Communication is a vital part of our lives. From Marconi’s first transatlantic transmission to the advent of satellite communication, intercontinental cables and optical fiber communication it has overridden our lives. Communication is either digital or analog and uses many techniques.
The most essential resource in communication is the channel via which communication takes place. Take a telephone network. The channel – let us assume wired connections – will be at a scarcity, that is, for a single channel there will be many users vying for it. In digital communication the analog data will be converted to digital bits and that should be transmitted. Let’s take an example for a 24-channel system, having a sampling rate of 8000 samples per second, 8 bits per sample, and a pulse width of approximately 0.625 µs. This means that the sampling interval is 1/8000 = 0.000125 s = 125 µs, and the period required for each pulse group is 8 x 0.625 = 5 µs. For each of the 8000 samples, 5 µs would be taken up by the pulse group (series of bits) and the remaining 120 µs would be made up of nothing. Thus to make use of the channel efficiently time division multiplexing is used.
What is time division multiplexing? It is a system where different groups of bits from different transmitters are sequenced one after the other in time and transmitted. Similarly the correct receiver is synchronized such that it receives the correct data. So in the above example then in one sample of 125 µs, 24 different bit samples can be transmitted each of 5 µs duration and the 25th bit sample for synchronization. To say in simpler terms, say 3 people want to use the same channel to communicate with 3 other people. They can share the channel on a time-shared basis. So the first two people will be talking till a certain time T1 and then till time T2 the next pair will use the channel for communication and so on.
Look at the circuit. It consists of a multiplexer (MUX) at the transmitting side and a demultiplexer (DEMUX) at the receiving side. A multiplexer is a switch which routes the input to one of its many outputs which is chosen depending on the binary number at its ‘select’ lines. The DEMUX works in the reverse manner to the MUX. Note that to show the synchronization the same pulse train has been given to the select lines – at the transmitter and receiver. In real life a sync bit will be sent along with the data bits which will instruct the multiplexer as to which receiver the particular data stream is to be transmitted. In synchronous multiplexing, the order of shifting of inputs in the common channel is also synchronized with that of the channels in the demultiplexer. Say as in the example, 5 µs for the bit stream means that the timing at the DEMUX such that the outputs will be shifted after every 5 µs.
For input you can give the bit stream of your choice and see whether the data from first input is received at the corresponding output and so on. The bit period in the circuit we are discussing is 1 ns. So for each nanosecond the communication shifts between channels and thus 8 lines are virtually active in the single channel. Since the time range we are speaking in is in microseconds and nanoseconds the shifting will be very fast and all the 8 lines that are in communication will not be aware of the change.
Note: The communication although digital will be modulated as PCM (Pulse Code Modulation) as there is no way a bit can be directly sent. The PCM waves will be sent via cable after modulation – done after multiplexing – and at the receiver it is demodulated back and demultiplexed in to the various channels, and at each channel the digital data is converted back to analog (like voice, for example) for the receiver.
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