Explain FDM and show how CCITT standards help in building the base band?

Explain FDM and show how CCITT standards help in building the base band?
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Answer :

Frequency Division Multiplexing: It is the process of combining several information channels by shifting their signals to different frequency groups within the spectrum so that they can be transmitted simultaneously over common transmission facility.

The bandwidth of a telephone speech signal is rather less than 4 kHz. Where as the available bandwidth on unloaded cables pair, is well above 100 kHz. It is therefore possible, using modulation techniques, to divide up the cable bandwidth so that a  number of telephone speech paths can be carried simultaneously along a single cable pair. The normal arrangement consists of 24 telephone channels per cable pair, the modulation into 24 channels being carried out in the two stages. In the first stage, 12 channels are multiplexed together to form what is commonly known as a basic group. The basic group arrangement is shown in Fig. Each of the 12 telephone signals are single side-band amplitude modulated on to carriers spaced at 4 kHz intervals from 64 kHz to 108 kHz. The lower side-band (LSB) is used in each case. The 12 base-band signals are therefore translated into the frequency band from 60 kHz 108 kHz as shown. The block diagram for the channel translating equipment is given in Fig. To form a 24-channel system, to basic groups are taken together. One basic group (B) is transmitted directly as it stands. The other basic group (A) is amplitude modulated on to a carrier are 120 kHz and the lower side-band is taken so as to occupy the frequency range from 12 kHz to 60 kHz as shown in Fig. by using two stages of modulation for basic group A, it is possible to reduce the physical size of the component required for the LSB filters of the Fig., since the lowest cut- off frequency required is at 64 kHz rather than 12 kHz if the whole block of 24 channels were assembled together in one stage of modulation. 

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