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Chapter 6

PhysicalLayer

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Distinguish between analog and digital data.Distinguish between analog and digital data.

Distinguish between analog and digital signals.Distinguish between analog and digital signals.

Understand the concept of bandwidth and the relationship Understand the concept of bandwidth and the relationship between bandwidth and data transmission speed.between bandwidth and data transmission speed.

Understand digital-to-digital, digital-to-analog, and analog-to-Understand digital-to-digital, digital-to-analog, and analog-to-digital encoding.digital encoding.

After reading this chapter, the reader should After reading this chapter, the reader should be able to:be able to:

OOBJECTIVESBJECTIVES

Understand multiplexing and the difference between a linkUnderstand multiplexing and the difference between a linkand a channel.and a channel.

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DIGITALDIGITALANDAND

ANALOGANALOG

6.16.1

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Figure 6-1

Digital and analog entities

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Figure 6-2

Digital data

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Figure 6-3

Analog data

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Figure 6-4

Digital signal

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Figure 6-5

Bit and bit interval

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Technical Focus:Technical Focus: Units of Bit RateUnits of Bit Rate

1 bps1 bps

1 kbps = 1000 bps1 kbps = 1000 bps

1 Mbps = 1,000,000 bps1 Mbps = 1,000,000 bps

1 Gbps = 1,000,000,000 bps1 Gbps = 1,000,000,000 bps

1 Tbps = 1,000,000,000,000 bps1 Tbps = 1,000,000,000,000 bps

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Figure 6-6

A sine wave

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Figure 6-7

Amplitude

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Figure 6-8

Period and frequency

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Technical Focus:Technical Focus: Units of FrequencyUnits of Frequency

1 Hz1 Hz

1 kHz = 1000 Hz1 kHz = 1000 Hz

1 MHz = 1,000,000 Hz1 MHz = 1,000,000 Hz

1 GHz = 1,000,000,000 Hz1 GHz = 1,000,000,000 Hz

1 THz = 1,000,000,000,000 Hz1 THz = 1,000,000,000,000 Hz

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Technical Focus:Technical Focus: Frequency and ChangeFrequency and Change

The concept of frequency is similar to the concept of change.

If a signal (or data) is changing rapidly, its frequency is higher. If it changes slowly, its frequency is lower.

When a signal changes 10 times per second, its frequency is 10 Hz; when a signal changes 1000 times per second, its frequency is 1000 Hz.

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Figure 6-9

Phase

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Zero frequency and infinite frequency

Figure 6-10

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Phase describes the position of a Phase describes the position of a waveform relative to other waveforms.waveform relative to other waveforms.

Note:Note:

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Business Focus:Business Focus: Two Familiar SignalsTwo Familiar Signals

A familiar signal in our daily lives is the electrical energy we use at home and at work. The signal we receive from the power company has an amplitude of 120 V and a frequency of 60 Hz (a simple analog signal). Another signal familiar to us is the power we get from a battery. It is an analog signal with an amplitude of 6 V (or 12 or 24) and a frequency of zero.

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Business Focus:Business Focus: The Bandwidth of Telephone LinesThe Bandwidth of Telephone Lines

The conventional line that connects a home or business to the telephone office has a bandwidth of 4 kHz. These lines were designed for carrying human voice, which normally has a bandwidth in this range. Human voice has a frequency that is normally between 0 and 4 kHz. The telephone lines are perfect for this purpose. However, if we try to send a digital signal, we are in trouble. A digital signal needs a very high bandwidth (theoretically infinite); it cannot be sent using these lines. We must either improve the quality of these lines or change our digital signal to a complex signal that needs only 4 kHz.

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TRANSFORMINGTRANSFORMINGDATADATA

TO SIGNALSTO SIGNALS

6.26.2

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Transforming data to signalsFigure 6-11

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Digital-to-digital encodingFigure 6-12

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A digital signal has a much higher A digital signal has a much higher bandwidth than an analog signal. bandwidth than an analog signal.

There is a need for a better media to There is a need for a better media to send a digital signal. send a digital signal.

Note:Note:

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Most LANs use digital-to-digital Most LANs use digital-to-digital encoding because the data stored in encoding because the data stored in

the computers are digital and the cable the computers are digital and the cable connecting them is capable of carrying connecting them is capable of carrying

digital signals.digital signals.

Note:Note:

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Digital encodingmethods

Figure 6-13

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Technical Focus:Technical Focus: Average Values in Digital SignalsAverage Values in Digital Signals

With one exception, all of the signals in Figure 6.3 have an average value of zero (the positive and negative values cancel each other in the long run). The first signal, unipolar, has a positive average value. This average value, sometimes called the residual value, cannot pass through some devices (such as a transformer). In this case, the receiver receives a signal that can be totally different from the one sent and results in an erroneous interpretation of data.

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Technical Focus:Technical Focus: Synchronization in Digital SignalsSynchronization in Digital Signals

To correctly interpret the signals received from the sender, the receiver’s bit intervals must correspond exactly to the sender’s bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver will interpret the signals differently than the sender intended. A self-synchronizing digital signal includes timing information in the data beingtransmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end ofthe bit interval. If the receiver’s clock is out of synchronization, these alerting points can reset the clock.

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Digital-to-analog modulationFigure 6-14

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ASKFigure 6-15

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FSKFigure 6-16

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PSKFigure 6-17

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Technical Focus:Technical Focus: Understanding Bit Rate and Baud RateUnderstanding Bit Rate and Baud Rate

A transportation analogy can clarify the concept of bauds and bits. A baud is analogous to a car; a bit is analogous to a passenger. A car can carry one or more passengers. If 1000 cars go from onepoint to another each carrying only one passenger (the driver), then 1000 passengers are transported. However, if each car carries four passengers (car pooling), then 4000 passengers aretransported. Note that the number of cars, not the number of passengers, determines the traffic and, therefore, the need for wider highways. Similarly, the number of bauds determines the required bandwidth, not the number of bits.

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Technical Focus:Technical Focus: Capacity of a ChannelCapacity of a Channel

We often need to know the capacity of a channel; that is, how fast can we send data over a specific medium? The answer was given by Shannon. Shannon proved that the number of bits that we can send through a channel depends on two factors: the bandwidth of the channel and the noise in the channel. Shannon came up with the following formula:

CB log2 (1signal-to-noise ratio)

C is the capacity in bits per second; B is the bandwidth.

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Analog-to-digital conversionFigure 6-18

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PCMFigure 6-19

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Technical Focus:Technical Focus: Sampling Rate and Nyquist TheoremSampling Rate and Nyquist Theorem

As you can see from the preceding figures, the accuracy of anydigital reproduction of an analog signal depends on the numberof samples taken. So the question is, how many samples aresufficient? This question was answered by Nyquist. His theorem states that the sampling rate must be at least twice the highest frequency of the original signal to ensure the accurate reproductionof the original analog signal. So if we want to sample a telephonevoice with a maximum frequency of 4000 Hz, we need a samplingrate of 8000 samples per second.

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TRANSMISSIONTRANSMISSIONMODESMODES

6.36.3

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Data transmissionFigure 6-20

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Parallel transmissionFigure 6-21

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Serial transmissionFigure 6-22

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In asynchronous transmission, we In asynchronous transmission, we send 1 start bit (0) at the beginning send 1 start bit (0) at the beginning

and 1 or more stop bits (1s) at the end and 1 or more stop bits (1s) at the end of each byte. There may be a gap of each byte. There may be a gap

between each byte.between each byte.

Note:Note:

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Asynchronous here means Asynchronous here means “asynchronous at the byte level,” but “asynchronous at the byte level,” but the bits are still synchronized; their the bits are still synchronized; their

durations are the same.durations are the same.

Note:Note:

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Asynchronous transmissionFigure 6-23

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In synchronous transmission, we send In synchronous transmission, we send bits one after another without bits one after another without start/stop bits or gaps. It is the start/stop bits or gaps. It is the

responsibility of the receiver to group responsibility of the receiver to group the bits.the bits.

Note:Note:

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Figure 6-24

Synchronous transmission

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LINELINECONFIGURATIONCONFIGURATION

6.46.4

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Line configuration defines the Line configuration defines the attachment of communication devices attachment of communication devices

to a link.to a link.

Note:Note:

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Figure 6-25

Point-to-point line configuration

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Figure 6-26

Multipoint line configuration

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DUPLEXITYDUPLEXITY

6.56.5

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Half-duplex modeFigure 6-27

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Full-duplex modeFigure 6-28

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MULTIPLEXING:MULTIPLEXING:SHARING THE MEDIASHARING THE MEDIA

6.66.6

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Multiplexing versus no multiplexingFigure 6-29

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Categories of multiplexingFigure 6-30

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FDMFigure 6-31

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FDM can only be used with analog FDM can only be used with analog signals.signals.

Note:Note:

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Technical Focus:Technical Focus: Use of FDM in Telephone SystemsUse of FDM in Telephone Systems

AT&T uses a hierarchical system to multiplex analog lines:

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Prisms in WDM multiplexing and demultiplexing

Figure 6-32

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TDMFigure 6-33

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TDM can be used only with digital TDM can be used only with digital signals. signals.

Note:Note:

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Synchronous TDMFigure 6-34

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Technical Focus:Technical Focus: Use of TDM in Telephone SystemsUse of TDM in Telephone Systems

AT&T uses a hierarchical system to multiplex digital lines:

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Asynchronous TDMFigure 6-35

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Multiplexing and inverse multiplexingFigure 6-36

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Technical Focus:Technical Focus: Use of TDM in ATM NetworksUse of TDM in ATM Networks

Asynchronous TDM is used today in the ATM network, a wide area network that we discuss in Chapter 11. ATM is acell network; the packets traveling through the networkare small packets called cells.