ideal for heating, cooling, security, lighting, and smoke and CO detector systems
ZigBee can use a mesh design – a ZigBee-enabled device can both accept and then pass on ZigBee signals

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Wireless Media (continued)
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Wireless Media (continued)
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Media Selection Criteria
Cost
Speed
Distance and expandability
Environment
Security

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Cost
Different types of costs
Initial cost – what does a particular type of medium cost to purchase? To install?
Maintenance / support cost
ROI (return on investment) – if one medium is cheaper to purchase and install but is not cost effective, where are the savings?

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Speed
Two different forms of speed:
Propagation speed – the time to send the first bit across the medium
This speed depends upon the medium
Airwaves and fiber are speed of light
Copper wire is two thirds the speed of light
Data transfer speed – the time to transmit the rest of the bits in the message
This speed is measured in bits per second

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Expandability and Distance
Certain media lend themselves more easily to expansion
Don’t forget right-of-way issue for conducted media and line-of-sight for certain wireless media

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Environment
Many types of environments are hazardous to certain media
Electromagnetic noise
Scintillation and movement
Extreme environmental conditions

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Security
If data must be secure during transmission, it is important that the medium not be easy to tap
Make the wire impervious to electromagnetic wiretapping
Encrypt the signal going over the medium

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Conducted Media in Action: Two Examples
First example – simple local area network
Hub typically used
To select proper medium, consider:
Cable distance
Data rate

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Conducted Media in Action:
Two Examples (continued)
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Conducted Media in Action:
Two Examples (continued)
Second example – company wishes to transmit data between buildings that are one mile apart
Is property between buildings owned by company?
If not consider using wireless
When making decision, need to consider:
Cost
Speed
Expandability and distance
Environment
Security

Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Wireless Media In Action: Three Examples
First example – you wish to connect two computers in your home to Internet, and want both computers to share a printer
Can purchase wireless network interface cards
May consider using Bluetooth devices
Second example – company wants to transmit data between two locations, Chicago and Los Angeles
Company considering two-way data communications service offered through VSAT satellite system

Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Wireless Media In Action:
Three Examples (continued)
Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Wireless Media In Action:
Three Examples (continued)
Third example – second company wishes to transmit data between offices two miles apart
Considering terrestrial microwave system

Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Wireless Media In Action:
Three Examples (continued)
Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Summary
All data communication media can be divided into two basic categories: (1) physical or conducted media, and (2) radiated or wireless media, such as satellite systems
The three types of conducted media are twisted pair, coaxial cable, and fiber-optic cable
Twisted pair and coaxial cable are both metal wires and are subject to electromagnetic interference
Fiber-optic cable is a glass wire and is impervious to electromagnetic interference
Experiences a lower noise level
Has best transmission speeds and long-distance performance of all conducted media

Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

*
Summary (continued)
Several basic groups of wireless media exist: terrestrial microwave transmissions, satellite transmissions, cellular telephone systems, infrared transmissions, WiMAX, Bluetooth, Wi-Fi, free space optics, ultra-wideband, near-field communications, and ZigBee
Each of the wireless technologies is designed for specific applications
When trying to select particular medium for an application, it helps to compare the different media using these six criteria: cost, speed, expandability and distance, right-of-way, environment, and security

Data Communications and Computer Networks: A Business User’s Approach, Eighth Edition
© 2016. Cengage Learning. All Rights Reserved.

*

Network
Models
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*

Chapter 2: Outline
2.1 Protocol Layering
2.2 TCP/IP Protocol Suite
2.3 OSI Model

1.#

Chapter 2: Objective

The first section introduces the concept of protocol layering using two scenarios. The section also discusses the two principles upon which the protocol layering is based. The first principle dictates that each layer needs to have two opposite tasks. The second principle dictates that the corresponding layers should be identical. The section ends with a brief discussion of logical connection between two identical layers in protocol layering. Throughout the book, we need to distinguish between logical and physical connections.

1.#

Chapter 2: Objective (continued)

The second section discusses the five layers of the TCP/IP protocol suite. We show how packets in each of the five layers (physical, data-link, network, transport, and application) are named. We also mention the addressing mechanism used in each layer. Each layer of the TCP/IP protocol suite is a subject of a part of the book. In other words, each layer is discussed in several chapters; this section is just an introduction and preparation.

The third section gives a brief discussion of the OSI model. This model was never implemented in practice, but a brief discussion of the model and its comparison with the TCP/IP protocol suite may be useful to better understand the TCP/IP protocol suite. In this section we also give a brief reason for the OSI model’s lack of success.

1.#

2.*

2-1 PROTOCOL LAYERING

A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.

1.#

2.*

2.1.1 Scenarios
Let us develop two simple scenarios to better understand the need for protocol layering.

In the first scenario, communication is so simple that it can occur in only one layer.

In the second, the communication between Maria and Ann takes place in three layers.

1.#

2.*
Figure 2.1: A single-layer protocol

1.#

2.*
Figure 2.2: A three-layer protocol

1.#

2.*

2.1.2 Principles of Protocol Layering
Let us discuss two principles of protocol layering.

The first principle dictates that if we want bidirectional communication, we need to make each layer so that it is able to perform two opposite tasks, one in each direction.

The second principle that we need to follow in protocol layering is that the two objects under each layer at both sites should be identical.

1.#

2.*

2.1.3 Logical Connections
After following the above two principles, we can think about logical connection between each layer as shown in Figure 2.3. This means that we have layer-to-layer communication. Maria and Ann can think that there is a logical (imaginary) connection at each layer through which they can send the object created from that layer. We will see that the concept of logical connection will help us better understand the task of layering we encounter in data communication and networking.

1.#

2.*
Figure 2.3: Logical connection between peer layers

1.#

2.*

2-2 TCP/IP PROTOCOL SUITE

A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.

1.#

2.*
Figure 2.4: Layers in the TCP/IP protocol suite

1.#

2.*

2.2.1 Layered Architecture
To show how the layers in the TCP/IP protocol suite are involved in communication between two hosts, we assume that we want to use the suite in a small internet made up of three LANs (links), each with a link-layer switch. We also assume that the links are connected by one router, as shown in Figure 2.5.

1.#

2.*
Figure 2.5: Communication through an internet

1.#

2.*

2.2.2 Layers in the TCP/IP Protocol Suite
After the above introduction, we briefly discuss the functions and duties of layers in the TCP/IP protocol suite. Each layer is discussed in detail in the next five parts of the book. To better understand the duties of each layer, we need to think about the logical connections between layers. Figure 2.6 shows logical connections in our simple internet.

1.#

2.*
Figure 2.6: Logical connections between layers in TCP/IP
Logical connections

1.#

2.*
Figure 2.7: Identical objects in the TCP/IP protocol suite
Identical objects (messages)
Identical objects (segment or user datagram)
Identical objects (datagram)
Identical objects (frame)
Identical objects (bits)
Identical objects (datagram)
Identical objects (frame)
Identical objects (bits)

1.#

2.*

2.2.3 Description of Each Layer
After understanding the concept of logical communication, we are ready to briefly discuss the duty of each layer. Our discussion in this chapter will be very brief, but we come back to the duty of each layer in next five parts of the book.

1.#

2.*

2.2.4 Encapsulation and Decapsulation
One of the important concepts in protocol layering in the Internet is encapsulation/ decapsulation. Figure 2.8 shows this concept for the small internet in Figure 2.5.

1.#

2.*
Figure 2.8: Encapsulation / Decapsulation

1.#

2.*

2.2.5 Addressing
It is worth mentioning another concept related to protocol layering in the Internet, addressing. As we discussed before, we have logical communication between pairs of layers in this model. Any communication that involves two parties needs two addresses: source address and destination address. Although it looks as if we need five pairs of addresses, one pair per layer, we normally have only four because the physical layer does not need addresses; the unit of data exchange at the physical layer is a bit, which definitely cannot have an address.

1.#

2.*
Figure 2.9: Addressing in the TCP/IP protocol suite

1.#

2.*

2.2.6 Multiplexing and Demultiplexing
Since the TCP/IP protocol suite uses several protocols at some layers, we can say that we have multiplexing at the source and demultiplexing at the destination. Multiplexing in this case means that a protocol at a layer can encapsulate a packet from several next-higher layer protocols (one at a time); demultiplexing means that a protocol can decapsulate and deliver a packet to several next-higher layer protocols (one at a time). Figure 2.10 shows the concept of multiplexing and demultiplexing at the three upper layers.

1.#

2.*
Figure 2.10: Multiplexing and demultiplexing

1.#

2.*

2-3 OSI MODEL

A word we hear all the time when we talk about the Internet is protocol. A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol; when the communication is complex, we need a protocol at each layer, or protocol layering.

1.#

2.*
Figure 2.11: The OSI model

1.#

2.*

2.3.1 OSI versus TCP/IP
When we compare the two models, we find that two layers, session and presentation, are missing from the TCP/IP protocol suite. These two layers were not added to the TCP/IP protocol suite after the publication of the OSI model. The application layer in the suite is usually considered to be the combination of three layers in the OSI model, as shown in Figure 2.12.

1.#

2.*
Figure 2.12: TCP/IP and OSI model

1.#

2.*

2.3.2 Lack of OSI Model’s Success
The OSI model appeared after the TCP/IP protocol suite. Most experts were at first excited and thought that the TCP/IP protocol would be fully replaced by the OSI model. This did not happen for several reasons, but we describe only three, which are agreed upon by all experts in the field.

1.#

Postal carrier facility

Network
Layer
Protocols
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*

Chapter 4: Outline
19.1 IPv4

19.2 ICMPv4

19.3 Mobile IP

19.#

: Objective

The first section discusses the IPv4 protocol. It first describes the IPv4 datagram format. It then explains the purpose of fragmentation in a datagram. The section then briefly discusses options fields and their purpose in a datagram. The section finally mentions some security issues in IPv4, which are addressed in Chapter 32.

The second section discusses ICMPv4, one of the auxiliary protocols used in the network layer to help IPv4. First, it briefly discusses the purpose of each option. The section then shows how ICMP can be used as a debugging tool. The section finally shows how the checksum is calculated for an ICMPv4 message.

19.#

Objective (continued)

The third section discusses the mobile IP, whose use is increasing every day when people temporarily move their computers from one place to another. The section first describes the issue of address change in this situation. It then shows the three phases involved in the process. The section finally explains the inefficiency involved in this process and some solutions.

19.#

19.*

19.1 NETWORK-LAYER PROTOCOLS

The network layer in version 4 can be thought of as one main protocol and three auxiliary ones. The main protocol, IPv4, is responsible for packetizing, forwarding, and delivery of a packet. The ICMPv4 helps IPv4 to handle some errors that may occur in delivery. The IGMP is used to help IPv4 in multicasting. ARP is used in address mapping.

19.#

19.*
Figure 19.1: Position of IP and other network-layer protocols in
TCP/IP protocol suite

19.#

19.*

19.19.1 Datagram Format
Packets used by the IP are called datagrams. Figure 19.2 shows the IPv4 datagram format. A datagram is a variable-length packet consisting of two parts: header and payload (data). The header is 20 to 60 bytes in length and contains information essential to routing and delivery. It is customary in TCP/IP to show the header in 4-byte sections..

19.#

19.*
Figure 19.2: IP datagram

19.#

19.*
Figure 19.3: Multiplexing and demultiplexing using the value of
the protocol field

19.#

An IPv4 packet has arrived with the first 8 bits as (01000010)2 The receiver discards the packet. Why?.

Solution
There is an error in this packet. The 4 leftmost bits (0100)2 show the version, which is correct. The next 4 bits (0010)2 show an invalid header length (2 × 4 = 8). The minimum number of bytes in the header must be 20. The packet has been corrupted in transmission.
19.*

Example 19.1

*

In an IPv4 packet, the value of HLEN is (1000)2. How many bytes of options are being carried by this packet?
Solution
The HLEN value is 8, which means the total number of bytes in the header is 8 × 4, or 32 bytes. The first 20 bytes are the base header, the next 12 bytes are the options.
19.*

Example 19.2

*

In an IPv4 packet, the value of HLEN is 5, and the value of the total length field is (0028)16. How many bytes of data are being carried by this packet?

Solution
The HLEN value is 5, which means the total number of bytes in the header is 5 × 4, or 20 bytes (no options). The total length is (0028)16 or 40 bytes, which means the packet is carrying 20 bytes of data (40 − 20).
19.*

Example 19.3

*

An IPv4 packet has arrived with the first few hexadecimal digits as shown.

How many hops can this packet travel before being dropped? The data belong to what upper-layer protocol?

Solution
To find the time-to-live field, we skip 8 bytes (16 hexadecimal digits). The time-to-live field is the ninth byte, which is (01)16. This means the packet can travel only one hop. The protocol field is the next byte (02)16, which means that the upper-layer protocol is IGMP.
19.*

Example 19.4

*

Figure 19.4 shows an example of a checksum calculation for an IPv4 header without options. The header is divided into 16-bit sections. All the sections are added and the sum is complemented after wrapping the leftmost digit. The result is inserted in the checksum field.

Note that the calculation of wrapped sum and checksum can also be done as follows in hexadecimal:
19.*

Example 19.5

*

19.*
Figure 19.4:

19.#

19.*

19.19.2 Fragmentation
A datagram can travel through different networks. Each router decapsulates the IP datagram from the frame it receives, processes it, and then encapsulates it in another frame. The format and size of the received frame depend on the protocol used by the physical network through which the frame has just traveled. The format and size of the sent frame depend on the protocol used by the physical network through which the frame is going to travel. For example, if a router connects a LAN to a WAN, it receives a frame in the LAN format and sends a frame in the WAN format.

19.#

19.*
Figure 19.5: Maximum transfer unit (MTU)

19.#

19.*
Figure 19.6: Fragmentation example

19.#

19.*
Figure 19.7: Detailed fragmentation example

19.#

A packet has arrived with an M bit value of 0. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented?

Solution
If the M bit is 0, it means that there are no more fragments; the fragment is the last one. However, we cannot say if the original packet was fragmented or not. A nonfragmented packet is considered the last fragment.
19.*

Example 19.6

*

A packet has arrived with an M bit value of 19. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented?

Solution
If the M bit is 1, it means that there is at least one more fragment. This fragment can be the first one or a middle one, but not the last one. We don’t know if it is the first one or a middle one; we need more information (the value of the fragmentation offset).
19.*

Example 19.7

*

A packet has arrived with an M bit value of 1 and a fragmentation offset value of 0. Is this the first fragment, the last fragment, or a middle fragment?

Solution
Because the M bit is 1, it is either the first fragment or a middle one. Because the offset value is 0, it is the first fragment.
19.*

Example 19.8

*

A packet has arrived in which the offset value is 100. What is the number of the first byte? Do we know the number of the last byte?

Solution
To find the number of the first byte, we multiply the offset value by 8. This means that the first byte number is 800. We cannot determine the number of the last byte unless we know the length of the data.
19.*

Example 19.9

*

A packet has arrived in which the offset value is 100, the value of HLEN is 5, and the value of the total length field is 100. What are the numbers of the first byte and the last byte?

Solution
The first byte number is 100 × 8 = 800. The total length is 100 bytes, and the header length is 20 bytes (5 × 4), which means that there are 80 bytes in this datagram. If the first byte number is 800, the last byte number must be 879.
19.*

Example 19.10

*

19.*

19.19.3 Options
The header of the IPv4 datagram is made of two parts: a fixed part and a variable part. The fixed part is 20 bytes long and was discussed in the previous section. The variable part comprises the options that can be a maximum of 40 bytes (in multiples of 4-bytes) to preserve the boundary of the header.

19.#

19.*

19.19.4 Security of IPv4 Datagrams
The IPv4 protocol, as well as the whole Internet, was started when the Internet users trusted each other. No security was provided for the IPv4 protocol. Today, however, the situation is different; the Internet is not secure anymore. Although we will discuss network security in general and IP security in particular in Chapters 31 and 32, here we give a brief idea about the security issues in IP protocol and the solutions. There are three security issues that are particularly applicable to the IP protocol: packet sniffing, packet modification, and IP spoofing.

19.#

19.*

19.2 ICMPv4

The IPv4 has no error-reporting or error-correcting mechanism. The IP protocol also lacks a mechanism for host and management queries. The Internet Control Message Protocol version 4 (ICMPv4) has been designed to compensate for the above two deficiencies.

19.#

19.*

19.2.1 MESSAGES
ICMP messages are divided into two broad categories: error-reporting messages and query messages. The error-reporting messages report problems that a router or a host (destination) may encounter when it processes an IP packet. The query messages, which occur in pairs, help a host or a network manager get specific information from a router or another host. For example, nodes can discover their neighbors. Also, hosts can discover and learn about routers on their network and routers can help a node redirect its messages.

19.#

19.*
Figure 19.8: General format of ICMP messages

19.#

19.*
Figure 19.9: Contents of data field for the error messages

19.#

19.*

19.2.2 Debugging Tools
There are several tools that can be used in the Internet for debugging. We can determine the viability of a host or router. We can trace the route of a packet. We introduce two tools that use ICMP for debugging: ping and traceroute.

19.#

The following shows how we send a ping message to the auniversity.edu site. We set the identifier field in the echo request and reply message and start the sequence number from 0; this number is incremented by one each time a new message is sent. Note that ping can calculate the round-trip time. It inserts the sending time in the data section of the message. When the packet arrives, it subtracts the arrival time from the departure time to get the round-trip time (rtt).
19.*

Example 19.11

*

19.*

Example 19.11(continued)

*

19.*
Figure 19.10: Example of traceroute program

19.#

19.*

19.2.3 ICMP Checksum
In ICMP the checksum is calculated over the entire message (header and data).

19.#

Figure 19.11 shows an example of checksum calculation for a simple echo-request message. We randomly chose the identifier to be 1 and the sequence number to be 9. The message is divided into 16-bit (2-byte) words. The words are added and the sum is complemented. Now the sender can put this value in the checksum field.
19.*

Example 19.12

*

19.*
Figure 19.11: Example of checksum calculation

19.#

19.*

19-3 MOBILE IP

In the last section of this chapter, we discuss mobile IP. As mobile and personal computers such as notebooks become increasingly popular, we need to think about mobile IP, the extension of IP protocol that allows mobile computers to be connected to the Internet at any location where the connection is possible. In this section, we discuss this issue.

19.#

19.*

19.3.1 Addressing
The main problem that must be solved in providing mobile communication using the IP protocol is addressing.

19.#

19.*
Figure 19.12: Home address and care-of address

19.#

19.*

19.3.2 Agents
To make the change of address transparent to the rest of the Internet requires a home agent and a foreign agent. Figure 19.13 shows the position of a home agent relative to the home network and a foreign agent relative to the foreign network..

19.#

19.*
Figure 19.13: Home agent and foreign agent

19.#

19.*

19.3.3 Three Phases
To communicate with a remote host, a mobile host goes through three phases: agent discovery, registration, and data transfer, as shown in Figure 19.14.

19.#

19.*
Figure19.14: Remote host and mobile host communication

19.#

19.*
Figure 19.14: Agent advertisement

19.#

Table 19.1: Code Bits
19.*

*

19.*
Figure 19.16: Registration request format

19.#

Table 19.2: Registration request flag field bits
19.*

*

19.*
Figure 19,17: Registration reply format

19.#

19.*
Figure 19.18: Data transfer

19.#

19.*

19.3.4 Inefficiency in Mobile IP
Communication involving mobile IP can be inefficient. The inefficiency can be severe or moderate. The severe case is called double crossing or 2X. The moderate case is triangle routing or dog-leg routing.

19.#

19.*
Figure 19.19: Double crossing

19.#

19.*
Figure 19.20: Triangle routing

19.#

Error-reporting messagesData sectionCodeChecksumType8 bits8 bits16 bitsRest of the header
Query messagesData sectionCodeChecksumType8 bits8 bits16 bitsIdentifierSequence number
1
2
3
1
2
1
2

Chapter 24
Transport Layer
Protocols
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*

Chapter 3: Outline
24.1 INTRODUCTION
24.2 UDP
24.3 TCP
24.4 SCTP

24.#

Chapter 24: Objective
The first section introduces the three transport-layer protocols in the Internet and gives some information common to all of them.

The second section concentrates on UDP, which is the simplest of the three protocols. UDP lacks many services we require from a transport-layer protocol, but its simplicity is very attractive to some applications, as we show.

The third section discusses TCP. The section first lists its services and features. Using a transition diagram, it then shows how TCP provides a connection-oriented service. The section then uses abstract windows to show how flow and error control are accomplished in TCP. Congestion control in TCP is discussed next, a topic that was discussed for the network layer.

24.#

Chapter 24: Objective (continued)
The fourth section discusses SCTP. The section first lists its services and features. It then shows how STCP creates an association. The section then shows how flow and error control are accomplished in SCTP using SACKs.

24.#

24.*

24-1 INTRODUCTION

After discussing the general principle behind the transport layer in the previous chapter, we concentrate on the transport protocols in the Internet in this chapter. Figure 24.1 shows the position of these three protocols in the TCP/IP protocol suite.

24.#

24.*
Figure 24.1: Position of transport-layer protocols in the TCP/IP
protocol suite

24.#

24.*

24.24.1 Services
Each protocol provides a different type of service and should be used appropriately.

24.#

24.*

24.24.2 Port Numbers
As discussed in the previous chapter, a transport-layer protocol usually has several responsibilities. One is to create a process-to-process communication; these protocols use port numbers to accomplish this. Port numbers provide end-to-end addresses at the transport layer and allow multiplexing and demultiplexing at this layer, just as IP addresses do at the network layer. Table 24.1 gives some common port numbers for all three protocols we discuss in this chapter.

24.#

Table 24.1: Some well-known ports used with UDP and TCP
24.*

*

24.*

24-2 UDP

The User Datagram Protocol (UDP) is a connectionless, unreliable transport protocol. If UDP is so powerless, why would a process want to use it? With the disadvantages come some advantages. UDP is a very simple protocol using a minimum of overhead.

24.#

24.*

24.2.1 User Datagram
UDP packets, called user datagrams, have a fixed-size header of 8 bytes made of four fields, each of 2 bytes (16 bits). Figure 24.2 shows the format of a user datagram. The first two fields define the source and destination port numbers. The third field defines the total length of the user datagram, header plus data. The 16 bits can define a total length of 0 to 65,535 bytes. However, the total length needs to be less because a UDP user datagram is stored in an IP datagram with the total length of 65,535 bytes. The last field can carry the optional checksum (explained later).

24.#

24.*
Figure 24.2: User datagram packet format

24.#

The following is the contents of a UDP header in hexadecimal format.
a. What is the source port number?
b. What is the destination port number?
c. What is the total length of the user datagram?
d. What is the length of the data?
e. Is the packet directed from a client to a server or vice
versa?
f. What is the client process?
24.*

Example 24.1

*

Solution
a. The source port number is the first four hexadecimal
digits (CB84)16 or 52100
b. The destination port number is the second four
hexadecimal digits (000D)16 or 13.
c. The third four hexadecimal digits (001C)16 define the
length of the whole UDP packet as 28 bytes.
d. The length of the data is the length of the whole packet
minus the length of the header, or 28 − 8 = 20 bytes.
e. Since the destination port number is 13 (well-known
port), the packet is from the client to the server.
f. The client process is the Daytime (see Table 3.1).
24.*

Example 24.1 (continued)

*

24.*

24.2.2 UDP Services
Earlier we discussed the general services provided by a transport-layer protocol. In this section, we discuss what portions of those general services are provided by UDP.

24.#

24.*
Figure 24.3: Pseudoheader for checksum calculation

24.#

What value is sent for the checksum in one of the following hypothetical situations?

a. The sender decides not to include the checksum.

b. The sender decides to include the checksum, but the
value of the sum is all 1s.

c. The sender decides to include the checksum, but the
value of the sum is all 0s.
24.*

Example 24.2

*

Solution
a. The value sent for the checksum field is all 0s to show
that the checksum is not calculated.
b. When the sender complements the sum, the result is all
0s; the sender complements the result again before
sending. The value sent for the checksum is all 1s. The
second complement operation is needed to avoid
confusion with the case in part a.
c. This situation never happens because it implies that the
value of every term included in the calculation of the
sum is all 0s, which is impossible; some fields in the
pseudoheader have nonzero values.
24.*

Example 24.2 (continued)

*

24.*

24.2.3 UDP Applications
Although UDP meets almost none of the criteria we mentioned earlier for a reliable transport-layer protocol, UDP is preferable for some applications. The reason is that some services may have some side effects that are either unacceptable or not preferable. An application designer sometimes needs to compromise to get the optimum. For example, in our daily life, we all know that a one-day delivery of a package by a carrier is more expensive than a three-day delivery. Although high speed and low cost are both desirable features in delivery of a parcel, they are in conflict with each other.

24.#

A client-server application such as DNS (see Chapter 26) uses the services of UDP because a client needs to send a short request to a server and to receive a quick response from it. The request and response can each fit in one user datagram. Since only one message is exchanged in each direction, the connectionless feature is not an issue; the client or server does not worry that messages are delivered out of order.
24.*

Example 24. 3

*

A client-server application such as SMTP (see Chapter 26), which is used in electronic mail, cannot use the services of UDP because a user can send a long e-mail message, which may include multimedia (images, audio, or video). If the application uses UDP and the message does not fit in one single user datagram, the message must be split by the application into different user datagrams. Here the connectionless service may create problems. The user datagrams may arrive and be delivered to the receiver application out of order. The receiver application may not be able to reorder the pieces. This means the connectionless service has a disadvantage for an application program that sends long messages.
24.*

Example 24.4

*

Assume we are downloading a very large text file from the Internet. We definitely need to use a transport layer that provides reliable service. We don’t want part of the file to be missing or corrupted when we open the file. The delay created between the deliveries of the parts is not an overriding concern for us; we wait until the whole file is composed before looking at it. In this case, UDP is not a suitable transport layer.
24.*

Example 25.5

*

Assume we are using a real-time interactive application, such as Skype. Audio and video are divided into frames and sent one after another. If the transport layer is supposed to resend a corrupted or lost frame, the synchronizing of the whole transmission may be lost. The viewer suddenly sees a blank screen and needs to wait until the second transmission arrives. This is not tolerable. However, if each small part of the screen is sent using one single user datagram, the receiving UDP can easily ignore the corrupted or lost packet and deliver the rest to the application program. That part of the screen is blank for a very short period of time, which most viewers do not even notice.
24.*

Example 25.6

*

24.*

24-3 TCP

Transmission Control Protocol (TCP) is a connection-oriented, reliable protocol. TCP explicitly defines connection establishment, data transfer, and connection teardown phases to provide a connection-oriented service. TCP uses a combination of GBN and SR protocols to provide reliability.

24.#

24.*

24.3.1 TCP Services
Before discussing TCP in detail, let us explain the services offered by TCP to the processes at the application layer.

24.#

24.*
Figure 24.4: Stream delivery

24.#

24.*
Figure 24.5: Sending and receiving buffers

24.#

24.*
Figure 24.6: TCP segments

24.#

24.*

24.3.2 TCP Features
To provide the services mentioned in the previous section, TCP has several features that are briefly summarized in this section and discussed later in detail.

24.#

Suppose a TCP connection is transferring a file of 5,000 bytes. The first byte is numbered 10,0024. What are the sequence numbers for each segment if data are sent in five segments, each carrying 1,000 bytes?
Solution
The following shows the sequence number for each segment:
24.*

Example 24.7

*

24.*

24.3.3 Segment
Before discussing TCP in more detail, let us discuss the TCP packets themselves. A packet in TCP is called a segment.

24.#

24.*
Figure 24.7: TCP segment format

24.#

24.*
Figure 24.8: Control field

24.#

24.*
Figure 24.9: Pseudoheader added to the TCP datagram

24.#

24.*

24.3.4 A TCP Connection
TCP is connection-oriented. All of the segments belonging to a message are then sent over this logical path. Using a single logical pathway for the entire message facilitates the acknowledgment process as well as retransmission of damaged or lost frames. You may wonder how TCP, which uses the services of IP, a connectionless protocol, can be connection-oriented. The point is that a TCP connection is logical, not physical. TCP operates at a higher level. TCP uses the services of IP to deliver individual segments to the receiver, but it controls the connection itself.

24.#

24.*
Figure 24.10: Connection establishment using three-way handshaking

24.#

24.*
Figure 24.11: Data transfer

24.#

24.*
Figure 24.12: Connection termination using three-way handshaking

24.#

24.*
Figure 24.13: Half-close
Categories of ICMPv6 messages

24.#

24.*

24.3.5 State Transition Diagram
To keep track of all the different events happening during connection establishment, connection termination, and data transfer, TCP is specified as the finite state machine (FSM) as shown in Figure 24.14.

24.#

24.*
Figure 24.14: State transition diagram

24.#

Table 24.2: States for TCP
24.*

*

24.*
Figure 24.15: Transition diagram with half-close connection termination

24.#

24.*
Figure 24.16: Time-line diagram for a common scenario

24.#

24.*

24.3.6 Windows in TCP
Before discussing data transfer in TCP and the issues such as flow, error, and congestion control, we describe the windows used in TCP. TCP uses two windows (send window and receive window) for each direction of data transfer, which means four windows for a bidirectional communication. To make the discussion simple, we make an unrealistic assumption that communication is only unidirectional. The bidirectional communication can be inferred using two unidirectional communications with piggybacking.

24.#

24.*
Figure 24.17: Send window in TCP

24.#

24.*
Figure 24.18: Receive window in TCP

24.#

24.*

24.3.7 Flow Control
As discussed before, flow control balances the rate a producer creates data with the rate a consumer can use the data. TCP separates flow control from error control. In this section we discuss flow control, ignoring error control. We assume that the logical channel between the sending and receiving TCP is error-free.

24.#

24.*
Figure 24.19: Data flow and flow control feedbacks in TCP

24.#

24.*
Figure 24.20: An example of flow control

24.#

Figure 3.58 shows the reason for this mandate.
Part a of the figure shows the values of the last acknowledgment and rwnd. Part b shows the situation in which the sender has sent bytes 206 to 214. Bytes 206 to 209 are acknowledged and purged. The new advertisement, however, defines the new value of rwnd as 4, in which
210 + 4 < 206 + 12. When the send window shrinks, it creates a problem: byte 214, which has already been sent, is outside the window. The relation discussed before forces the receiver to maintain the right-hand wall of the window to be as shown in part a, because the receiver does not know which of the bytes 210 to 217 has already been sent. described above. 24.* Example 3.18 * 24.* Figure 24.21: Example 3.18 24.# 24.* 24.3.8 Error Control TCP is a reliable transport-layer protocol. This means that an application program that delivers a stream of data to TCP relies on TCP to deliver the entire stream to the application program on the other end in order, without error, and without any part lost or duplicated. 24.# 24.* Figure 24.22: Simplified FSM for the TCP sender side 24.# 24.* Figure 24.23: Simplified FSM for the TCP receiver side 24.# 24.* Figure 24.24: Normal operation 24.# 24.* Figure 24.25: Lost segment 24.# 24.* Figure 24.26: Fast retransmission 24.# 24.* Figure 24.27: Lost acknowledgment 24.# 24.* Figure 24.28: Lost acknowledgment corrected by resending a segment 24.# 24.* 24.3.9 TCP Congestion Control TCP uses different policies to handle the congestion in the network. We describe these policies in this section. 24.# 24.* Figure 24.29: Slow start, exponential increase 24.# 24.* Figure 24.30: Congestion avoidance, additive increase 24.# 24.* Figure 24.31: FSM for Taho TCP 24.# Figure 24.32 shows an example of congestion control in a Taho TCP. TCP starts data transfer and sets the ssthresh variable to an ambitious value of 16 MSS. TCP begins at the slow-start (SS) state with the cwnd = 24. The congestion window grows exponentially, but a time-out occurs after the third RTT (before reaching the threshold). TCP assumes that there is congestion in the network. It immediately sets the new ssthresh = 4 MSS (half of the current cwnd, which is 8) and begins a new slow start (SA) state with cwnd = 1 MSS. The congestion grows exponentially until it reaches the newly set threshold. TCP now moves to the congestion avoidance (CA) state and the congestion window grows additively until it reaches cwnd = 12 MSS. 24.* Example 24.9 * At this moment, three duplicate ACKs arrive, another indication of the congestion in the network. TCP again halves the value of ssthresh to 6 MSS and begins a new slow-start (SS) state. The exponential growth of the cwnd continues. After RTT 15, the size of cwnd is 4 MSS. After sending four segments and receiving only two ACKs, the size of the window reaches the ssthresh (6) and the TCP moves to the congestion avoidance state. The data transfer now continues in the congestion avoidance (CA) state until the connection is terminated after RTT 20. 24.* Example 24.9 (continued) * 24.* Figure 24.32: Example of Taho TCP 24.# 24.* Figure 24.33: FSM for Reno TCP 24.# Figure 24.34 shows the same situation as Figure 3.69, but in Reno TCP. The changes in the congestion window are the same until RTT 13 when three duplicate ACKs arrive. At this moment, Reno TCP drops the ssthresh to 6 MSS, but it sets the cwnd to a much higher value (ssthresh + 3 = 9 MSS) instead of 1 MSS. It now moves to the fast recovery state. We assume that two more duplicate ACKs arrive until RTT 15, where cwnd grows exponentially. In this moment, a new ACK (not duplicate) arrives that announces the receipt of the lost segment. It now moves to the congestion avoidance state, but first deflates the congestion window to 6 MSS as though ignoring the whole fast-recovery state and moving back to the previous track. 24.* Example 24.10 * 24.* Figure 24.34: Example of a Reno TCP 24.# 24.* Figure 24.35: Additive increase, multiplicative decrease (AIMD) 24.# If MSS = 10 KB (kilobytes) and RTT = 100 ms in Figure 3.72, we can calculate the throughput as shown below. 24.* Example 24.11 * Let us give a hypothetical example. Figure 3.73 shows part of a connection. The figure shows the connection establishment and part of the data transfer phases. 24. When the SYN segment is sent, there is no value for RTTM, RTTS, or RTTD. The value of RTO is set to 6.00 seconds. The following shows the value of these variables at this moment: 24.* Example 24.12 * 2. When the SYN+ACK segment arrives, RTTM is measured and is equal to 24.5 seconds. The following shows the values of these variables: 24.* Example 24.12 (continued) * 3. When the first data segment is sent, a new RTT measurement starts. Note that the sender does not start an RTT measurement when it sends the ACK segment, because it does not consume a sequence number and there is no time-out. No RTT measurement starts for the second data segment because a measurement is already in progress. 24.* Example 24.12 (continued) * 24.* 24.3.10 TCP Timers To perform their operations smoothly, most TCP implementations use at least four timers: retransmission, persistence, keepalive, and TIME-WAIT. 24.# 24.* Figure 24.36: Example 3.22 24.# Figure 24.37 is a continuation of the previous example. There is retransmission and Karn’s algorithm is applied. The first segment in the figure is sent, but lost. The RTO timer expires after 4.74 seconds. The segment is retransmitted and the timer is set to 9.48, twice the previous value of RTO. This time an ACK is received before the time-out. We wait until we send a new segment and receive the ACK for it before recalculating the RTO (Karn’s algorithm). 24.* Example 24.13 * 24.* Figure 24.37: Example 3.23 24.# 24.* 24.3.11 Options The TCP header can have up to 40 bytes of optional information. Options convey additional information to the destination or align other options. These options are included on the book website for further reference. 24.# 24.* 24-4 SCTP Stream Control Transmission Protocol (SCTP) is a new transport-layer protocol designed to combine some features of UDP and TCP in an effort to create a protocol for multimedia communication. 24.# 24.* 24.4.1 SCTP Services Before discussing the operation of SCTP, let us explain the services offered by SCTP to the application-layer processes. 24.# 24.* Figure 24.38 : Multiple-stream concept 24.# 24.* Figure 24.39 : Multihoming concept 24.# 24.* 24.4.2 SCTP Features The following shows the general features of SCTP. Transmission Sequence Number (TSN) Stream Identifier (SI) Stream Sequence Number (SSN) 24.# 24.* Figure 24.40 : Comparison between a TCP segment and an SCTP packet 24.# 24.* Figure 24.41 : Packets, data chunks, and streams 24.# 24.* 24.4.3 Packet Format An SCTP packet has a mandatory general header and a set of blocks called chunks. There are two types of chunks: control chunks and data chunks. A control chunk controls and maintains the association; a data chunk carries user data. In a packet, the control chunks come before the data chunks. Figure 24.42 shows the general format of an SCTP packet. 24.# 24.* Figure 24.43 : SCTP packet format 24.# 24.* Figure 8.50 : General header 24.# 24.* Figure 24.44 : Common layout of a chunk 24.# Table 24.3: Chunks 24.* * 24.* 24.4.4 An SCTP Association SCTP, like TCP, is a connection-oriented protocol. However, a connection in SCTP is called an association to emphasize multihoming. 24.# 24.* Figure 24.45: Four-way handshaking 24.# 24.* Figure 24.46 : Association termination 24.# 24.* 24.4.5 Flow Control Flow control in SCTP is similar to that in TCP. In SCTP, we need to handle two units of data, the byte and the chunk. The values of rwnd and cwnd are expressed in bytes; the values of TSN and acknowledgments are expressed in chunks. To show the concept, we make some unrealistic assumptions. We assume that there is never congestion in the network and that the network is error free. 24.# 24.* Figure 24.47: Flow control, receiver site 24.# 24.* Figure 24.48: Flow control, sender site 24.# 24.* 24.4.6 Error Control SCTP, like TCP, is a reliable transport-layer protocol. It uses a SACK chunk to report the state of the receiver buffer to the sender. Each implementation uses a different set of entities and timers for the receiver and sender sites. We use a very simple design to convey the concept to the reader. 24.# 24.* Figure 24.49 : Error control, receiver site 24.# 24.* Figure 24.50 : Error control, sender site 24.# 24.* Figure 24.51: New state at the sender site after receiving a SACK chunk 24.# Sending �processReceiving�processStream of bytes Stream of bytes Sending �process Receiving�process Transport Layer Protocols Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. * Outline 24.1 INTRODUCTION 24.2 UDP 24.3 TCP 24.4 SCTP 24.# Objective The first section introduces the three transport-layer protocols in the Internet and gives some information common to all of them. The second section concentrates on UDP, which is the simplest of the three protocols. UDP lacks many services we require from a transport-layer protocol, but its simplicity is very attractive to some applications, as we show. The third section discusses TCP. The section first lists its services and features. Using a transition diagram, it then shows how TCP provides a connection-oriented service. The section then uses abstract windows to show how flow and error control are accomplished in TCP. Congestion control in TCP is discussed next, a topic that was discussed for the network layer. 24.# Objective (continued) The fourth section discusses SCTP. The section first lists its services and features. It then shows how STCP creates an association. The section then shows how flow and error control are accomplished in SCTP using SACKs. 24.# 24.* 24-1 INTRODUCTION After discussing the general principle behind the transport layer in the previous chapter, we concentrate on the transport protocols in the Internet in this chapter. Figure 24.1 shows the position of these three protocols in the TCP/IP protocol suite. 24.# 24.* Figure 24.1: Position of transport-layer protocols in the TCP/IP protocol suite 24.# 24.* 24.24.1 Services Each protocol provides a different type of service and should be used appropriately. 24.# 24.* 24.24.2 Port Numbers As discussed in the previous chapter, a transport-layer protocol usually has several responsibilities. One is to create a process-to-process communication; these protocols use port numbers to accomplish this. Port numbers provide end-to-end addresses at the transport layer and allow multiplexing and demultiplexing at this layer, just as IP addresses do at the network layer. Table 24.1 gives some common port numbers for all three protocols we discuss in this chapter. 24.# Table 24.1: Some well-known ports used with UDP and TCP 24.* * 24.* 24-2 UDP The User Datagram Protocol (UDP) is a connectionless, unreliable transport protocol. If UDP is so powerless, why would a process want to use it? With the disadvantages come some advantages. UDP is a very simple protocol using a minimum of overhead. 24.# 24.* 24.2.1 User Datagram UDP packets, called user datagrams, have a fixed-size header of 8 bytes made of four fields, each of 2 bytes (16 bits). Figure 24.2 shows the format of a user datagram. The first two fields define the source and destination port numbers. The third field defines the total length of the user datagram, header plus data. The 16 bits can define a total length of 0 to 65,535 bytes. However, the total length needs to be less because a UDP user datagram is stored in an IP datagram with the total length of 65,535 bytes. The last field can carry the optional checksum (explained later). 24.# 24.* Figure 24.2: User datagram packet format 24.# The following is the contents of a UDP header in hexadecimal format. a. What is the source port number? b. What is the destination port number? c. What is the total length of the user datagram? d. What is the length of the data? e. Is the packet directed from a client to a server or vice versa? f. What is the client process? 24.* Example 24.1 * Solution a. The source port number is the first four hexadecimal digits (CB84)16 or 52100 b. The destination port number is the second four hexadecimal digits (000D)16 or 13. c. The third four hexadecimal digits (001C)16 define the length of the whole UDP packet as 28 bytes. d. The length of the data is the length of the whole packet minus the length of the header, or 28 − 8 = 20 bytes. e. Since the destination port number is 13 (well-known port), the packet is from the client to the server. f. The client process is the Daytime (see Table 3.1). 24.* Example 24.1 (continued) * 24.* 24.2.2 UDP Services Earlier we discussed the general services provided by a transport-layer protocol. In this section, we discuss what portions of those general services are provided by UDP. 24.# 24.* Figure 24.3: Pseudoheader for checksum calculation 24.# What value is sent for the checksum in one of the following hypothetical situations? a. The sender decides not to include the checksum. b. The sender decides to include the checksum, but the value of the sum is all 1s. c. The sender decides to include the checksum, but the value of the sum is all 0s. 24.* Example 24.2 * Solution a. The value sent for the checksum field is all 0s to show that the checksum is not calculated. b. When the sender complements the sum, the result is all 0s; the sender complements the result again before sending. The value sent for the checksum is all 1s. The second complement operation is needed to avoid confusion with the case in part a. c. This situation never happens because it implies that the value of every term included in the calculation of the sum is all 0s, which is impossible; some fields in the pseudoheader have nonzero values. 24.* Example 24.2 (continued) * 24.* 24.2.3 UDP Applications Although UDP meets almost none of the criteria we mentioned earlier for a reliable transport-layer protocol, UDP is preferable for some applications. The reason is that some services may have some side effects that are either unacceptable or not preferable. An application designer sometimes needs to compromise to get the optimum. For example, in our daily life, we all know that a one-day delivery of a package by a carrier is more expensive than a three-day delivery. Although high speed and low cost are both desirable features in delivery of a parcel, they are in conflict with each other. 24.# A client-server application such as DNS (see Chapter 26) uses the services of UDP because a client needs to send a short request to a server and to receive a quick response from it. The request and response can each fit in one user datagram. Since only one message is exchanged in each direction, the connectionless feature is not an issue; the client or server does not worry that messages are delivered out of order. 24.* Example 24. 3 * A client-server application such as SMTP (see Chapter 26), which is used in electronic mail, cannot use the services of UDP because a user can send a long e-mail message, which may include multimedia (images, audio, or video). If the application uses UDP and the message does not fit in one single user datagram, the message must be split by the application into different user datagrams. Here the connectionless service may create problems. The user datagrams may arrive and be delivered to the receiver application out of order. The receiver application may not be able to reorder the pieces. This means the connectionless service has a disadvantage for an application program that sends long messages. 24.* Example 24.4 * Assume we are downloading a very large text file from the Internet. We definitely need to use a transport layer that provides reliable service. We don’t want part of the file to be missing or corrupted when we open the file. The delay created between the deliveries of the parts is not an overriding concern for us; we wait until the whole file is composed before looking at it. In this case, UDP is not a suitable transport layer. 24.* Example 25.5 * Assume we are using a real-time interactive application, such as Skype. Audio and video are divided into frames and sent one after another. If the transport layer is supposed to resend a corrupted or lost frame, the synchronizing of the whole transmission may be lost. The viewer suddenly sees a blank screen and needs to wait until the second transmission arrives. This is not tolerable. However, if each small part of the screen is sent using one single user datagram, the receiving UDP can easily ignore the corrupted or lost packet and deliver the rest to the application program. That part of the screen is blank for a very short period of time, which most viewers do not even notice. 24.* Example 25.6 * 24.* 24-3 TCP Transmission Control Protocol (TCP) is a connection-oriented, reliable protocol. TCP explicitly defines connection establishment, data transfer, and connection teardown phases to provide a connection-oriented service. TCP uses a combination of GBN and SR protocols to provide reliability. 24.# 24.* 24.3.1 TCP Services Before discussing TCP in detail, let us explain the services offered by TCP to the processes at the application layer. 24.# 24.* Figure 24.4: Stream delivery 24.# 24.* Figure 24.5: Sending and receiving buffers 24.# 24.* Figure 24.6: TCP segments 24.# 24.* 24.3.2 TCP Features To provide the services mentioned in the previous section, TCP has several features that are briefly summarized in this section and discussed later in detail. 24.# Suppose a TCP connection is transferring a file of 5,000 bytes. The first byte is numbered 10,0024. What are the sequence numbers for each segment if data are sent in five segments, each carrying 1,000 bytes? Solution The following shows the sequence number for each segment: 24.* Example 24.7 * 24.* 24.3.3 Segment Before discussing TCP in more detail, let us discuss the TCP packets themselves. A packet in TCP is called a segment. 24.# 24.* Figure 24.7: TCP segment format 24.# 24.* Figure 24.8: Control field 24.# 24.* Figure 24.9: Pseudoheader added to the TCP datagram 24.# 24.* 24.3.4 A TCP Connection TCP is connection-oriented. All of the segments belonging to a message are then sent over this logical path. Using a single logical pathway for the entire message facilitates the acknowledgment process as well as retransmission of damaged or lost frames. You may wonder how TCP, which uses the services of IP, a connectionless protocol, can be connection-oriented. The point is that a TCP connection is logical, not physical. TCP operates at a higher level. TCP uses the services of IP to deliver individual segments to the receiver, but it controls the connection itself. 24.# 24.* Figure 24.10: Connection establishment using three-way handshaking 24.# 24.* Figure 24.11: Data transfer 24.# 24.* Figure 24.12: Connection termination using three-way handshaking 24.# 24.* Figure 24.13: Half-close Categories of ICMPv6 messages 24.# 24.* 24.3.5 State Transition Diagram To keep track of all the different events happening during connection establishment, connection termination, and data transfer, TCP is specified as the finite state machine (FSM) as shown in Figure 24.14. 24.# 24.* Figure 24.14: State transition diagram 24.# Table 24.2: States for TCP 24.* * 24.* Figure 24.15: Transition diagram with half-close connection termination 24.# 24.* Figure 24.16: Time-line diagram for a common scenario 24.# 24.* 24.3.6 Windows in TCP Before discussing data transfer in TCP and the issues such as flow, error, and congestion control, we describe the windows used in TCP. TCP uses two windows (send window and receive window) for each direction of data transfer, which means four windows for a bidirectional communication. To make the discussion simple, we make an unrealistic assumption that communication is only unidirectional. The bidirectional communication can be inferred using two unidirectional communications with piggybacking. 24.# 24.* Figure 24.17: Send window in TCP 24.# 24.* Figure 24.18: Receive window in TCP 24.# 24.* 24.3.7 Flow Control As discussed before, flow control balances the rate a producer creates data with the rate a consumer can use the data. TCP separates flow control from error control. In this section we discuss flow control, ignoring error control. We assume that the logical channel between the sending and receiving TCP is error-free. 24.# 24.* Figure 24.19: Data flow and flow control feedbacks in TCP 24.# 24.* Figure 24.20: An example of flow control 24.# Figure 3.58 shows the reason for this mandate. Part a of the figure shows the values of the last acknowledgment and rwnd. Part b shows the situation in which the sender has sent bytes 206 to 214. Bytes 206 to 209 are acknowledged and purged. The new advertisement, however, defines the new value of rwnd as 4, in which 210 + 4 < 206 + 12. When the send window shrinks, it creates a problem: byte 214, which has already been sent, is outside the window. The relation discussed before forces the receiver to maintain the right-hand wall of the window to be as shown in part a, because the receiver does not know which of the bytes 210 to 217 has already been sent. described above. 24.* Example 3.18 * 24.* Figure 24.21: Example 3.18 24.# 24.* 24.3.8 Error Control TCP is a reliable transport-layer protocol. This means that an application program that delivers a stream of data to TCP relies on TCP to deliver the entire stream to the application program on the other end in order, without error, and without any part lost or duplicated. 24.# 24.* Figure 24.22: Simplified FSM for the TCP sender side 24.# 24.* Figure 24.23: Simplified FSM for the TCP receiver side 24.# 24.* Figure 24.24: Normal operation 24.# 24.* Figure 24.25: Lost segment 24.# 24.* Figure 24.26: Fast retransmission 24.# 24.* Figure 24.27: Lost acknowledgment 24.# 24.* Figure 24.28: Lost acknowledgment corrected by resending a segment 24.# 24.* 24.3.9 TCP Congestion Control TCP uses different policies to handle the congestion in the network. We describe these policies in this section. 24.# 24.* Figure 24.29: Slow start, exponential increase 24.# 24.* Figure 24.30: Congestion avoidance, additive increase 24.# 24.* Figure 24.31: FSM for Taho TCP 24.# Figure 24.32 shows an example of congestion control in a Taho TCP. TCP starts data transfer and sets the ssthresh variable to an ambitious value of 16 MSS. TCP begins at the slow-start (SS) state with the cwnd = 24. The congestion window grows exponentially, but a time-out occurs after the third RTT (before reaching the threshold). TCP assumes that there is congestion in the network. It immediately sets the new ssthresh = 4 MSS (half of the current cwnd, which is 8) and begins a new slow start (SA) state with cwnd = 1 MSS. The congestion grows exponentially until it reaches the newly set threshold. TCP now moves to the congestion avoidance (CA) state and the congestion window grows additively until it reaches cwnd = 12 MSS. 24.* Example 24.9 * At this moment, three duplicate ACKs arrive, another indication of the congestion in the network. TCP again halves the value of ssthresh to 6 MSS and begins a new slow-start (SS) state. The exponential growth of the cwnd continues. After RTT 15, the size of cwnd is 4 MSS. After sending four segments and receiving only two ACKs, the size of the window reaches the ssthresh (6) and the TCP moves to the congestion avoidance state. The data transfer now continues in the congestion avoidance (CA) state until the connection is terminated after RTT 20. 24.* Example 24.9 (continued) * 24.* Figure 24.32: Example of Taho TCP 24.# 24.* Figure 24.33: FSM for Reno TCP 24.# Figure 24.34 shows the same situation as Figure 3.69, but in Reno TCP. The changes in the congestion window are the same until RTT 13 when three duplicate ACKs arrive. At this moment, Reno TCP drops the ssthresh to 6 MSS, but it sets the cwnd to a much higher value (ssthresh + 3 = 9 MSS) instead of 1 MSS. It now moves to the fast recovery state. We assume that two more duplicate ACKs arrive until RTT 15, where cwnd grows exponentially. In this moment, a new ACK (not duplicate) arrives that announces the receipt of the lost segment. It now moves to the congestion avoidance state, but first deflates the congestion window to 6 MSS as though ignoring the whole fast-recovery state and moving back to the previous track. 24.* Example 24.10 * 24.* Figure 24.34: Example of a Reno TCP 24.# 24.* Figure 24.35: Additive increase, multiplicative decrease (AIMD) 24.# If MSS = 10 KB (kilobytes) and RTT = 100 ms in Figure 3.72, we can calculate the throughput as shown below. 24.* Example 24.11 * Let us give a hypothetical example. Figure 3.73 shows part of a connection. The figure shows the connection establishment and part of the data transfer phases. 24. When the SYN segment is sent, there is no value for RTTM, RTTS, or RTTD. The value of RTO is set to 6.00 seconds. The following shows the value of these variables at this moment: 24.* Example 24.12 * 2. When the SYN+ACK segment arrives, RTTM is measured and is equal to 24.5 seconds. The following shows the values of these variables: 24.* Example 24.12 (continued) * 3. When the first data segment is sent, a new RTT measurement starts. Note that the sender does not start an RTT measurement when it sends the ACK segment, because it does not consume a sequence number and there is no time-out. No RTT measurement starts for the second data segment because a measurement is already in progress. 24.* Example 24.12 (continued) * 24.* 24.3.10 TCP Timers To perform their operations smoothly, most TCP implementations use at least four timers: retransmission, persistence, keepalive, and TIME-WAIT. 24.# 24.* Figure 24.36: Example 3.22 24.# Figure 24.37 is a continuation of the previous example. There is retransmission and Karn’s algorithm is applied. The first segment in the figure is sent, but lost. The RTO timer expires after 4.74 seconds. The segment is retransmitted and the timer is set to 9.48, twice the previous value of RTO. This time an ACK is received before the time-out. We wait until we send a new segment and receive the ACK for it before recalculating the RTO (Karn’s algorithm). 24.* Example 24.13 * 24.* Figure 24.37: Example 3.23 24.# 24.* 24.3.11 Options The TCP header can have up to 40 bytes of optional information. Options convey additional information to the destination or align other options. These options are included on the book website for further reference. 24.# 24.* 24-4 SCTP Stream Control Transmission Protocol (SCTP) is a new transport-layer protocol designed to combine some features of UDP and TCP in an effort to create a protocol for multimedia communication. 24.# 24.* 24.4.1 SCTP Services Before discussing the operation of SCTP, let us explain the services offered by SCTP to the application-layer processes. 24.# 24.* Figure 24.38 : Multiple-stream concept 24.# 24.* Figure 24.39 : Multihoming concept 24.# 24.* 24.4.2 SCTP Features The following shows the general features of SCTP. Transmission Sequence Number (TSN) Stream Identifier (SI) Stream Sequence Number (SSN) 24.# 24.* Figure 24.40 : Comparison between a TCP segment and an SCTP packet 24.# 24.* Figure 24.41 : Packets, data chunks, and streams 24.# 24.* 24.4.3 Packet Format An SCTP packet has a mandatory general header and a set of blocks called chunks. There are two types of chunks: control chunks and data chunks. A control chunk controls and maintains the association; a data chunk carries user data. In a packet, the control chunks come before the data chunks. Figure 24.42 shows the general format of an SCTP packet. 24.# 24.* Figure 24.43 : SCTP packet format 24.# 24.* Figure 8.50 : General header 24.# 24.* Figure 24.44 : Common layout of a chunk 24.# Table 24.3: Chunks 24.* * 24.* 24.4.4 An SCTP Association SCTP, like TCP, is a connection-oriented protocol. However, a connection in SCTP is called an association to emphasize multihoming. 24.# 24.* Figure 24.45: Four-way handshaking 24.# 24.* Figure 24.46 : Association termination 24.# 24.* 24.4.5 Flow Control Flow control in SCTP is similar to that in TCP. In SCTP, we need to handle two units of data, the byte and the chunk. The values of rwnd and cwnd are expressed in bytes; the values of TSN and acknowledgments are expressed in chunks. To show the concept, we make some unrealistic assumptions. We assume that there is never congestion in the network and that the network is error free. 24.# 24.* Figure 24.47: Flow control, receiver site 24.# 24.* Figure 24.48: Flow control, sender site 24.# 24.* 24.4.6 Error Control SCTP, like TCP, is a reliable transport-layer protocol. It uses a SACK chunk to report the state of the receiver buffer to the sender. Each implementation uses a different set of entities and timers for the receiver and sender sites. We use a very simple design to convey the concept to the reader. 24.# 24.* Figure 24.49 : Error control, receiver site 24.# 24.* Figure 24.50 : Error control, sender site 24.# 24.* Figure 24.51: New state at the sender site after receiving a SACK chunk 24.# Sending �processReceiving�processStream of bytes Stream of bytes Sending �process Receiving�process