The maximum window size for data transmission using the

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The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence number is

1. 2ⁿ
2. 2^{n – 1}
3. 2ⁿ – 1
4. 2^{n – 2}

Correct Option: B

Let us assume the following
I Communication between two Stations; Station A and Station B
II The size of Window to be 8 (0, 1, 2, 3, 4, 5, 6, 7)
Now, the communication between the two stations would be as follows
1. Station A sends frame 0 to Station B
2. Station B receive frame 0 and sends an acknowledgement (ack) as RR1
3. Station A sends frame 1 to Station B
4. Station B receive frame 1 and sends an acknowledgement (ack) as RR2
5. Station A sends frame 2 to Station B
6. Station B receive frame 2 and sends an acknowledgement (ack) as RR3
7. Station A sends frame 3 to Station B
8. Station B receive frame 3 and sends an acknowledgement (ack) as RR4
9. Station A sends frame 5 to Station B
10. Station B receive frame 5 and sends an acknowledgement (ack) as RR6
11. Station A sends frame 6 to Station B
12. Station B receive frame 6 and sends an acknowledgement (ack) as RR7
13. Now, RR7 is lost in the transmission. Since, Station A will send frame 7 only after receiving RR7, so it will Time Out as RR7 is already lost.
14. Now, A times out retransmit frame 0
15. But B was expecting frame 7, so it considers frame 7 as Lost and accept frame 0 as a new frame This problem is overcome by limiting the maximum windows size to 2ⁿ – 1.