MULTIPLEXING Habib Youssef, Ph.D firstname.lastname@example.org Department of Computer Engineering King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia COMPUTER NETWORK
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.
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.
Multiplexing (Cont.) • Time Division Multiplexing The stations take turns, each one periodically getting the entire bandwidth for a short interval of time.
Multiplexing of Communications Links Modem Modem MUX MUX CPU Remote terminals
Time Division Multiplexing • Each user gets the channel’s full capacity for a period of time • Each user gets a time slot in each frame Start User1 User2 User3 User N 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”
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
Statistical Time Division Multiplexing (Cont.) • For example Control field Data field Port no. Character (5) (8) Frame of tagged characters
Statistical Time Division Multiplexing (Cont.) • Statistical multiplexing can be generalized to produce packet switching • More control information • Multiple characters of data
Typical Statistical Multiplexer (STAT MUX) Example Printer MUX MUX CPU Modem Modem Terminal Supervisory Terminal