(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.