How to detect source errors?
In order make sure that the frames are delivered properly, the receiver should inform the sender about incoming frames using positive and negative acknowledgements. On the sender end the receipt of a +ve acknowledgement implies that the frame has here at the destination safely while the receipt of a -ve acknowledgement means that an error has occurred in the frame and it needs to be retransmitted. On the other hand, this scheme is too simplistic as if a noise fracture causes the frame to disappear completely, the receiver will not react at all and the sender would hang up forever waiting for an acknowledgement. To overcome this disadvantage, timers are introduced into the data link layer. When the sender transmits a frame or packet it also at the same time initializes a timer. The timer is set to go off after a interval long enough for the frame to reach the dest. be processed there, and have the acknowledgement circulate back to the sender. If the frame is received properly the positive acknowledgment arrives before the timer runs out and so the timer is cancelled. If yet either the frame or the acknowledgement is lost the timer will go off and the sender possibly will retransmit the frame. Since numerous transmission of frames can cause the receiver to admit the same frame and pass it to the network layer more than once, sequence numbers are normally assigned to the outgoing frames.
The kind of acknowledgements that are sent can be classified as follows:
- Cumulative acknowledgements: A single most acknowledgement informing the sender that all the frames upto a certain number has been received.
- Selective acknowledgements:It is a Acknowledgement for a particular frame.
They may be also classified as:
- Individual acknowledgements: It is Individual acknowledgement for each frame.
- Group acknowledgements: It is a bit-map that specifies the acknowledgements of a range of frame numbers.
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