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

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

3. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 DIGITALANDANALOGDIGITALANDANALOG 6.1

4. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-1 Digital and analog entities The data we use in data communications can also be analog or digital.

5. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-2 Digital data

6. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-3 Analog data Analog data is information that is continuous.

7. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-4 Digital signal

8. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-5 Bit and bit interval The bit interval is the time required to send one single bit. The bit rate is the number of bit intervals per second.

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

10. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-6 A sine wave The sine wave is the most fundamental form of an analog signal. Each cycle consists of a single arc above the time axis followed by a single arc below it. Sine waves can be fully described by three characteristics: amplitude, period or frequency, and phase.

11. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-7 Amplitude The amplitudeof a signal is the value of the signalat any point on the wave. The amplitude of a signal is the value of the signal at any point on the wave.

12. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-8 Period and frequency Period refers to the amount of time, in seconds, a signal needs to complete one cycle. Frequencyrefers to the number of periods in one second. Frequency refers to the number of periods in one second. The frequencyof a signal is its number of cycles per second. The frequency of a signal is its number of cycles per second.

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

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

15. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-9 Phase Phase describes the position of a waveform relative to other waveforms.

16. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Zero frequency and infinite frequency Figure 6-10

17. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Phase describes the position of a waveform relative to other waveforms. Note:

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

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

20. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 TRANSFORMINGDATA TO SIGNALS TRANSFORMINGDATA 6.2

21. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Transforming data to signals Figure 6-11

22. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Digital-to-digital encoding Figure 6-12 Figure shows this concept. The data, in the form of 0s and 1s, are represented by digital signals and sent through the media.

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

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

25. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Digital encoding methods Figure 6-13

26. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Average 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.

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

28. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Digital-to-analog modulation Figure 6-14 The physical layer needs to convert digital data to analog signals. A sine wave is defined by three characteristics: amplitude, frequency, and phase. Any of these three characteristics can be altered, giving us at least three mechanisms:  Amplitude shift keying(ASK) Frequencyshift keying(FSK)  Frequency shift keying(FSK) Phase shift keying(PSK)  Phase shift keying(PSK)

29. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 ASK Figure 6-15 In amplitude shift keying (ASK), the amplitude of the carrier signal is varied to represent binary 1 or 0. The speed of transmission using ASK is limited by the physical characteristics of the transmission medium. 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.

30. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 FSK Figure 6-16 In frequencyshift keying (FSK), the frequency of the carrier signal is varied to represent binary 1 or 0. In frequency shift keying (FSK), the frequency of the carrier signal is varied to represent binary 1 or 0. FSK avoids most of the noise problems of ASK. Because the receiving device is looking for specific frequency changes over a given number of periods, it can ignore amplitude spikes.

31. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 PSK Figure 6-17 In phase shift keying (PSK), the phase of the carrier is varied to represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase changes.

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

33. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Analog-to-digital conversion Figure 6-18 This is the case when long distance telephone companies send voice over a digital network. There are two major reasons for using digital signals in long distances telephony.  digital signals are more noise resistant.  digital networks (such as the Internet) can be used for voice as well as data.

34. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 PCM Figure 6-19

35. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Number of bit per second Digitized voice 8000 sample/sec 256 levels (8 bit per sample) Bandwidth required for digital voice= 8000*8=64Kbps

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

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

38. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 TRANSMISSIONMODESTRANSMISSIONMODES 6.3

39. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Data transmission Figure 6-20 The transmission of binary data across a link can be accomplished either in parallel mode or serial. In parallel mode, multiple bits are sent with each clock pulse. In serial mode, 1 bits is sent with each clock pulse. While there is only one way to send parallel While there is only one way to send parallel data, there are two subclasses of serial transmission  synchronous  asynchronous

40. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Parallel transmission Figure 6-21

41. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Serial transmission Figure 6-22

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

43. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same. Note:

44. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Asynchronous transmission Figure 6-23

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

46. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-24 Synchronous transmission

47. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 LINECONFIGURATIONLINECONFIGURATION 6.4

48. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Line configuration defines the attachment of communication devices to a link. Note:

49. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-25 Point-to-point line configuration A point-to-point line configuration provides a dedicated link between two devices.

50. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 6-26 Multipoint line configuration A multipoint (also called multidrop) line configuration is one in which more than two specific devices share a single link.

51. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 DUPLEXITYDUPLEXITY 6.5

52. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Half-duplex mode Figure 6-27 A half-duplex mode is like a one-lane road with two- directional traffic.

53. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Full-duplex mode Figure 6-28 A full-duplex mode is like a two-way street traffic flowing in both directions at the same time.

54. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 MULTIPLEXING: SHARING THE MEDIA MULTIPLEXING: 6.6

55. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Multiplexing versus no multiplexing Figure 6-29 Figure shows two possible ways of linking four pairs of devices:  Figure a, each pair has its own link.  Figure b, transmissions between the pairs are multiplexed ;the same four pairs share the capacity of a single link.

56. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Categories of multiplexing Figure 6-30

57. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 FDM Figure 6-31 Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted.

58. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 FDM can only be used with analog signals. Note:

59. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Use of FDM in Telephone Systems AT&T uses a hierarchical system to multiplex analog lines:

60. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Prisms in WDM multiplexing and demultiplexing Figure 6-32 Wave-division multiplexing (WDM) is conceptually the same as FDM, except that the multiplexing and demultiplexing involve light signals transmitted through fiber-optic channels.

61. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 TDM Figure 6-33 Time-division multiplexing (TDM) is a digital process that can be applied when the data capacity of the transmission medium is greater than the data rate required by the sending and receiving devices.

62. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 TDM can be used only with digital signals. Note:

63. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Synchronous TDM Figure 6-34

64. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Use of TDM in Telephone Systems AT&T uses a hierarchical system to multiplex digital lines:

65. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Asynchronous TDM Figure 6-35

66. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Multiplexing and inverse multiplexing Figure 6-36

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