Station A needs to send a message consisting of 9 packets to Station B using a…

The sliding window size is 3, and the go-back-n protocol retransmits all packets starting from the lost one. Packets 5 and 10 are lost (every 5th packet), but only packets 1 to 9 need to be sent. Packet 5 is lost during its first transmission, so packets 5, 6, and 7 are retransmitted. After the window slides, packet 10 is not sent initially but would be part of a later transmission. However, since the total message has only 9 packets, packet 10 is not part of the original transmission. But in retransmissions, when packet 5 is lost and retransmitted, the window continues. After successful transmission of packets 1-4 and retransmission of 5-7, the next window includes packet 8 and 9. However, since every 5th packet is lost, packet 10 would be the next to be lost if sent. But since only 9 packets are needed, packet 10 is not transmitted initially. However, in the retransmission of packets 5-7, packet 10 is not involved. The key insight is that packet 5 is lost in its first transmission, so packets 5-7 are retransmitted. After that, the window slides and sends 8 and 9. But packet 5 is retransmitted again in the next window, which causes it to be lost again. This leads to multiple retransmissions of packets 5-7, and eventually packet 10 is not sent. The total number of transmissions is calculated as follows: initial transmission of packets 1-9 (9 transmissions), retransmission of packet 5 and subsequent packets in the window, leading to additional transmissions. The correct total is 16.