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3.6.1. Space Division Multiplexing (SDM)

In space division multiplexing more than one physical transmission path are grouped together. A telephone cable consisting of hundreds (or thousands) of twisted pair constitutes a space division multiplexed system, wire pair cables are constructed containing many hundreds of wire pairs. Several coaxial tubes bound together in one cable is also an example of space division switching.

A very large number of separate telephone calls can be transmitted together down a coaxial system. Single wire pair commonly carries 12 or 24 voice channels. But one single coaxial tube commonly carries 3600 and the higher capacity one can carry 10800. In India, the telephone cables (5 or 10 lbs per mile) containing copper wires pairs of 20, 40, 60, 80, 100, 1000 and 5 per cent extra used in various towns and cities. For trunk cable lines, 70 - 300 lb copper wire are used. With coaxial cable 10800 two way telephone conversation are possible.

(a) Unshielded twistedpair (UTP) cable

(b) Shielded twisted-pair (STP) cable

Plastic insulator Conductor

(b) Shielded twisted-pair (STP) cable

PVC coating

Wiremesh shield (c) Coaxial cable

Fig. 3.20. Twisted pair cable and coaxial cable.

PVC coating

Wiremesh shield (c) Coaxial cable

Fig. 3.20. Twisted pair cable and coaxial cable.

The space division switching is not limited to voice frequency circuits alone. Many high capacity transmission systems using either frequency or time division multiplexing can also be space division multiplexed.

3.6.2. Frequency Division Multiplexing (FDM)

FDM is a broad band analog transmission technique in which multiple signals are transmitted over a single cable simultaneously as shown in Fig. 3.21 (a). FDM systems divides the available BW of the transmission medium into a number of narrow band or sub channels. The channels are sent over a common path by modulation each channel to different carrier frequency (higher frequency). The signal thus occupies a relatively narrow bandwidth which is a part of a much wider bandwidth transmitted. Each speech channel occupies 4 kHz of the available bandwidth. Fig. 3.21 (b) shows FDM in telephone transmission.

30 kHz

40 kHz

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