In this module, you learned that some communication protocols implement error correction, which is a mechanism that both detects and corrects errors that occur between the sender and receiver of a message. You also learned that one common method of error correction is automatic repeat request $($ARQ$),$ in which the sender and receiver detect potential errors in each protocol data unit $($PDU$)$ sent and retransmit them when appropriate.

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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 $3.$

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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.

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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:

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$1.$ Sender transmits PDU$1$

$2.$ Receiver receives PDU$1$ and transmits an ACK

$3.$ Sender receives the ACK

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Below are the scenarios for which you should give numbered steps. As previously noted, please ensure that your solutions implement continuous ARQ as opposed to stop$-$and$-$wait ARQ.

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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$.$