With time division multiplexing, transmissions from multiple sources occur on same facility but not at the same time. Transmissions from various sources are interleaved in time domain.
Two Channel PCM-TDM System
Block Diagram shows the simplified block diagram for a PCM carrier system comprised of two DS-0 channels that have been time division multiplexed. Each channel input is sampled at an 8KHz rate and then converted to an eight bit PCM code. While PCM code for channel 1 is being transmitted. Channel 2 is sampled and converted to a PCM code. While the PCM code from channel 2 is being transmitted, the next sample is taken from channel 1 and converted to PCM code.
The process continues and samples are taken alternately from each channel, converted to PCM codes and transmitted.
Multiplexer is simply an electronically controlled digital switch with two inputs and one output. Channel 1 and channel 2 are alternately selected and connected to transmission line through the multiplexer.
One 8-bit PCM code from each channel (16-bits total) is called a TDM frame, and time it takes to transmit one TDM frame is called frame time. Frame time is reciprocal of sample rate (1/fs) or 1/8000 =125μs.
The PCM code for each channel occupies a fixed time slot within the total TDM frame.
With Two channel system , one sample is taken from each channel during each frame, and time allocated to transmit PCM bits from each channel is equal to one half the total frame time. Therefore eight bits from each channel must be transmitted during each frame (a total of 16 PCM bits per frame). Thus line speed at output of multiplexer is
Each channel is producing and transmitting only 64Kbps, the bits must be clocked out onto line at a 128KHz rate to allow eight bits from each channel to be transmitted in a 1211μs time slot.