Answer to Question #201990 in Computer Networks for Muqeet Waraich

Question

Ex

a)    How a damaged or lost frame is recovered in a communication between a sender and a receiver using Selective Repeat ARQ. Explain with the help of flow diagram.                        

b)    Bandwidth of the line is 2 Mbps, and 1 bit takes 30 ms to make a round trip. What is the efficiency of the link if a frame size is 1500 bits? Use Go Back-N protocol. Sender is allowed to send 15 frames at a time.

Answer 1

a) In the selective repeat protocol, sender sends the number of segment which is equal to the window size.Let the size of the window be n, then the sender will send all the n segment at once and waits for the acknowledgement.

If the sender receives the negative acknowledge for any particular segment, then sender will resend that particular segment not the entire window like GO BACK N.

Each segment has it's own timeout interval, hence sender will wait for timeout time and if the acknowledgement not received in the given specified timeout, then sender will resend it.

b)

Bandwidth of the line = 2MBPS

Round trip time = 30ms

Frame size = 1500 bits

Number of frame send at a time = 15

Round trip time for the packet $=2T_p=30ms\times 1500$

$=45sec$

Transmission time $(t)=\frac{L}{B}=\frac{1500}{2MBPS}=0.75ms$

Efficiency $=\frac{W\times T_t}{T_t+2T_p}$

$=\frac{15\times 0.00075\times 100}{0.00075+45}$

$=0.024\%$