Ace Test Series: Computer Networks - Stop And Wait

Question

Comments

c is the correct answer. because time out timer just resolve the deadlock possibility either in case data packet get lost or ack get lost or whatever. Sender will retransmitted the data just after the time out.

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@gauravkc,

Let me clear the idea behind the "Data packet missing problem". This problem would be arise in flow control policies in DLL. 

Let us consider S-> sender and R -> Receiver. 

S transmit the data (immediately start Timeout timer) and it accepted by the R and then R send the acknowledgment(ack1) to S but it does not reach to the S (because of Some delay by medium). but Sender side timer is expire the sender S re-transmit the same data again but it will discarded by R because of duplicate data problem( which is resolved by sequence number to data) and R will think that ohh! I think acknowledgment is lost in between then R re-send the ack2 for the same previous accepted packet. suppose it accepted by S and S send the another packet, and waiting for current packet ack. from R. but suppose now, previous delayed ack1 reached to the S but S was waiting for current transmitted packet ack but got previous packet ack.(which is already accepted by S)  . So this problem is Data packet missing problem

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we have Time Out  Timer for Dead lock problem, Data packet loss and Acknowledgement loss problem

but initially we used time out timer for deadlock problem so answer should be option c

Answer 1

Answer should be (C)

Reason behind it.

Suppose sender 'S' want to transmit the data to receiver 'R' . S transmitted the data but it lost in between in medium ( do not reach to R).

Here you can see S waiting for acknowledgement from R and At the same time R is waiting for data, so both are simultaneously waiting for data and acknowledgement. So deadlock problem arises. We can solve this problem by Timeout timer. How?, Okk let me tell you!

As S transmit the data immediately start time out timer. If data is lost in between. R waiting for data and S waiting for acknowledgement but data is lost. So after timer expire. S understand that ohh! Data is lost in between so S retransmit the data again. So this deadlock problem is resolved.

Comments

see this

https://gateoverflow.in/187694/hamming-distance-question

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@gauravkc,

Let me clear the idea behind the "Data packet missing problem". This problem would be arise in flow control policies in DLL. 

Let us consider S-> sender and R -> Receiver. 

S transmit the data (immediately start Timeout timer) and it accepted by the R and then R send the acknowledgment(ack1) to S but it does not reach to the S (because of Some delay by medium). but Sender side timer is expire the sender S re-transmit the same data again but it will discarded by R because of duplicate data problem( which is resolved by sequence number to data) and R will think that ohh! I think acknowledgment is lost in between then R re-send the ack2 for the same previous accepted packet. suppose it accepted by S and S send the another packet, and waiting for current packet ack. from R. but suppose now, previous delayed ack1 reached to the S but S was waiting for current transmitted packet ack but got previous packet ack.(which is already accepted by S)  . So this problem is Data packet missing problem