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Simple Time Division Multiplexing

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					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
  – 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
                                             Slot not Used
  –   Inexpensive to implement
                    A                       B

                    A
       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
  –              is
      But STDMA now B    cost-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               Channel


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




  Frequency         A        B       Channel

				
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posted:9/30/2012
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