In time division multiplexing (TDM), the data transmission of the individual connections takes place in a defined multiplex frame in which there is a fixed time slot for each transmission channel. The individual time slots are processed one after the other. When a time unit has elapsed, transmission is briefly interrupted for the active channel; the transmission bandwidth is then available to the next user.
The frame length depends on the number of terminals to be connected or is specified in the international standards for PCM transmission systems for use in public networks. Unlike frequency division multiplexing, time division multiplexing divides the available bandwidth into defined time slots. Time division multiplex works exclusively digitally, i.e. several digital input signals are concentrated into a common digital output signal. The order in which the data from the individual terminals is transmitted is also defined. Depending on the type of time division multiplex, the data is interleaved either bit by bit, character by character or block by block. In each case, a group of bits, characters or blocks is combined to form a frame, with the beginning and end of this frame still being specially marked to facilitate synchronization with the remote terminal.
The different TDM methodssynchronous time division multiplex (STDM) and asynchronous time division multiplex (ATDM). In synchronous time division multiplex, transmission frames are defined, each consisting of a certain number of time slots of a fixed size. Each user is assigned a specific time slot within the transmission frame during which he can transmit or receive.
A transmission channel is uniquely identified by the position of the time slot within the transmission frame. The time slot of a channel is always synchronous with respect to the transmission frame. Hence the term synchronous time division multiplexasynchronous time division multiplexing, the data streams to be transmitted are converted into information units of fixed or variable length and transmitted asynchronously. The information units are assigned via channel identifiers, with which each data packet is provided. Asynchronous time division multiplexing is therefore also sometimes referred to as address multiplexing. If variable-length data packets are used for transmission, this is referred to as data packet switching, while fixed-length data packets are referred to as cell switching.
A variant of time division multiplex is time division duplex (TDD).