Explanation:

In the Go-Back-N (GBN) protocol, when the timer of a packet times out, all the packets from that point onwards in the sender's window need to be retransmitted. This is because the sender does not know which packets have been successfully received by the receiver and which packets have been lost or corrupted. Therefore, it assumes that all the packets in the current window are lost and retransmits them.

However, in the Selective Repeat (SR) protocol, the sender resends only those packets which are lost or corrupted. It maintains a separate timer for each packet in the window. When the timer of a specific packet times out, only that particular packet is retransmitted, rather than retransmitting the entire window. This is possible because the receiver in the SR protocol sends individual acknowledgments for each successfully received packet. Therefore, the sender can accurately identify which packets need to be resent.

Comparison between GBN and SR:

1. Go-Back-N (GBN):
- In GBN, the sender maintains a sliding window of packets.
- When the timer for the oldest unacknowledged packet expires, all the packets in the window are retransmitted.
- The receiver acknowledges the correctly received packets cumulatively, indicating the highest consecutive packet it has received.
- If a packet is lost or corrupted, the receiver discards it and waits for the next expected packet.
- GBN provides a cumulative acknowledgment scheme, reducing the number of acknowledgments sent by the receiver.

2. Selective Repeat (SR):
- In SR, the sender maintains a sliding window of packets.
- Each packet in the window has its own individual timer.
- When the timer for a specific packet expires, only that packet is retransmitted.
- The receiver acknowledges each successfully received packet individually.
- If a packet is lost or corrupted, the receiver discards it and sends a negative acknowledgment (NAK) for that specific packet, requesting its retransmission.
- SR provides a selective acknowledgment scheme, allowing the sender to accurately identify and retransmit only the lost or corrupted packets.

Conclusion:

In summary, the Go-Back-N protocol retransmits the entire window of packets when the timer expires, while the Selective Repeat protocol retransmits only the specific packet that has timed out. The Selective Repeat protocol provides more efficiency and reduces unnecessary retransmissions compared to the Go-Back-N protocol.