6.1 Chapter 6 Bandwidth Utilization: Multiplexing and Spreading Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6.2 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. Note

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. Frequency-Division Multiplexing Wavelength-Division Multiplexing Synchronous Time-Division Multiplexing Statistical Time-Division Multiplexing Topics discussed in this section:

6.4 Figure 6.1 Dividing a link into channels

6.5 Figure 6.2 Categories of multiplexing

6.6 Figure 6.3 Frequency-division multiplexing

6.7 FDM is an analog multiplexing technique that combines analog signals. Note

6.8 Figure 6.4 FDM process

6.9 Figure 6.5 FDM demultiplexing example

6.10 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. Example 6.1

6.11 Figure 6.6 Example 6.1

6.12 Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 × × 10 = 540 kHz, as shown in Figure 6.7. Example 6.2

6.13 Figure 6.7 Example 6.2

6.14 Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration, using FDM. Solution The satellite channel is analog. We divide it into four channels, each channel having a 250-kHz bandwidth. Each digital channel of 1 Mbps is modulated such that each 4 bits is modulated to 1 Hz. One solution is 16-QAM modulation. Figure 6.8 shows one possible configuration. Example 6.3

6.15 Figure 6.8 Example 6.3

6.16 Figure 6.9 Analog hierarchy

6.17 The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously? Solution Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users. Example 6.4

6.18 Figure 6.10 Wavelength-division multiplexing

6.19 WDM is an analog multiplexing technique to combine optical signals. Note

6.20 Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

6.21 Figure 6.12 TDM Each connection occupies a portion of time in the link

6.22 TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one. Note

6.23 Figure 6.13 Synchronous time-division multiplexing Each input connection has an allotment in the output regardless of the data

6.24 Figure 6.14 Example 6.6

6.25 Figure 6.15 Interleaving

6.26 Four channels are multiplexed using TDM. If each channel sends 100 bytes /s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link. Solution The multiplexer is shown in Figure Each frame carries 1 byte from each channel; the size of each frame, therefore, is 4 bytes, or 32 bits. Because each channel is sending 100 bytes/s and a frame carries 1 byte from each channel, the frame rate must be 100 frames per second. The bit rate is 100 × 32, or 3200 bps. Example 6.8

6.27 Figure 6.16 Example 6.8

6.28 A multiplexer combines four 100-kbps channels using a time slot of 2 bits. Show the output with four arbitrary inputs. What is the frame rate? What is the frame duration? What is the bit rate? What is the bit duration? Solution Figure 6.17 shows the output for four arbitrary inputs. The link carries 50,000 frames per second. The frame duration is therefore 1/50,000 s or 20 μs. The frame rate is 50,000 frames per second, and each frame carries 8 bits; the bit rate is 50,000 × 8 = 400,000 bits or 400 kbps. The bit duration is 1/400,000 s, or 2.5 μs. Example 6.9

6.29 Figure 6.17 Example 6.9

6.30 Figure 6.18 Empty slots

6.31 Figure 6.19 Multilevel multiplexing Data rate management What if the data rates of all input lines are not same Data rate of input lines is a multiple of others

6.32 Figure 6.20 Multiple-slot multiplexing Allotting more than one slot in a frame to a single input line

6.33 Figure 6.21 Pulse stuffing Make highest data rate dominate and dummy bits to the lower rates.

6.34 Figure 6.22 Framing bits

6.35 Figure 6.23 Digital hierarchy Digital Signal Service

6.36 Table 6.1 DS and T line rates

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. Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS) Topics discussed in this section:

6.38 Figure 6.26 TDM slot comparison Statistical TDM

6.39 Figure 6.27 Spread spectrum

6.40 Figure 6.28 Frequency hopping spread spectrum (FHSS)

6.41 Figure 6.29 Frequency selection in FHSS

6.42 Figure 6.30 FHSS cycles

6.43 Figure 6.31 Bandwidth sharing

6.44 Figure 6.32 DSSS

6.45 Figure 6.33 DSSS example