McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 6 Physical Layer

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

Chapter 6

PhysicalLayer


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.


DIGITALDIGITALANDAND

ANALOGANALOG

DIGITALDIGITALANDAND

ANALOGANALOG

6.16.1


Figure 6-1

Digital and analog entities

The data we use in data communications can The data we use in data communications can also be analog or digital.also be analog or digital.


Figure 6-2

Digital data


Figure 6-3

Analog data

Analog data is information that is continuous.Analog data is information that is continuous.


Figure 6-4

Digital signal


Figure 6-5

Bit and bit interval

The bit interval is the time required to send one single bit.The bit interval is the time required to send one single bit.

The bit rate is the number of bit intervals per second.The bit rate is the number of bit intervals per second.


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


Figure 6-6

A sine wave

The sine wave is the most fundamental form of The sine wave is the most fundamental form of an analog signal.an analog signal.

Each cycle consists of a single arc above the time Each cycle consists of a single arc above the time axis followed by a single arc below it.axis followed by a single arc below it.

Sine waves can be fully described by three characteristics:Sine waves can be fully described by three characteristics:amplitudeamplitude, , period period oror frequency frequency, and , and phasephase..


Figure 6-7

Amplitude

The amplitudeThe amplitude of a signal is the value of the of a signal is the value of the signalsignal at any point on the wave.at any point on the wave.


Figure 6-8

Period and frequencyPeriod refers to the amount of time, in seconds, a signal Period refers to the amount of time, in seconds, a signal needs to complete one cycle.needs to complete one cycle.

FrequencyFrequency refers to the number of periods in one refers to the number of periods in one second.second.The frequencyThe frequency of of a signala signal is its number of cycles per is its number of cycles per second.second.


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


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.


Figure 6-9

Phase

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


Zero frequency and infinite frequency

Figure 6-10


Phase describes the Phase describes the position of a waveform position of a waveform

relative to other relative to other waveforms.waveforms.

Note:Note:


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.


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.


TRANSFORMINGTRANSFORMINGDATADATA

TO SIGNALSTO SIGNALS

TRANSFORMINGTRANSFORMINGDATADATA

TO SIGNALSTO SIGNALS

6.26.2


Transforming data to signalsFigure 6-11


Digital-to-digital encodingFigure 6-12

Figure shows this concept. The data, in the form Figure shows this concept. The data, in the form of 0s and 1s, are represented by digital signals of 0s and 1s, are represented by digital signals and sent through the media.and sent through the media.


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

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

Note:Note:


Most LANs use digital-to-Most LANs use digital-to-digital encoding because digital encoding because

the data stored in the the data stored in the computers are digital and computers are digital and the cable connecting them the cable connecting them

is capable of carrying is capable of carrying digital signals.digital signals.

Note:Note:


Digital encodingmethods

Figure 6-13


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

With one exception, all of the signals in Figure 16.3 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.


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.


Digital-to-analog modulation

Figure 6-14

The physical layer needs to convert digital data to analog The physical layer needs to convert digital data to analog signals.signals.

A sine wave is defined by three characteristics: A sine wave is defined by three characteristics: amplitudeamplitude, , frequencyfrequency, and , and phasephase..

Any of these three characteristics can be altered, giving us at Any of these three characteristics can be altered, giving us at least three mechanisms:least three mechanisms: Amplitude shift keying(ASK)Amplitude shift keying(ASK) FrequencyFrequency shift keying(FSK)shift keying(FSK) Phase shift keying(PSK)Phase shift keying(PSK)


ASKFigure 6-15

In amplitude shift keying (ASK), the amplitude of the carrier signal is In amplitude shift keying (ASK), the amplitude of the carrier signal is varied to represent binary 1 or 0.varied to represent binary 1 or 0.

The speed of transmission using ASK is limited by the physical The speed of transmission using ASK is limited by the physical characteristics of the transmission medium.characteristics of the transmission medium.

ASK transmission is highly susceptible to noise interference.ASK transmission is highly susceptible to noise interference.

A 0 may be change to a 1, and a 1 to 0. Noise usually affects amplitude. A 0 may be change to a 1, and a 1 to 0. Noise usually affects amplitude.


FSKFigure 6-16

In frequencyIn frequency shift keying (FSK), the frequency of the carrier signal is shift keying (FSK), the frequency of the carrier signal is varied to represent binary 1 or 0.varied to represent binary 1 or 0.

FSK avoids most of the noise problems of ASK. Because the receiving FSK avoids most of the noise problems of ASK. Because the receiving device is looking for specific frequency changes over a given number device is looking for specific frequency changes over a given number of periods, it can ignore amplitude spikes.of periods, it can ignore amplitude spikes.


PSKFigure 6-17

In phase shift keying (PSK), the In phase shift keying (PSK), the phasephase of the carrier is varied to of the carrier is varied to represent binary 1 or 0.represent binary 1 or 0.

Both peak amplitude and frequency remain constant as the phase Both peak amplitude and frequency remain constant as the phase changes.changes.


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.


Analog-to-digital conversionFigure 6-18

This is the case when long distance telephone companies send This is the case when long distance telephone companies send voice over a digital network.voice over a digital network.

There are two major reasons for using digital signals in long There are two major reasons for using digital signals in long distances telephony.distances telephony. digital signals are more noise resistant.digital signals are more noise resistant. digital networks (such as the Internet) can be used for voice as digital networks (such as the Internet) can be used for voice as well as data.well as data.


PCMFigure 6-19


Number of bit per second

• Digitized voice

• 8000 sample/sec

• 256 levels (8 bit per sample)

• Bandwidth required for digital voice=

• 8000*8=64Kbps


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.


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.


TRANSMISSIONTRANSMISSIONMODESMODES

TRANSMISSIONTRANSMISSIONMODESMODES

6.36.3


Data transmissionFigure 6-20

The transmission of binary data across a link can be The transmission of binary data across a link can be accomplished either in parallel mode or serial.accomplished either in parallel mode or serial.

In parallel mode, multiple bits are sent with each clock pulse.In parallel mode, multiple bits are sent with each clock pulse.

In In serialserial mode, 1 bits is sent with each clock pulse. mode, 1 bits is sent with each clock pulse.

While there is only one way to send parallelWhile there is only one way to send parallel data, there are two subclasses of serial transmission synchronoussynchronous asynchronousasynchronous


Parallel transmissionFigure 6-21


Serial transmissionFigure 6-22


In asynchronous In asynchronous transmission, we send 1 transmission, we send 1

start bit (0) at the start bit (0) at the beginning and 1 or more beginning and 1 or more

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

gap between each byte.gap between each byte.

Note:Note:


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

synchronized; their synchronized; their durations are the same.durations are the same.

Note:Note:


Asynchronous transmissionFigure 6-23


In synchronous In synchronous transmission, we send bits transmission, we send bits one after another without one after another without

start/stop bits or gaps. It is start/stop bits or gaps. It is the responsibility of the the responsibility of the

receiver to group the bits.receiver to group the bits.

Note:Note:


Figure 6-24

Synchronous transmission


LINELINECONFIGURATIONCONFIGURATION

LINELINECONFIGURATIONCONFIGURATION

6.46.4


Line configuration defines Line configuration defines the attachment of the attachment of

communication devices to a communication devices to a link.link.

Note:Note:


Figure 6-25

Point-to-point line configuration

A point-to-point line configuration provides a dedicated A point-to-point line configuration provides a dedicated link between two devices.link between two devices.


Figure 6-26

Multipoint line configuration

A multipoint (also called multidrop) line configuration is A multipoint (also called multidrop) line configuration is one in which more than two specific devices share a single one in which more than two specific devices share a single link.link.


DUPLEXITYDUPLEXITYDUPLEXITYDUPLEXITY

6.56.5


Half-duplex modeFigure 6-27

A half-duplex mode is like a one-lane road with two-A half-duplex mode is like a one-lane road with two-directional traffic.directional traffic.


Full-duplex modeFigure 6-28

A full-duplex mode is like a two-way street traffic flowing A full-duplex mode is like a two-way street traffic flowing in both directions at the same time.in both directions at the same time.


MULTIPLEXING:MULTIPLEXING:SHARING THE MEDIASHARING THE MEDIA

MULTIPLEXING:MULTIPLEXING:SHARING THE MEDIASHARING THE MEDIA

6.66.6


Multiplexing versus no multiplexing

Figure 6-29

Figure shows two possible ways of linking four pairs of devices:Figure shows two possible ways of linking four pairs of devices: Figure a Figure a , each pair has its own link., each pair has its own link. Figure b Figure b , transmissions between the pairs are multiplexed ;the same four , transmissions between the pairs are multiplexed ;the same four pairs share the capacity of a single link.pairs share the capacity of a single link.


Categories of multiplexingFigure 6-30


FDMFigure 6-31

Frequency-division multiplexing (FDM) is an analog Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to is greater than the combined bandwidths of the signals to be transmitted.be transmitted.


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

Note:Note:


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:


Prisms in WDM multiplexing and demultiplexing

Figure 6-32

Wave-division multiplexing (WDM) is conceptually the Wave-division multiplexing (WDM) is conceptually the same as FDM, except that the multiplexing and same as FDM, except that the multiplexing and demultiplexing involve light signals transmitted through demultiplexing involve light signals transmitted through fiber-optic channels.fiber-optic channels.


TDMFigure 6-33

Time-division multiplexing (TDM) is a digital process that Time-division multiplexing (TDM) is a digital process that can be applied when the data capacity of the transmission can be applied when the data capacity of the transmission medium is greater than the data rate required by the medium is greater than the data rate required by the sending and receiving devices.sending and receiving devices.


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

Note:Note:


Synchronous TDMFigure 6-34


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:


Asynchronous TDMFigure 6-35


Multiplexing and inverse multiplexingFigure 6-36


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.

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McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 6 Physical Layer