Đang tải... (xem toàn văn)
Chapter 6 - Bandwidth utilization: Multiplexing and spreading. In this chapter we will show how we can use the available bandwidth efficiently. We discuss two separate, but related topics, multiplexing and spreading.
Chapter Bandwidth Utilization: Multiplexing and Spreading 6.1 Copyright © The McGrawHill Companies, Inc. Permission required for reproduction or display Note Bandwidth utilization is the wise use of available bandwidth to achieve specific goals Efficiency can be achieved by multiplexing; privacy and anti-jamming can be achieved by spreading 6.2 6-1 MULTIPLEXING Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic Topics discussed in this section: FrequencyDivision Multiplexing WavelengthDivision Multiplexing Synchronous TimeDivision Multiplexing Statistical TimeDivision Multiplexing 6.3 Figure 6.1 Dividing a link into channels 6.4 Figure 6.2 Categories of multiplexing 6.5 Figure 6.3 Frequencydivision multiplexing 6.6 Note FDM is an analog multiplexing technique that combines analog signals 6.7 Figure 6.4 FDM process 6.8 Figure 6.5 FDM demultiplexing example 6.9 Example 6.1 Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6 We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one Then we combine them as shown in Figure 6.6 6.10 Example 6.11 Two channels, one with a bit rate of 100 kbps and another with a bit rate of 200 kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link? Solution We can allocate one slot to the first channel and two slots to the second channel Each frame carries bits The frame rate is 100,000 frames per second because it carries bit from the first channel The bit rate is 100,000 frames/s × bits per frame, or 300 kbps 6.44 Figure 6.23 Digital hierarchy 6.45 Table 6.1 DS and T line rates 6.46 Figure 6.24 T1 line for multiplexing telephone lines 6.47 Figure 6.25 T1 frame structure 6.48 Table 6.2 E line rates 6.49 Figure 6.26 TDM slot comparison 6.50 6-1 SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy Topics discussed in this section: Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS) 6.51 Figure 6.27 Spread spectrum 6.52 Figure 6.28 Frequency hopping spread spectrum (FHSS) 6.53 Figure 6.29 Frequency selection in FHSS 6.54 Figure 6.30 FHSS cycles 6.55 Figure 6.31 Bandwidth sharing 6.56 Figure 6.32 DSSS 6.57 Figure 6.33 DSSS example 6.58 ... a different bandwidth, as shown in Figure 6. 6 We use the 2 0- to 24-kHz bandwidth for the first channel, the 2 4- to 28-kHz bandwidth for the second channel, and the 2 8- to 32-kHz bandwidth for... 2.5 μs 6. 35 Figure? ?6. 17 Example? ?6. 9 6. 36 Figure? ?6. 18 Empty slots 6. 37 Figure? ?6. 19 Multilevel multiplexing 6. 38 Figure? ?6. 20 Multipleslot multiplexing 6. 39 Figure? ?6. 21 Pulse stuffing 6. 40 Figure? ?6. 22 Framing bits... kbps 6. 44 Figure? ?6. 23 Digital hierarchy 6. 45 Table? ?6. 1 DS? ?and? ?T line rates 6. 46 Figure? ?6. 24 T1 line for multiplexing telephone lines 6. 47 Figure? ?6. 25 T1 frame structure 6. 48 Table? ?6. 2 E line rates