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Multiplexing
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Getting Started
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  • The Concept:
    • Multiple messages/signals on a single (shared) channel
  • Roles:
    • Multiplexer (MUX) - combines the messages/signals
    • Demultiplexer (DEMUX) - splits the messages/signals
Multiplexing on Circuits
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  • Frequency-Division Multiplexing (FDM):
    • A frequency band is devoted to each connection (e.g., analog cable television)
  • Time-Division Multiplexing (TDM):
    • Each connection is allocated a time slot
Multiplexing on the Internet
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  • Network/Internetwork Layer (IP):
    • Responsible for routing and delivering frames/datagrams
  • Transport Layer:
    • Responsible for (among other things) delivering the segments to the appropriate process/application (which is identified by fields in the segment's header)
    • The receiver identifies the process/application and delivers the segment (i.e., demultiplexes)
    • The sender includes the destination process/application fields in the header and hands the segment off to the network layer (i.e., multiplexes)
An Analogy
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  • The Setting:
    • Several people live at the same address
  • Multiplexing:
    • The local post office bundles all mail to the same address
  • Delivery:
    • The letter carrier delivers the bundle to the address
  • Demultiplexing:
    • Someone at the address divides the bundle and puts it into different stacks
Ports
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  • Types:
    • Source - used to identify the process/application that sent the segment
    • Destination - used to identify the receiving process/application
  • Details (UDP and TCP):
    • Two octets (i.e., 0-65535)
    • "Well-known" port numbers are in the range 0-1023 (e.g., FTP uses 20 and 21, HTTP uses 80)
Sockets
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  • The Concept:
    • What you "plug into" to get segments where they need to go
  • UDP:
    • A socket is specified by a two-tuple consisting of the receiving IP address and port
  • TCP:
    • A socket is specified by a four-tuple consisting of the sending IP address and port and the receiving IP address and port
There's Always More to Learn
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