Answer to Question #317687 in Computer Networks for vavanshu

Question

Q1. (a) Find CRC code for message bits to be transmitted which is 11001110 with generator value 1011 known to receiver. (5 marks)

(b) Explain in detail Go-Back-N ARQ Protocol with timeline diagram. (5 marks)

Answer 1

a)

• The generator polynomial G(x) = x⁴ + x + 1 is encoded as 10011.
• Clearly, the generator polynomial consists of 5 bits.
• So, a string of 4 zeroes is appended to the bit stream to be transmitted.
• The resulting bit stream is 11010110110000.
  

Now, the binary division is performed as-

From here, CRC = 1110.

Now,

• The code word to be transmitted is obtained by replacing the last 4 zeroes of 11010110110000 with the CRC.
• Thus, the code word transmitted to the receiver = 11010110111110.

b)Go-Back-N ARQ (Go-Back-N automatic repeat request)

This is a flow control protocol where the sender continues to send several frames specified by a window size even without receiving feedback from the receiver node. It can be said that it's a special case of the general sliding window protocol where the transmitter or sender window size is N and the receiver's window size is 1, which means that it can transmit N frames to the receiving node before waiting for a feedback.

Now the receiver's duty is to keep the track of the sequence number of the next frame it expects to receive and sends the feedback after every data packet it receives. Once the sender has sent all the frames in its window, it makes sure that it has then received all the feedbacks of the transmitted data packets, if in any case, it hasn't received a feedback of any data packet and the time out timer expires, it then resends all the data packets again, starting from the lost data packet to the final data packet.

Sender window size (WS)

In Go-Back-N ARQ, N is the sender window size, which we can see in the above example was 5. Now, here N should be greater than 1 in order to implement pipelining. If N=1, then our system reduces to Stop & Wait protocol.

Now the efficiency of Go-Back-N ARQ = N/(1+2a), where a = tp/tt.

Where tp is propagation delay and tt is the transmission delay

Also, tt = D/B; and here D = data size and B = bandwidth

And tp = d/s, here d = distance and s = wave propagation speed.

Now to find the effective bandwidth (or throughput),

Effective bandwidth = efficiency * bandwidth, which means,

Effective bandwidth = (N/(1+2a)) *B