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All multiplex equipment in the FDM hierarchy uses SSB modulation. Each level of the hierarchy is implemented using a set of standard FDM modules. The multiplex equipment is independent of particular transmission media.

FDM principle. FDM for 12 telephone channels (group multiplex level) is considered for example. The signals pass through 12 low pass filters (LPF) to remove any high frequency components. The LPF outputs are mudulated on 12 separate carrier signals separated by 4 kHz. The modulations in all FDM hierarchy is single side band (SSB) modulation. The output of each of the 12 modulations must be filtered (band pass filter) to avoid the interference with each other. BPF are used to restrict each signal to the allocated 4 kHz band. The principle of operation is shown in Fig. 3.22.

When the signal is received, converse process takes place as shown. In fact, the modulators and demodulators are combined into 12 single units to permit two way transmissions.

Signal 1

Signal 2

Signal 12

Signal 1

Signal 2

Signal 12

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