H

Fig. 3.23. Principle of TDM.

If analog signals are to be multiplexed, the signals should be sampled. The analog signals are digitized by CODEC (coder/decoder) device. It produces 7 or 8 bit number. Sampling is 8000 per sec (125 mirco sec/sample) to capture all the information from the 4 kHz telephone channel bandwidth. Pulse cone modulation (PCM) technique is used in TDM. T1 carrier is the widespread method (in North America and Japan) T1 consists of 24 voice channels multiplexed together. Each channel has 8 bits, 7 for information and 1 for signalling. The output frame of 125 micro sec consists of 24 x 8 = 192 bits plus one extra bit for framing.

Fig. 3.24. The T1 carrier format.

Bit 8

(signalling) Bit 1 (framing code)

Fig. 3.24. The T1 carrier format.

A PAM or PPM can be employed for TDM. These methods are now obsolete as the transmitted pulses dispersed due to attenuation and delay distortion. They spread in time and interfere with the pulses of adjacent channels, thus causing interchannel cross talk. The PCM technique used in TDM eliminates this problem. Typical PCM system for TDM transmission is shown.

Receiving gates

• O '

Coder

Medium

Decoder

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