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

2. McGraw-Hill©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.

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

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

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

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

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

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

9. McGraw-Hill©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

10. McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 6-6 A sine wave Amplitude Period / frequency phase

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

12. McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 6-8 Period and frequency Hz: periods/second

13. McGraw-Hill©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

14. McGraw-Hill©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.

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

16. McGraw-Hill©The McGraw-Hill Companies, Inc. Zero frequency and infinite frequency Figure 6-10 A signal does not change has zero frequency A vertical spike represents a sudden change ( theoretically in no time)

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

18. McGraw-Hill©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.

19. McGraw-Hill©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.

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

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

22. McGraw-Hill©The McGraw-Hill Companies, Inc. Digital-to-digital encoding Figure 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)

23. McGraw-Hill©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:

24. McGraw-Hill©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:

25. McGraw-Hill©The McGraw-Hill Companies, Inc. Digital encoding methods 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

26. McGraw-Hill©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.

27. McGraw-Hill©The McGraw-Hill Companies, Inc. Digital-to-analog modulation Figure 6-14 傳輸屆介質只能傳送類比訊號時

28. McGraw-Hill©The McGraw-Hill Companies, Inc. ASK Figure 6-15 用不同振幅代表 0 與 1, 在一般語音傳輸, 速度可達 1200bps

29. McGraw-Hill©The McGraw-Hill Companies, Inc. FSK Figure 6-16 比較不受電流強度變化的影響

30. McGraw-Hill©The McGraw-Hill Companies, Inc. PSK Figure 6-17 比 FSK 更穩定, 語音可達 9600bps

31. McGraw-Hill©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.

32. McGraw-Hill©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.

33. McGraw-Hill©The McGraw-Hill Companies, Inc. Analog-to-digital conversion Figure 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

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

35. McGraw-Hill©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.

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

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

38. McGraw-Hill©The McGraw-Hill Companies, Inc. Parallel transmission Figure 6-21 優 : 速度 劣 : 成本 適合短距離傳輸

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

40. McGraw-Hill©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:

41. McGraw-Hill©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:

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

43. McGraw-Hill©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:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

65. McGraw-Hill©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.