Time Division Multiplexing (TDM)

Time Division Multiplexing (TDM)

 Time Division Multiplexing (TDM)

TDM is a digital multiplexing technique that allows several connections to share the high bandwidth of a link.Instead of sharing a portion of the bandwidth as in FDM,time is shared in TDM.  Each connection occupies a portion of time in the link. i.e, each channel has a time slice assigned to it.                                

Interleaving in TDM

TDM can be visualized as two fast rotating switches,one on the multiplexing side and the other on the demultiplexing side.The switches are synchronized and rotate at the same speed,but in opposite directions.On the multiplexing side,as the switch opens in front of a connection,that connection has the opportunity to send a unit onto the path.This process is called interleaving.On the demultiplexing side,as the switch opens in front of a connection,that connection has the opportunity to receive a unit from the path.

Synchronizing

The implementation of TDM is not as easy as that of FDM.Synchronization between the multiplexer and demultiplexer is a major issue.If the multiplexer and the demultiplexer are out of synchronization a bit belonging to one channel may be received by the wrong channel.For this reason,one or more synchronization bits are usually added to the beginning of each frame.These bits,called framing bits,follow a pattern,frame to frame,that allows the demultiplexer to synchronize with the incoming stream so that it can separate the time slots accurately.