7.3.1 Synchronization And Framing

Accurate synchronization is fundamental to the operation of any TDM system, since the correct association of time slots with their corresponding channels depends entirely on precise alignment between transmitter and receiver timing. De-multiplexing of the pulse streams is accomplished by the output commutator, which must be synchronized accurately with the input commutator process. If a loss of synchronism occurs between the input and output commutators the pulse samples are gated to the wrong output resulting in confusion of the output message waveforms. Perhaps the most significant problem in TDM systems is the acquisition and tracking of synchronization information. Most TDM systems transmit synchronization signals by occasionally inserting frame synchronization (sync) pulses into the transmission, which enable the receiving circuitry to lock-on to the timing of the transmitter. Other systems use very stable clocks at both the transmitter and receiver which, after an initial sync pulse, can remain synchronized for long periods of time.

Demultiplexing at the receiver requires accurate synchronization between the input and output commutators. Loss of synchronization causes pulse samples to be routed to incorrect outputs, resulting in distortion of all recovered signals. To maintain synchronization, TDM systems transmit frame synchronization (sync) signals. These are inserted at regular intervals into the data stream—usually at the beginning of each frame—to allow the receiver to align its timing circuitry with the transmitter.