05_Intro to DataCom

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2000

Introduction to Data Communications

1

A Communications Model

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Data Communications Model

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1-1 DATA COMMUNICATIONS1-1 DATA COMMUNICATIONS

The term The term telecommunicationtelecommunication means communication at a means communication at a distance. The word distance. The word datadata refers to information presented refers to information presented in whatever form is agreed upon by the parties creating in whatever form is agreed upon by the parties creating and using the data. and using the data. Data communicationsData communications are the are the exchange of data between two devices via some form of exchange of data between two devices via some form of transmission medium such as a wire cable. transmission medium such as a wire cable.

ComponentsData RepresentationData Flow

Topics discussed in this section:Topics discussed in this section:

1.5

Figure 1.1 Five components of data communication

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Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

Transmission Medium

Guided Media (Cable/Wire) Twisted Pair : UTP & STP Coaxial Optical Fiber / Fiber Optic

Unguided Media (Wireless) Terrestrial Microwave Satellite Microwave

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Data and Signals

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

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To be transmitted, data must be transformed to electromagnetic signals.

Note

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3-1 ANALOG AND DIGITAL3-1 ANALOG AND DIGITAL

Data can be Data can be analoganalog or or digitaldigital. The term . The term analog dataanalog data refers to information that is continuous; refers to information that is continuous; digital datadigital data refers to information that has discrete states. refers to information that has discrete states. Analog data take on continuous values. Digital data Analog data take on continuous values. Digital data take on discrete values.take on discrete values.

Analog and Digital DataAnalog and Digital SignalsPeriodic and Nonperiodic Signals


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Note

Data can be analog or digital. Analog data are continuous and take

continuous values.Digital data have discrete states and

take discrete values.

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Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital

signals can have only a limited number of values.

Note

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Figure 3.1 Comparison of analog and digital signals

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In data communications, we commonly use periodic analog signals and

nonperiodic digital signals.

Note

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3-2 PERIODIC ANALOG SIGNALS3-2 PERIODIC ANALOG SIGNALS

Periodic analog signals can be classified as Periodic analog signals can be classified as simplesimple or or compositecomposite. A simple periodic analog . A simple periodic analog signal, a signal, a sine wavesine wave, cannot be decomposed into , cannot be decomposed into simpler signals. A compositesimpler signals. A composite periodic analog periodic analog signal is composed of multiple sine waves.signal is composed of multiple sine waves.

Sine WaveWavelengthTime and Frequency DomainComposite SignalsBandwidth


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Figure 3.2 A sine wave

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We discuss a mathematical approach to sine waves in Appendix C.

Note

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The power in your house can be represented by a sine wave with a peak amplitude of 155 to 170 V. However, it is common knowledge that the voltage of the power in U.S. homes is 110 to 120 V. This discrepancy is due to the fact that these are root mean square (rms) values. The signal is squared and then the average amplitude is calculated. The peak value is equal to 2½ × rms value.

Example 3.1

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Figure 3.3 Two signals with the same phase and frequency, but different amplitudes

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The voltage of a battery is a constant; this constant value can be considered a sine wave, as we will see later. For example, the peak value of an AA battery is normally1.5 V.

Example 3.2

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Frequency and period are the inverse of each other.

Note

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Figure 3.4 Two signals with the same amplitude and phase, but different frequencies

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Table 3.1 Units of period and frequency

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The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows:

Example 3.3

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Express a period of 100 ms in microseconds.

Example 3.4

SolutionFrom Table 3.1 we find the equivalents of 1 ms (1 ms is 10−3 s) and 1 s (1 s is 106 μs). We make the following substitutions:.

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The period of a signal is 100 ms. What is its frequency in kilohertz?

Example 3.5

SolutionFirst we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz).

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Frequency is the rate of change with respect to time.

Change in a short span of timemeans high frequency.

Change over a long span of time means low frequency.

Note

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If a signal does not change at all, its frequency is zero.

If a signal changes instantaneously, its frequency is infinite.

Note

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Phase describes the position of the waveform relative to time 0.

Note

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Figure 3.5 Three sine waves with the same amplitude and frequency, but different phases

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A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?

Example 3.6

SolutionWe know that 1 complete cycle is 360°. Therefore, 1/6 cycle is

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Figure 3.6 Wavelength and period

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Figure 3.7 The time-domain and frequency-domain plots of a sine wave

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A complete sine wave in the time domain can be represented by one

single spike in the frequency domain.

Note

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The frequency domain is more compact and useful when we are dealing with more than one sine wave. For example, Figure 3.8 shows three sine waves, each with different amplitude and frequency. All can be represented by three spikes in the frequency domain.

Example 3.7

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Figure 3.8 The time domain and frequency domain of three sine waves

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A single-frequency sine wave is not useful in data communications;

we need to send a composite signal, a signal made of many simple sine waves.

Note

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According to Fourier analysis, any composite signal is a combination of

simple sine waves with different frequencies, amplitudes, and phases.

Fourier analysis is discussed in Appendix C.

Note

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If the composite signal is periodic, the decomposition gives a series of signals

with discrete frequencies; if the composite signal is nonperiodic,

the decomposition gives a combination of sine waves with continuous

frequencies.

Note

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Figure 3.9 shows a periodic composite signal with frequency f. This type of signal is not typical of those found in data communications. We can consider it to be three alarm systems, each with a different frequency. The analysis of this signal can give us a good understanding of how to decompose signals.

Example 3.8

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Figure 3.9 A composite periodic signal

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Figure 3.10 Decomposition of a composite periodic signal in the time and frequency domains

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Figure 3.11 shows a nonperiodic composite signal. It can be the signal created by a microphone or a telephone set when a word or two is pronounced. In this case, the composite signal cannot be periodic, because that implies that we are repeating the same word or words with exactly the same tone.

Example 3.9

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Figure 3.11 The time and frequency domains of a nonperiodic signal

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The bandwidth of a composite signal is the difference between the

highest and the lowest frequencies contained in that signal.

Note

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Figure 3.12 The bandwidth of periodic and nonperiodic composite signals

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If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then

Example 3.10

The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz (see Figure 3.13).

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Figure 3.13 The bandwidth for Example 3.10

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A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all frequencies of the same amplitude.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then

Example 3.11

The spectrum contains all integer frequencies. We show this by a series of spikes (see Figure 3.14).

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A nonperiodic composite signal has a bandwidth of 200 kHz, with a middle frequency of 140 kHz and peak amplitude of 20 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal.

SolutionThe lowest frequency must be at 40 kHz and the highest at 240 kHz. Figure 3.15 shows the frequency domain and the bandwidth.

Example 3.12

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An example of a nonperiodic composite signal is the signal propagated by an AM radio station. In the United States, each AM radio station is assigned a 10-kHz bandwidth. The total bandwidth dedicated to AM radio ranges from 530 to 1700 kHz. We will show the rationale behind this 10-kHz bandwidth in Chapter 5.

Example 3.13

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Another example of a nonperiodic composite signal is the signal propagated by an FM radio station. In the United States, each FM radio station is assigned a 200-kHz bandwidth. The total bandwidth dedicated to FM radio ranges from 88 to 108 MHz. We will show the rationale behind this 200-kHz bandwidth in Chapter 5.

Example 3.14

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Another example of a nonperiodic composite signal is the signal received by an old-fashioned analog black-and-white TV. A TV screen is made up of pixels. If we assume a resolution of 525 × 700, we have 367,500 pixels per screen. If we scan the screen 30 times per second, this is 367,500 × 30 = 11,025,000 pixels per second. The worst-case scenario is alternating black and white pixels. We can send 2 pixels per cycle. Therefore, we need 11,025,000 / 2 = 5,512,500 cycles per second, or Hz. The bandwidth needed is 5.5125 MHz.

Example 3.15

Acoustic Spectrum (Analog)

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Audio Signals

freq range 20Hz-20kHz (speech 100Hz-7kHz) easily converted into electromagnetic signals varying volume converted to varying voltage can limit frequency range for voice channel to

300-3400Hz

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3-6 PERFORMANCE3-6 PERFORMANCE

One important issue in networking is the One important issue in networking is the performanceperformance of the network—how good is it? We of the network—how good is it? We discuss quality of service, an overall discuss quality of service, an overall measurement of network performance, in greater measurement of network performance, in greater detail in Chapter 24. In this section, we introduce detail in Chapter 24. In this section, we introduce terms that we need for future chapters.terms that we need for future chapters.

BandwidthThroughputLatency (Delay)Bandwidth-Delay Product


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In networking, we use the term bandwidth in two contexts.

❏ The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.

❏ The second, bandwidth in bits per second, refers to the speed of bit transmission in a channel or link.

Note

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The bandwidth of a subscriber line is 4 kHz for voice or data. The bandwidth of this line for data transmissioncan be up to 56,000 bps using a sophisticated modem to change the digital signal to analog.

Example 3.42

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If the telephone company improves the quality of the line and increases the bandwidth to 8 kHz, we can send 112,000 bps by using the same technology as mentioned in Example 3.42.

Example 3.43

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A network with bandwidth of 10 Mbps can pass only an average of 12,000 frames per minute with each frame carrying an average of 10,000 bits. What is the throughput of this network?

SolutionWe can calculate the throughput as

Example 3.44

The throughput is almost one-fifth of the bandwidth in this case.

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What is the propagation time if the distance between the two points is 12,000 km? Assume the propagation speed to be 2.4 × 108 m/s in cable.

SolutionWe can calculate the propagation time as

Example 3.45

The example shows that a bit can go over the Atlantic Ocean in only 50 ms if there is a direct cable between the source and the destination.

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What are the propagation time and the transmission time for a 2.5-kbyte message (an e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.

SolutionWe can calculate the propagation and transmission time as shown on the next slide:

Example 3.46

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Note that in this case, because the message is short and the bandwidth is high, the dominant factor is the propagation time, not the transmission time. The transmission time can be ignored.

Example 3.46 (continued)

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What are the propagation time and the transmission time for a 5-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.

SolutionWe can calculate the propagation and transmission times as shown on the next slide.

Example 3.47

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Note that in this case, because the message is very long and the bandwidth is not very high, the dominant factor is the transmission time, not the propagation time. The propagation time can be ignored.

Example 3.47 (continued)

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Figure 3.31 Filling the link with bits for case 1

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We can think about the link between two points as a pipe. The cross section of the pipe represents the bandwidth, and the length of the pipe represents the delay. We can say the volume of the pipe defines the bandwidth-delay product, as shown in Figure 3.33.

Example 3.48

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Figure 3.32 Filling the link with bits in case 2

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The bandwidth-delay product defines the number of bits that can fill the link.

Note

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Figure 3.33 Concept of bandwidth-delay product

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Transmission Media

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

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Figure 7.1 Transmission medium and physical layer

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Figure 7.2 Classes of transmission media

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7-1 GUIDED MEDIA7-1 GUIDED MEDIA

Guided media, which are those that provide a conduit Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.coaxial cable, and fiber-optic cable.

Twisted-Pair CableCoaxial CableFiber-Optic Cable


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Figure 7.3 Twisted-pair cable

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Figure 7.4 UTP and STP cables

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Table 7.1 Categories of unshielded twisted-pair cables

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Figure 7.5 UTP connector

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Figure 7.6 UTP performance

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Figure 7.7 Coaxial cable

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Table 7.2 Categories of coaxial cables

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Figure 7.8 BNC connectors

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Figure 7.9 Coaxial cable performance

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Figure 7.10 Bending of light ray

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Figure 7.11 Optical fiber

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Figure 7.12 Propagation modes

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Figure 7.13 Modes

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Table 7.3 Fiber types

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Figure 7.14 Fiber construction

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Figure 7.15 Fiber-optic cable connectors

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Figure 7.16 Optical fiber performance

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7-2 UNGUIDED MEDIA: WIRELESS7-2 UNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic waves Unguided media transport electromagnetic waves without using a physical conductor. This type of without using a physical conductor. This type of communication is often referred to as wireless communication is often referred to as wireless communication.communication.

Radio WavesMicrowavesInfrared


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Figure 7.17 Electromagnetic spectrum for wireless communication

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Figure 7.18 Propagation methods

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Table 7.4 Bands

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Figure 7.19 Wireless transmission waves

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Figure 7.20 Omnidirectional antenna

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Radio waves are used for multicast communications, such as radio and

television, and paging systems.

Note

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Figure 7.21 Unidirectional antennas

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Microwaves are used for unicast communication such as cellular telephones, satellite networks,

and wireless LANs.

Note

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Infrared signals can be used for short-range communication in a closed area

using line-of-sight propagation.

Note

Satellite Point to Point Link

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Satellite Broadcast Link

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Copyright 2005 John Wiley & Sons, Inc1 - 107

Datacom Basics

Broadband Communications

Telecommunicationstransmission of voice, video, data, - imply longer distances- broader term

Data Communicationsmovement of computer information by means of electrical or optical transmission systems

convergence

05_Intro to DataCom

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