Data Communications Networking Devices - 4th Ed.pdf

6.1 MULTIPLEXERS _______________________________________________________________ 629Figure 6.2 Frequency division multiplexing.Since the channel sets modulate the line atspeci®ed frequencies, no modems are required at remote locationsPhysically, an FDM contains a channel set for each data channel as well ascommon logic, as shown in Figure 6.2. Each channel set contains a transmitter andreceiver tuned to a speci®c frequency, with bits being indicated by the presence orabsence of signals at each of the channel's assigned frequencies. In FDM, the widthof each frequency band determines the transmission rate capacity of the channel,and the total bandwidth of the line is a limiting factor in determining the totalnumber or mix of channels that can be serviced. Although a multipoint operation isillustrated in Figure 6.2, FDM equipment can also be utilized for the multiplexingof data between two locations on a point-to-point circuit. Data rates up to 1200 bpscan be multiplexed by an FDM connected to a voice grade analog line. TypicalFDM channel spacings required at different data rates for operation on a voicegradeline are listed in Table 6.2.Table 6.2 FDM voice-grade leased linechannel spacingsSpeedbps)SpacingHz)75 120110 170150 240300 480450 720600 9601200 1800