Multiplexing

Multiplexing mixes the signals of different conversations over a single transmission line to reduce costs

Why Multiplex? Economies of Scale –64 kbps lines carry a single 64 kbps signal –T1 lines can multiplex 24 such signals –Yet T1 lines cost only about 3-5 times as much as 64 kbps lines Example: Suppose you have ten 64 kbps signals –This will require ten 64 kbps lines –But one T1 line will carry them for only 3-5 times the cost of a single 64 kbps line

Why Multiplex? Data transmission tends to be bursty –Uses capacity of a line only a small fraction of the time Signal A Signal B Multiplexing allows several conversations to share a single trunk line, lowering the cost for each

Economics of Multiplexing Cost Savings –Economies of scale in transmission lines –Multiplexing to lower costs for bursty traffic Cost Increases –Multiplexing costs money for multiplexers/demultiplexers at the two ends Net Savings –Usually is very high

Time Division Multiplexing Time is divided into short periods –In each period, one frame is sent Frame times are further divided –Each subdivision is a slot Slot Frame

Simple Time Division Multiplexing (TDM) In simple time division multiplexing, each conversation is given one slot per frame –Guaranteed the slot –Slot is wasted if the connection does not use it –Wasteful but still brings economies of scale –Inexpensive to implement AB A Slot not Used

Statistical Time Division Multiplexing (STDM) Still Frames and Slots But statistical TDM assigns slots as needed –Connections that need more slots get them –More efficient use of line than simple TDM –More expensive to implement –But STDM is now cost-effective ABAA Frame

Frequency Division Multiplexing (FDM) Signals are sent in different channels –Signals sent in different channels do not interfere –Brings economies of scale –Used in radio transmission A B Frequency Channel

Combining TDM and FM Use Simple or statistical TDM Within a Channel Frequency Channel AB