Thursday, September 11, 2008

Lecture 3

Lecture 3


  • MAC
    • Acronym: Medium Access Control
  • To prevent collisions the host computer would poll each terminal based on its MAC address and at that point they would communicate
  • Frame
    • Kept information on where it is going
    • And where its source is
  • Modification 2
    • Use multiplexing to
      • Transmit multiple messages simultaneously and
      • To detect communication errors
    • Multiplexers provide a second approach for sharing the communication line
    • CRC was used to detect errors
    • At this point every frame had a header, CRC and data
    • CRC
      • Acronym: Cyclic Redundancy Check

Second Generation of Networks


  • the second generation of networks were Computer to computer networks
    • as cost of computers dropped, dumb terminals were replaced by PC's
    • interconnecting computers were required to support
      • file transferring
      • remote telnet to allow remote application
      • parallel processing to execute a single program over multiple computers
  • ARPANET was the first WAN connecting universities
    • Operated using packet switching
    • Each message is converted into several smaller packets
    • At the destination computer the packets are combined into the original message from the host
    • Acronym:
      Advanced Research Projects Agency Net
    • Missing packets / corrupt packets became a concern
  • Internet is the interconnection of many networks
    • Resulted in compatibility issues with speeds of networks and bandwidth
    • Standards needed to be created to connect the networks seamlessly


Comparison of Switching Techniques


  • Circuit switching (designed for phone networks)
    • End to end path is established between transmitter and receiver
    • Complete blocks transmitted and once complete, circuit is terminated
    • Transmitter and Receiver were inaccessible for the duration of the connection
    • Definition Trunk: a major line connection in a telephone network
  • Message switching (designed for telegraphic networks)
    • No physical path is established between Transmitter and Receiver
    • Connection is established between the Transmitter and first switching office (router)
    • Entire block of data is transmitted to the switching office
    • Block is forwarded one hop at a time
    • No limit on block size, switching stations inaccessible for duration of transfer
  • Packet switching (used in internet)
    • A tight limit is placed on maximum block size
    • Data is broken in different sub-blocks and each sub-block is transmitted one hop at a time, on after the other
    • Message switching and packet switching are very alike
    • Packet switching is quicker because the original data is broken into packets and the length of time the message takes to send everything at once is broken into fragments

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