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

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

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


Figure 6-2

Digital data


Figure 6-3

Analog data


Figure 6-4

Digital signal


Figure 6-5

Bit and bit interval


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

1 1 bpsbps

1 1 kbps = 1000 bpskbps = 1000 bps

1 1 Mbps = 1,000,000 bpsMbps = 1,000,000 bps

1 1 Gbps = 1,000,000,000 bpsGbps = 1,000,000,000 bps

1 1 Tbps = 1,000,000,000,000 bpsTbps = 1,000,000,000,000 bps


Figure 6-6

A sine wave

AmplitudePeriod / frequencyphase


Figure 6-7

Amplitude


Figure 6-8

Period and frequency

Hz: periods/second


Technical Focus:Technical Focus: Units of FrequencyUnits of Frequency

1 1 HzHz

1 1 kHz = 1000 HzkHz = 1000 Hz

1 1 MHz = 1,000,000 HzMHz = 1,000,000 Hz

1 1 GHz = 1,000,000,000 HzGHz = 1,000,000,000 Hz

1 1 THz = 1,000,000,000,000 HzTHz = 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


Zero frequency and infinite frequency

Figure 6-10

A signal does not change has zero frequencyA vertical spike represents a sudden change ( theoretically in no time)


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

Line coding : 將 binary information 轉換成數位訊號的方法數位訊號是一連串不連續的電壓脈衝 (voltage pulses)


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:


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:


Digital encodingmethods

Figure 6-13

(Nonreturn to zero)

(Return to zero)

Polar : positive value and negative value

Unipolar : positive value and 0

Bipolar : positive ,negative and zero

訊號改變時反向

電壓無改變 : 0, 電壓有改變 : 1

訊號中點電壓高到低 : 0 電壓低到高 : 1

每一個 bit time 都會回到 0 (-1)-(0) : 0 (1) – (0) :1


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

傳輸屆介質只能傳送類比訊號時


ASKFigure 6-15

用不同振幅代表 0 與 1, 在一般語音傳輸 , 速度可達 1200bps


FSKFigure 6-16

比較不受電流強度變化的影響


PSKFigure 6-17

比 FSK 更穩定 , 語音可達 9600bps


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.


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.


Analog-to-digital conversionFigure 6-18

PCM(pulse code modulation) : 若將連續產生的訊號 , 在固定時間間隔抽樣 , 而抽樣速度高於訊號最高有效頻率的 2 倍 , 則抽樣所得的不連續資料 , 可以用來重建原來的連續訊號e.g. 用數位訊號來傳遞語音資料 , 取樣速度至少要是類比資料頻率的兩倍 2 x 4kHz = 8000 次 / 秒 , 所取到的值以 8bits 表示 , 則一般承載語音的數位線路傳輸速率約為 8 x 8000= 64kbps


PCMFigure 6-19


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.


TRANSMISSIONTRANSMISSIONMODESMODES

TRANSMISSIONTRANSMISSIONMODESMODES

6.36.3


Data transmissionFigure 6-20


Parallel transmissionFigure 6-21

優 : 速度劣 : 成本適合短距離傳輸


Serial transmissionFigure 6-22


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:


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:


Asynchronous transmissionFigure 6-23


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:


Figure 6-24

Synchronous transmission


LINELINECONFIGURATIONCONFIGURATION

LINELINECONFIGURATIONCONFIGURATION

6.46.4


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

to a link.to a link.

Note:Note:


Figure 6-25

Point-to-point line configuration


Figure 6-26

Multipoint line configuration


DUPLEXITYDUPLEXITYDUPLEXITYDUPLEXITY

6.56.5


Half-duplex modeFigure 6-27


Full-duplex modeFigure 6-28


MULTIPLEXING:MULTIPLEXING:SHARING THE MEDIASHARING THE MEDIA

MULTIPLEXING:MULTIPLEXING:SHARING THE MEDIASHARING THE MEDIA

6.66.6


Multiplexing versus no multiplexing

Figure 6-29


Categories of multiplexingFigure 6-30


FDMFigure 6-31


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


TDMFigure 6-33


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

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

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