The basic principle behind time division
multiplexing is that the user has access to a modem operating at a rate several times that required to support his own data throughput,
such that he can send his information in a time slot that is shorter than his own message
transaction. Other users can then be assigned similar time slots on the same channel.
Clearly if the data rate on the channel is w bits/second, and each individual user
requires only b bits/second, then the system can support w/b simultaneous
users.
In many TDM systems, users are assigned a time slot for the duration of their call whether
they require it or not. So, for example, if the user is generating voice traffic, or
typing at a keyboard, a time slot will be assigned regularly regardless of whether the
person is speaking or a key has been pressed, and it is very likely that the channel
capacity is being 'wasted' (this is equally true of FDMA systems).
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