Question

Using CRC method, calculate the Checksummed message to send throw the medium if:

• Message = 101011
• Cx=x3+x2+1

Q2) Explain how the ‘Stop and Wait’ flow control method is working?

Q3) Explain the differences between Go-Back-N and Selective Repeat sliding window flow control methods?

Using CRC method, calculate the Checksummed message to send throw the medium if:

• Message = 101011
• Cx=x3+x2+1

Q2) Explain how the ‘Stop and Wait’ flow control method is working?

Q3) Explain the differences between Go-Back-N and Selective Repeat sliding window flow control methods?

Answer 1

Question 2:

The working of a stop and wait protocol:

• The sender node sends a data packet to the receiver node.

• Then, waits for the feedback of the transmitted packet.

• As soon as the receiver node receives a data packet it starts processing it.

• Then, the receiver node sends the feedback to the sender node (about the received data packet).

• After receiving the feedback, if the feedback is positive then the sender node sends the next data packet otherwise resends the damaged packet.

Stop & Wait Protocol performs better for LANs than WANs because the efficiency of the protocol is inversely proportional to the distance between the sender & receiver node and as we all know that the distance is less in LANs as compared to WANs.

But this protocol has some issues, one such issue is the probability of occurrence of deadlock, which is very high due to loss in a data packet or loss in feedback which can lead to infinite waiting, where the sender will keep on sending the data packet if the feedback is lost.

These steps are given below

Analysis:

• Sender puts the data packet on the transmission link.
• Data packet propagates towards the receiver’s end.
• Data packet reaches the receiver and waits in its buffer.
• Receiver processes the data packet.
• Receiver puts the acknowledgement on the transmission link.
• Acknowledgement propagates towards the sender’s end.
• Acknowledgement reaches the sender and waits in its buffer.
• Sender processes the acknowledgement.

Steps:

Total Time:

Total time taken in sending one data packet = (Transmission delay + Propagation delay + Queuing delay + Processing delay)_packet + (Transmission delay + Propagation delay + Queuing delay + Processing delay)_ACK

Advantages of Stop and Wait:

• It's simple and each frame is checked and acknowledged well.

Disadvantages of Stop and Wait:

• Only one frame can be in transmission at a time.
• It is inefficient, if the distance between devices is long. Reason is propagation delay is much longer than the transmission delay.
• The time spent for waiting acknowledgements between each frame can add significant amount to the total transmission time.

Question 3:

S. No.	Keys	GoBackN protocol	Selective Repeat protocol
1	Sender window size	In GoBackN protocol, Sender window size is N.	In Selective Repeat protocol, Sender window size is N.
2	Receiver Window size	In GoBackN protocol, Receiver window size is 1.	In Selective Repeat protocol, Receiver window size is N.
3	Minimum Sequence Number	In GoBackN protocol, Minimum Sequence Number is N+1 where N is number of packets sent.	In Selective Repeat protocol, Minimum Sequence Number is 2N where N is number of packets sent.
4	Efficiency	In GoBackN protocol, Efficiency formular is N/(1+2*a) where a is ratio of propagation delay vs transmission delay and N is number of packets sent.	In Selective Repeat protocol, Efficiency formular is N/(1+2*a) where a is ratio of propagation delay vs transmission delay and N is number of packets sent.
5	Acknowledgement Type	In GoBackN protocol, Acknowledgement type is cumulative.	In Selective Repeat protocol, Acknowledgement type is individual.
6	Supported Order	In GoBackN protocol, in-order delivery only are accepted at receiver end.	In Selective Repeat protocol, out-of-order deliveries also can be accepted at receiver end.
7	Retransmissions	In GoBackN protocol, in case of packet drop,numbers of retransmitions are N.	In Selective Repeat protocol, in case of packet drop,number of retransmition is 1.