MULTIPLEXING

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MULTIPLEXING

• Habib Youssef, Ph.D
• youssef_at_ccse.kfupm.edu.sa
• Department of Computer Engineering
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

COMPUTER NETWORK
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Multiplexing

• It costs about the same amount of money to
  install and maintain a high bandwidth cable as a
  low bandwidth wire between two stations
• Need for multiplexing techniques to share a
  single communication channel between multiple
  stations.

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Multiplexing (Cont.)

• Two classes of multiplexing schemes
• Frequency Division Multiplexing (FDM)
• The frequency spectrum is divided among the
  logical channel, with each station having
  exclusive possession of its frequency band.
  Filters limit the usable bandwidth per channel.

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Multiplexing (Cont.)

• Time Division Multiplexing
• The stations take turns, each one periodically
  getting the entire bandwidth for a short interval
  of time.

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Multiplexing of Communications Links
Modem
Modem
MUX
MUX
CPU
Remote terminals
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Time Division Multiplexing

• Each user gets the channels full capacity for a
  period of time
• Each user gets a time slot in each frame

Start
User N
User1
User2
User3
Start
User1
One Frame

• One character of user data is sent in each slot
• If a user has nothing to send, the slot contains
  null

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Statistical Time Division Multiplexing (STDM)

• Few users fill every slot assigned to them
• This results in wasted slots
• A better approach is statistical TDM
• It operates as follows
• A user character is tagged with the port number

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Statistical Time Division Multiplexing (Cont.)

• For example

Data field
Control field
Port no.
Character
(5)
(8)
Frame of tagged characters
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Statistical Time Division Multiplexing (Cont.)

• Statistical multiplexing can be generalized to
  produce packet switching
• More control information
• Multiple characters of data

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Typical Statistical Multiplexer (STAT MUX) Example
Printer
MUX
MUX
CPU
Modem
Modem
Terminal
Supervisory Terminal