Space Division Multiplexing SDM Frequency Division Multiplexing FDM Time Division Multiplexing TDM

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2.3.1. Space Division Multiplexing SDM

SDM is the simplest and crudest form of multiplexing. It involves grouping many separate wires into a common cable enclosure. A cable that has, for example, 50 twisted pairs inside it can support 50 channels. There is therefore a one-to-one correspondence between physical and logical channels see Figure 2.18. SDM has the unique advantage of not requiring any multiplexing equipment. It is usually combined with other multiplexing techniques to better utilize the individual physical channels. Figure 2.18 Space division multiplexing. cable enclosure

2.3.2. Frequency Division Multiplexing FDM

In FDM, the frequency bandwidth of the line is divided into a number of partitions, each of which is used as a separate logical channel. Radio and TV broadcasting represent the oldest examples of FDM. To avoid neighboring channels from interfering with one another, the extreme ends of the channel frequencies are left unused to provide a gap. For example, a line that has a bandwidth of 30 kHz can be divided into 3 times 10 kHz channels, each of which consists of 8 kHz of bandwidth for data and two gaps of 1 kHz on either side. FDM requires special multiplexingdemultiplexing hardware MUX at either end of the line see Figure 2.19. Figure 2.19 Frequency division multiplexing. MUX MUX 8kHz 8kHz 30kHz time 30kHz 20kHz 10kHz time 8kHz www.pragsoft.com Chapter 2: The Physical Layer 29

2.3.3. Time Division Multiplexing TDM

In TDM, each logical channel is allocated a time slot to transmit over a shared physical channel. For example, each logical channel may be given a 5 millisecond time slot to transmit, during which time it will have the entire bandwidth of the line to itself. Like FDM, TDM requires special multiplexingdemultiplexing hardware MUX at either end of the line see Figure 2.20. Because the channels are spread across time, some means of initial synchronization is also needed. Basically, the receiving end needs to know which time slot belongs to the first channel when the connection is established, and can work everything else out from this reference point. Figure 2.20 Time division multiplexing. MUX MUX 101 001 011 011 001 101 time slot

2.3.4. Concentration