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Lecture Data communications and networking: Chapter 4 - Behrouz A. Forouzan

A computer network is designed to send information from one point to another. This information needs to be converted to either a digital signal or an analog signal for transmission. In this chapter, we discuss the first choice, conversion to digital signals. | Chapter 4 Digital Transmission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4. 4-1 DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed. Line Coding Line Coding Schemes Block Coding Scrambling Topics discussed in this section: 4. Figure 4.1 Line coding and decoding 4. Figure 4.2 Signal element versus data element 4. A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? Solution We assume that the average value of c is 1/2 . The baud rate is then Example 4.1 4. Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. Note 4. The maximum data rate of a channel (see Chapter 3) is Nmax = 2 × B × log2 L (defined by the Nyquist formula). Does this agree with the previous formula for Nmax? Solution A signal with L levels actually can carry log2L bits per level. If each level corresponds to one signal element and we assume the average case (c = 1/2), then we have Example 4.2 4. Figure 4.3 Effect of lack of synchronization 4. In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps? Solution At 1 kbps, the receiver receives 1001 bps instead of 1000 bps. Example 4.3 At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps. 4. Figure 4.4 Line coding schemes 4. Figure 4.5 Unipolar NRZ scheme 4. Figure 4.6 Polar NRZ-L and NRZ-I schemes 4. In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I . | Chapter 4 Digital Transmission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4. 4-1 DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed. Line Coding Line Coding Schemes Block Coding Scrambling Topics discussed in this section: 4. Figure 4.1 Line coding and decoding 4. Figure 4.2 Signal element versus data element 4. A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? Solution We assume that the average value of c is 1/2 . The baud rate is then Example 4.1 4. Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. .