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Intro to Local Area networks

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• "Local Area" networks (LANs)
  • Fact:

    Local Area networks spans a small geographic area

    Example:

    An (office) building (≤ 500 m ~= 1/3 mile) (E.g.: MSC - this building...)

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  • Consequence of a short distance:

    Transmissions takes very short time to reach all nodes in the network

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• Important property of LANs
  • Important property:

    When a node starts transmitting a message (data frame): the transmission will be "heard" (sensed) by all nodes: before the node finish transmitting:

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  • Therefore:

    When a node "hears" (senses) a transmission: Then its own transmission will very likely cause a collision:

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• Carrier sensing in LAN

  • Fact:

    Carrier sensing can reduce the collision probability in Local Area Networks

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• A property of LANs
  • Fact:

    Message transmission over 500 m of coaxial wire or twisted pairs is virtually error free (if there are no collision)

  • Design decision:

    Due to the extremely low error rate: The data link layer of LAN protocols does not use sliding window (ACK) schemes (I.e., retransmission methods are not built in to the data link layer)

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  • Notice

    Although errors are extremely rare: There can be collision on a CSMA network The sender will still have to recover from these rare frame losses The (design) decision is: Do not implement reliability in the data link layer (Because it is not worth the trouble --- frame losses are rare) The reliability of the data transmissions will be handle in the Transport Layer

    Example:

    The TCP protocol (in the Transport Layer) has time out and retransmit functions So: The TCP layer will provide the reliabilty functionality for the CSMA protocol