How speech is transmitted in a digital switching environment using PCM/TDM?

A digital carrier system is a communications system which uses digital pulses quite than analogue signals to encode information. Figure demonstrates the block diagram for a digital carrier system. Such digital carrier system T1 uses PCM-encoded samples by 24 voice band channels for transmission over optical fibre transmission line or a single metallic wire pair. All voice-band channels have a band width of around from 300 Hz to 3000 Hz. Again, the multiplexer is only a digital switch with 24 independent inputs and one time division multiplexed output. The output signals of PCM by the 24 voice-band channels are sequentially selected and linked through the multiplexer to the transmission line.

Whenever a T1 carrier system, D-type (digital) channel banks perform the encoding, sampling and multiplexing of 24 voice-band channels, all channels has an eight-bit PCM code and is sampled 8000 times a second. All channels are sampled at similar rate. Figure illustrates the channel sampling sequence for a 24-channel T1 digital carrier system. As the figure shows all channel is sampled once each frame but not at similar time. Each channel's sample is offset from the previous channel's sample through 1/24 of the total frame time. Hence, one 64-kbps PCM-encoded sample is transmitted for each voice-band channel throughout all frames (a frame time of 1/8000 = 125 µs). Afterwards, an additional bit (termed as the framing bit) is added to all frames.

The framing bit happens once per frame (8000-bps rate) and is recovered into the receiver, where this is used to keep frame and sample synchronization among the TDM transmitter and receiver.

FIG - Two-channel PCM-TDM system