Multiplexing (MC)

Multiplexing

Multiplexing (or muxing) is a way of sending multiple signals or streams of information over a communications link at the same time in the form of a single, complex signal; the receiver recovers the separate signals, a process called demultiplexing (or demuxing).

Time Division Multiplexing

Time division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency. Time division multiplexing (TDM) is also known as a digital circuit switched. Synchronous TDM works by the muliplexor giving exactly the same amount of time to each device connected to it. This time slice is allocated even if a device has nothing to transmit. This is wasteful in that there will be many times when allocated time slots are not being used. Therefore, the use of Synchronous TDM does not guarantee maximum line usage and efficiency.

Statistical TDM For example, a busy laser printer shared by many users might need to receive or transmit data 80-90% of the time at a much higher transmission rate than a seldom-used, data-entry computer attached to the same T-1 line. With TDM, even though the printer's transmission needs are greater, both devices would still be allocated the same duration of time to transmit or receive data. In comparison to TDM, the STDM method analyses statistics related to the typical workload of each input device (printer, fax, computer) and determines on-the-fly how much time each device should be allocated for data transmission on the cable or line. In the above example, STDM would allocate more time to the group printer, based on its past and current transmission needs and less time to the data-entry computer. Many believe the STDM method is a more efficient use of total bandwidth available than the TDM method.

Code Division Multiplexing

Code Division Multiplexing (CDM) uses identifying codes to distinguish one signal from another on a shared medium. Each signal is assigned a sequence of bits called the spreading code that is combined with the original signal to produce a new stream of encoded data; a receiver that knows the code can retrieve the original signal by subtracting out the spreading code (a process called dispreading). CDM is widely used in digital television and radio broadcasting and in 3G mobile cellular networks. Where CDM allows multiple signals from multiple sources, it is called Code-Division Multiple Access (CDMA).