In terms of data link control and physical layer concepts how are error and flow control accomplished in....?

Question

In terms of data link control and physical layer concepts how are error and flow control accomplished in synchronous time division multiplexing?

Answer 1

In the 7 layer ISO model, the data link layer (layer 2) is responsible for attaching a Frame Check Sequence to each packet for error control. This FCS is used to detect errors injected by the physical layer (layer 1), and prevents corrupted data from being passed up the stack to higher layers. Note that layer 2 is not responsible for correcting the errors. Higher layers do that. Layer 2 just discards any packet with an invalid checksum.
In HDLC, layer 3 is responsible for both flow control, and for requesting retransmission of lost or errored frames.
This is accomplished by numbering each frame and using the concept of a sliding window to allow multiple frames to be outstanding at once. If a frame is skipped or comes in out of order, a request is sent requesting the correct frame be resent. Once all of the packets in the window have been sent, no more will go out until acknowledgements are received. The size of the window provides flow control for each channel, and the numbering of the frames provides error detection/correction.

Answer 2

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