In this part, you are given different scenarios for which the sender initially transmits some PDUs, and different situations occur after initial transmission. Continuous ARQ is being used with a sliding window of $3.$

For each scenario, indicate a complete and numbered series of steps which describe exactly how the receiver ultimately successfully receives all transmitted PDUs. 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.

Follow the example below when giving your series of steps. If the scenario were to be $$the sender transmits a single PDU without error$,$ the numbered of steps would be something like this:

$.$ Sender transmits PDU$1$

$.$ Receiver receives PDU$1$ and transmits an ACK

$.$ Sender receives the ACK

Below are the scenarios for which you should give numbered steps.

$$ Scenario $1$: The sender transmits only two PDUs; the first PDU is affected by an error, and the second PDU is transmitted error$-$free.

$$ Scenario $2$: The sender transmits three PDUs without error.

$$ Scenario $3$: The sender transmits a single PDU without error, but the PDU is lost before it arrives at the receiver.

$$ Scenario $4$: Four PDUs are transmitted by the sender without error. However, the acknowledgment $($ACK$)$ of the third PDU sent by the receiver is lost before it arrives at the sender$.$