FDM GROUPS

All voice—grade channels have a bandwidth of 0 to 4 kHz. In order to be transmitted as part of a broadband signal, the voice channel is mixed with a carrier frequency. A mixer is a nonlinear circuit that produces the original signal and the sum and difference frequencies of two input wave forms. When a balanced modulator is used as the mixer’s output, all frequencies above the difference frequency are filtered out, so that the mixer/filter circuit only gives out the difference frequency. [ see Figure 1]

Figure 2 shows the basic building block of groups of channels used on the analog telephone lines. These Channel Groups use FDM to convey 12 voice channels through one trunk line with a 48,000 Hz bandwidth, each voice channel can convey voice signals or data communications signals.

           
              The telephone communication system has a set of standards that specify the frequency allocations for a broadband FDM system. The standard begins with twelve voice channels each mixed with carrier signals that are 4 kHz apart. These carrier signals form a single group channel by linearly summing the outputs of several mixers as shown in Figure  2.

            Each mixer mixes a voice channel (0 to 4 kHz) with a carrier signal. The carrier signals are 4 kHz apart, placing each channel adjacent to the next group, ranging from 60 kHz to 108 kHz. This 48 kHz bandwidth is verified by multiplying 12 channels times 4 kHz per channel, resulting a 48 kHz. Notice that the 60-yo-108 kHz bandwidth is well above the telephone systems 300Hz to 3 kHz bandwidth. This group channel is meant to be sent by mediums other than voice grade telephone lines. It could be sent on a fiber optic cable that has a larger bandwidth or using a radio microwave or a satellite transmission link. Since, both these methods have very large bandwidths compared with 48 kHz. As such, larger groupings are created in similar manner to take full advantage if these large bandwidth of the communication links.

Frequency Division Multiplexing Subgroups

By repeating the process of creating groups, larger grouping are developed as they transmission frequencies increase. Five Groups (sixty channels) are assembled to form a subgroup, which files a bandwidth between 312 and 552 kHz. Actual bandwidth (552 – 312 = 240 kHz) equals the five groups (5 X 48), which is turn is equivalent to the sixty channels (60 X 4 kHz) that make up the five groups. The next level formed from ten super-groups is the Master-group. An additional carrier pilot at 2,840 kHz is inserted at the master-group level for amplitude regulation at that level. [See Figure 3]

            A Jumbo group is composed of six master-groups, each master-group encompasses ten super-groups, one super-group contains five groups, each with twelve channels. A Jumbo Mux (multiplexer) combines three Jumbo groups into a single channel. By performing a little multiplication, it is can be seen that 3 x 6 x 10 x 5 x 12 = 10,800 channels are carried by a Jumbo Mux system.

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