Data Link Layer Services-Error Control
Error Control
• The next problem to deal with is, who to make sure all frames are eventually delivered to the network layer at the destination, and in proper order.
• The usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at he other end of the line.
• One complication with this is that the frame may vanish completely, in which case, the receiver will not react at all, since it has no reason to reactThis possibility is dealt with by introducing timers into the data link layer. When the sender transmits a frame, it generally also starts a timer. The timer is set to go off after an interval long enough for the frame to reach the destination machine. If the frame or acknowledgment is lost
the timer will go off. The obvious solution is to transmit the frame again. This creates the problem of possible sending frames multiple times. To prevent this from happening, it is generally necessary to assign sequence
numbers to outgoing frames, so that the receiver can distinguish retransmission from originals.
