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Introduction: the perspective of error

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• Protocol
  • What is a protocol:

    Protocol = an (a priori) agreed action and response between 2 or multiple parties used to accomplish a task

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  • Protocol interactions:

    Actions can cause the occurence of certain events Example: A frame transmission (= action) will cause a reception event at the receiver Events can cause a certain response Example: A reception event at the receiver can cause the transmission of an acknowledge frame (= response)

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• A warning on the study of protocols
  • Communication protocol:

    Communication protocol = a set of procedure/rules that provide reliable communication between a sender and a receiver

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  • Warning:

    When you study a communication protocol: You are aware of all events that has happened In particular: You are aware of the events at the sender You are also aware of the events at the receiver In contrast: The sender is not aware of the events at the receiver The receiver is not aware of the events at the sender

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  • Warning:

    Make sure that you take a note of this fact when we study communication protocols I call this the "omniscient" trap: You have an "omniscient" view But the sender/receiver themselves do not !!!

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• My analogy of computer communication
  • Computer communication:

    A transmits a frame to B: Analogy: A and B are behind 2 tall walls A and B are separated by a river A throws rocks over the wall and river to B

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• The point of view of the receiver
  • The point of view of the receiver is as follows:

    Specifically:

    The receiver is not aware of any event that has occured within the blackbox !!! Because the wall prevents the view of the event !!!

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• Example of the point of view of the receiver
  • Example:

    Sender transmits a frame The frame is received correctly by the receiver: This result in: Receiver receives a frame In the analogy: The receiver gets a rock

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• Example of the omniscience trap

  • Example:

    Scenario 1: The sender transmits a frame The frame is lost (e.g., contains bit errors) Result: The observed event by the receiver is: No frame was received !!! Scenario 2: The sender did not perform any transmissions Result: The observed event by the receiver is: No frame was received !!!

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  • Conclusion:

    The receiver cannot tell the difference between scenario 1: and scenario 2: The "omniscient" trap: Scenario 1 and 2 is indistingusihable to the receiver However: YOU can tell the difference Warning: To understand a communication protocol, you must force yourself to consider Scenario 1 and 2 to be identical !!! (Because the protocol must work correctly for the receiver (and not for you !!!) The receiver could not know that the sender has transmitted a frame !!!)