Multiplexing Multiplexing Multiplexing mixes the signals of different

Multiplexing

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 – T 1 lines can multiplex 24 such signals – Yet T 1 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 T 1 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 Slot not Used – Inexpensive to implement A A B

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 STDMAis now Bcost-effective A A 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 Frequency A B Channel

Combining TDM and FM • Use Simple or statistical TDM Within a Channel Frequency A B Channel