Computer Networks - Andrew Tanenbaum - The Data Link Layer

Question

Consider a bidirectional link that uses Selective Repeat ARQ with a window size of N = 7. Suppose that all frames are one unit long and use a time-out value of 2. Assume that the one-way propagation delay is 0.5 time unit, the processing times are negligible, and the ACK transmission time is one unit long. Assuming station A and B begin with their sequence numbers set to zero, show the pattern of transmissions and associated state transitions (and sequence numbers) for the following sequences of events.

a) Station A sends 8 frames in a row, starting at t = 0. All frames are received correctly.
b) Station A sends 8 frame in a row, starting at t = 0. All frames are received correctly, except frame 3 is lost.

Answer 1

(a) In this scenario, all frames are successfully received by Station B, and no retransmissions are necessary. The pattern of transmissions and associated state transitions are as follows:

time (in units) | station A             | station B
----------------|----------------------|----------------------
0               | send(0)               | receive(0), ACK(0)
1               | send(1)               | receive(1), ACK(1)
2               | send(2)               | receive(2), ACK(2)
3               | send(3)               | receive(3), ACK(3)
4               | send(4)               | receive(4), ACK(4)
5               | send(5)               | receive(5), ACK(5)
6               | send(6)               | receive(6), ACK(6)
7               | send(7)               | receive(7), ACK(7)
 

(b) In this scenario, frame 3 is lost, and Station A needs to retransmit it after the timeout value of 2 time units. The pattern of transmissions and associated state transitions are as follows:

time (in units) | station A             | station B
----------------|----------------------|----------------------
0               | send(0)               | receive(0), ACK(0)
1               | send(1)               | receive(1), ACK(1)
2               | send(2)               | receive(2), ACK(2)
3               | timeout(3), send(3)   | receive(4), ACK(4)
4               | send(4)               | receive(5), ACK(5)
5               | send(5)               | receive(6), ACK(6)
6               | send(6)               | receive(7), ACK(7)
7               | send(7)               |
8               | timeout(3), send(3)   | receive(3), ACK(8)
9               | send(4)               |
10              | send(5)               |
11              | send(6)               |
12              | send(7)               |
13              |                       | receive(4), ACK(4)
14              |                       | receive(5), ACK(5)
15              |                       | receive(6), ACK(6)
16              |                       | receive(7), ACK(7)
At time 3, Station A times out waiting for the ACK for frame 3 and retransmits it. At time 8, Station A times out again waiting for the ACK for frame 3 and retransmits it. Meanwhile, Station B receives frames 4-7 and sends cumulative ACKs for them. At time 13, Station A receives the ACK for frame 4 and sends frames 4-7 again. At time 17, Station B receives frames 3-7 and sends ACKs for them.

Comments

wrong because selective repeat protocol is given .independent ack is sent by b