Mumbai University > Information Technology > Sem4 > Computer Networks

Marks: 5M

Year: May 18

Disadvantage of Stop-and-Wait

1. In stop-and-wait, at any point in time, there is only one frame that is sent and waiting to be acknowledged.

2. This is not a good use of transmission medium. To improve efficiency, multiple frames should be in transition while waiting for ACK.

3. Two protocol use the above concept

, – Go-Back-N ARQ

– Selective Repeat ARQ

1. Efficiency is very less.
2. Only one frame is sent at a time.
3. Timer should be set for each individual frame.
4. No pipelining.
5. Sender window size is one(disadvantage over go back and ARQ).
6. Receiver window size is one(disadvantage over selective repeat ARQ).

1. One problem is when the ACK sent by the receiver is damaged or lost. In this case, the sender doesn't receive the ACK, times out, and sends the frame again. Now the receiver has two copies of the same frame, and doesn't know if the second one is a duplicate frame or the next frame of the sequence carrying identical DATA

2. Another problem is when the transmission medium has such a long latency that the sender's timeout runs out before the frame reaches the receiver. In this case the sender re-sends the same packet. Eventually the receiver gets two copies of the same frame, and sends an ACK for each one. The sender, waiting for a single ACK, receives two ACKs, which may cause problems if it assumes that the second ACK is for the next frame in the sequence.