# 8.3 Time Division Multiple Access (TDMA)

## Basic system operation

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). |

In order to maximize the use of a channel resource under these circumstances, packet based transmission is now common on wired links, where the user is not given a fixed repeated time slot, but rather allocated a time slot 'on demand'. This system works well provided time slot availability can be guaranteed for real-time applications – video, voice, and so on. It also involves quite a high penalty in signalling overheads.