1. Multiplexing

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

3. 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

4. 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

5. 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

6. 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

7. 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 Slot not Used A B A

8. 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 A B A A Frame

9. 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 Channel Frequency B

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