8.1. The Need for Flow Control

Both LANs and LAN switches are connectionless in nature. Frames are sent between end stations on a single LAN or across a switched catenet on a best-effort basis. There is no concept of a virtual circuit, and no guarantee that any given frame will be successfully delivered either to its intended recipient or to any intervening switch. Frames are transferred without error to a high degree of probability, but there is no absolute assurance of success. More specifically, there are no mechanisms provided to:

  • Recover from frame errors: There are no error-control mechanisms that invoke retransmissions in the event of frame corruption within the Data Link layer. Any such error recovery is typically implemented within the Transport layer or the application itself.

  • Ensure that adequate resources (i.e., buffers) are available to receive frames: Even if a frame arrives error free, there is no assurance that there will be a buffer available in which to store the frame.

In the event of a bit error, receiver buffer unavailability, or any other abnormal occurrence, a receiver simply discards the frame without providing any notification of the fact. This allows LAN interfaces to be built at very low cost; a connectionless system is much simpler to implement than a system that includes mechanisms for error recovery and flow control within the Data Link.

The probability of bit errors on LANs is extremely low. Ethernet specifies a bit error rate (BER) of 10−8 in the ...

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