In previous article Framing we had seen than, sender transmits packets in the form of frame to the receiver. But there is a travel of frame between sender to the receiver. So there is chances of error to occur in this travel.
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Now in this article we will see how error is control in data link layer.
With positive and negative acknowledgement about a frame.
- If the sender receives a positive acknowledgement about a frame, the frame has received safely.
- A negative acknowledgement means that something has gone wrong and the frame must be transmitted again.
What if frame not received by receiver?
- Here timer concept is used.
- When the sender transmits a frame, it also starts a timer.
- Before expiry of this timer acknowledgement from receiver must reach to the sender.
- In absent of acknowledgement sender will again transmit the frame.
What is acknowledgement sent by receiver not reached to sender?
- In this case may possible sender sent multiple frames.
- To prevent from receiving same frames by receiver sender assign a sequence number to the frame.
- By seeing sequence number receiver identifies that its duplicate frame.