McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Chapter 6 Physical Layer

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Distinguish between analog and digital data. Distinguish between analog and digital signals. Understand the concept of bandwidth and the relationship between bandwidth and data transmission speed. Understand digital-to-digital, digital-to-analog, and analog-to- digital encoding. After reading this chapter, the reader should be able to: O BJECTIVES Understand multiplexing and the difference between a link and a channel.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. DIGITALANDANALOGDIGITALANDANALOG 6.1

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-1 Digital and analog entities

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-2 Digital data

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-3 Analog data

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-4 Digital signal

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-5 Bit and bit interval

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Units of Bit Rate 1 bps 1 kbps = 1000 bps 1 Mbps = 1,000,000 bps 1 Gbps = 1,000,000,000 bps 1 Tbps = 1,000,000,000,000 bps

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-6 A sine wave

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-7 Amplitude

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-8 Period and frequency

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Units of Frequency 1 Hz 1 kHz = 1000 Hz 1 MHz = 1,000,000 Hz 1 GHz = 1,000,000,000 Hz 1 THz = 1,000,000,000,000 Hz

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Frequency 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.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-9 Phase

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Zero frequency and infinite frequency Figure 6-10

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Phase describes the position of a waveform relative to other waveforms. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Business Focus: Two 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.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Business Focus: The 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.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. TRANSFORMINGDATA TO SIGNALS TRANSFORMINGDATA 6.2

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Transforming data to signals Figure 6-11

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Digital-to-digital encoding Figure 6-12

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. A digital signal has a much higher bandwidth than an analog signal. There is a need for a better media to send a digital signal. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Most LANs use digital-to-digital encoding because the data stored in the computers are digital and the cable connecting them is capable of carrying digital signals. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Digital encoding methods Figure 6-13

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Average 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.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Synchronization 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 being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the bit interval. If the receiver’s clock is out of synchronization, these alerting points can reset the clock.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Digital-to-analog modulation Figure 6-14

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. ASK Figure 6-15

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. FSK Figure 6-16

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. PSK Figure 6-17

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Understanding 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 one point 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 are transported. 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.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Capacity 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: C  B log 2 (1  signal-to-noise ratio) C is the capacity in bits per second; B is the bandwidth.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Analog-to-digital conversion Figure 6-18

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. PCM Figure 6-19

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Sampling Rate and Nyquist Theorem As you can see from the preceding figures, the accuracy of any digital reproduction of an analog signal depends on the number of samples taken. So the question is, how many samples are sufficient? 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 reproduction of the original analog signal. So if we want to sample a telephone voice with a maximum frequency of 4000 Hz, we need a sampling rate of 8000 samples per second.

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. TRANSMISSIONMODESTRANSMISSIONMODES 6.3

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Data transmission Figure 6-20

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Parallel transmission Figure 6-21

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Serial transmission Figure 6-22

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Asynchronous transmission Figure 6-23

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. In synchronous transmission, we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-24 Synchronous transmission

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. LINECONFIGURATIONLINECONFIGURATION 6.4

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Line configuration defines the attachment of communication devices to a link. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-25 Point-to-point line configuration

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Figure 6-26 Multipoint line configuration

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. DUPLEXITYDUPLEXITY 6.5

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Half-duplex mode Figure 6-27

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Full-duplex mode Figure 6-28

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. MULTIPLEXING: SHARING THE MEDIA MULTIPLEXING: 6.6

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Multiplexing versus no multiplexing Figure 6-29

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Categories of multiplexing Figure 6-30

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. FDM Figure 6-31

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. FDM can only be used with analog signals. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Use of FDM in Telephone Systems AT&T uses a hierarchical system to multiplex analog lines:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Prisms in WDM multiplexing and demultiplexing Figure 6-32

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. TDM Figure 6-33

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. TDM can be used only with digital signals. Note:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Synchronous TDM Figure 6-34

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Use of TDM in Telephone Systems AT&T uses a hierarchical system to multiplex digital lines:

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Asynchronous TDM Figure 6-35

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Multiplexing and inverse multiplexing Figure 6-36

McGraw-Hill©2003 The McGraw-Hill Companies, Inc. Technical Focus: Use 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 a cell network; the packets traveling through the network are small packets called cells.