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Chapter 6

Chapter 6. Physical Layer. O BJECTIVES. 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.

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Chapter 6

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  1. Chapter 6 Physical Layer

  2. OBJECTIVES 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. Understand multiplexing and the difference between a link and a channel. Distinguish between analog and digital data. After reading this chapter, the reader should be able to:

  3. 6.1 DIGITAL AND ANALOG

  4. Figure 6-1 The data we use in data communications can also be analog or digital. Digital and analog entities

  5. Figure 6-2 Digital data

  6. Figure 6-3 Analog data is information that is continuous. Analog data

  7. Figure 6-4 Digital signal

  8. Figure 6-5 The bit interval is the time required to send one single bit. The bit rate is the number of bit intervals per second. Bit and bit interval

  9. 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 Technical Focus:Units of Bit Rate

  10. Figure 6-6 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. A sine wave

  11. Figure 6-7 The amplitudeof a signal is the value of the signalat any point on the wave. Amplitude

  12. Figure 6-8 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. The frequencyof a signal is its number of cycles per second. Period and frequency

  13. 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 Technical Focus:Units of Frequency

  14. 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. Figure 6-9 Phase describes the position of a waveform relative to other waveforms. Phase

  16. Figure 6-10 Zero frequency and infinite frequency

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

  18. 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. 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. 6.2 TRANSFORMING DATA TO SIGNALS

  21. Figure 6-11 Transforming data to signals

  22. 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. Digital-to-digital encoding

  23. Note: 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.

  24. Note: 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.

  25. Figure 6-13 Digital encoding methods

  26. 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. 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. 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) • Phase shift keying(PSK) Digital-to-analog modulation

  29. 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. ASK

  30. Figure 6-16 In frequencyshift 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. FSK

  31. 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. PSK

  32. 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. 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. Analog-to-digital conversion

  34. Figure 6-19 PCM

  35. Digitized voice 8000 sample/sec 256 levels (8 bit per sample) Bandwidth required for digital voice= 8000*8=64Kbps Number of bit per second

  36. 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. 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 log2 (1 + signal-to-noise ratio) C is the capacity in bits per second; B is the bandwidth.

  38. 6.3 TRANSMISSION MODES

  39. 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 data, there are two subclasses of serial transmission • synchronous • asynchronous Data transmission

  40. Figure 6-21 Parallel transmission

  41. Figure 6-22 Serial transmission

  42. Note: 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.

  43. Note: Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.

  44. Figure 6-23 Asynchronous transmission

  45. Note: 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.

  46. Figure 6-24 Synchronous transmission

  47. 6.4 LINE CONFIGURATION

  48. Note: Line configuration defines the attachment of communication devices to a link.

  49. Figure 6-25 A point-to-point line configuration provides a dedicated link between two devices. Point-to-point line configuration

  50. Figure 6-26 A multipoint (also called multidrop) line configuration is one in which more than two specific devices share a single link. Multipoint line configuration

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