the bold is my attempt at answering the question. Please correct as necessary.

In this part, you are given different scenarios for which the sender initially transmits some PDUs, and different situations occur after initial transmission. For each scenario, indicate a complete and numbered series of steps which describe exactly how the receiver ultimately successfully receives all transmitted PDUs, assuming continuous ARQ is being used with a sliding window of 2. Recall that continuous ARQ is capable of retransmitting PDUs when necessary. In your steps, the sender should successfully receive acknowledgment (ACK) from the receiver for all transmitted PDUs. The final step for each scenario will thus be receipt of the last ACK.

The sender transmits two PDUS, and only the second PDU has an error.

1. Sender sends PDU1 and PDU2 to receiverReceiver receives PDU1 without errors and sends ACK1 to sender 2. Receiver receives PDU2 with errors and sends NACK2 to sender 3. Sender receives ACK1 and NACK2 and sends PDU3 and resends PDU2, but at the same time sends PDU4 4. Receiver receives PDU2, PDU3, and PDU4 and detects no errors 5. Receiver sends ACK2 and ACK3 6. Sender receives ACK2 and ACK3