Stop and Wait ARQ Process Steps | Generated by AI

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Question: In the error control technology of data communication, Stop-and-Wait ARQ is one of the common methods for error detection and retransmission. The diagram below describes the process of using Stop-and-Wait ARQ when the sender and receiver transmit data. Please write out the content of items ① to ⑥ in the diagram based on the provided reference answer logic.

Answer:

Based on the reference answer provided and the standard mechanics of the Stop-and-Wait ARQ protocol (specifically a variant utilizing NAK - Negative Acknowledgment), here is the detailed deduction and explanation for items ① through ⑥:

The Sequence of Events:

  1. Context: The sender has likely just sent PKT1 and received an acknowledgment, so it is now attempting to send PKT2.
  2. Item ①: ACK
    • Logic: Before sending new data (PKT2), the sender might be receiving a final acknowledgment for the previous packet (PKT1), or this represents the successful acknowledgment of a previous step allowing the sequence to continue. In the context of the reference answer starting with ACK, it implies the channel is clear or the previous transaction was successfully closed.
    • Content: ACK (Acknowledgment)
  3. Item ②: PKT2
    • Logic: Upon receiving the ACK (or as the next step in the sequence), the sender transmits the second data packet.
    • Content: PKT2 (Packet 2)
  4. Item ③: NAK
    • Logic: The receiver gets PKT2 but detects an error (e.g., via CRC check). Instead of staying silent (which would rely on a timeout), this specific protocol variation sends a NAK (Negative Acknowledgment) to immediately inform the sender that PKT2 was corrupted and needs retransmission.
    • Content: NAK (Negative Acknowledgment)
  5. Item ④: PKT2
    • Logic: Upon receiving the NAK, the sender immediately retransmits the corrupted packet. It does not move to the next sequence number yet; it must resend PKT2.
    • Content: PKT2 (Retransmission of Packet 2)
  6. Item ⑤: ACK
    • Logic: The receiver successfully receives the retransmitted PKT2. It verifies the integrity, accepts the data, and sends a positive acknowledgment.
    • Content: ACK (Acknowledgment for PKT2)
  7. Item ⑥: PKT3
    • Logic: Now that PKT2 is confirmed, the sender proceeds to send the next packet in the sequence.
    • Content: PKT3 (Packet 3)

Summary of the Flow:

This scenario highlights the efficiency of using NAKs in Stop-and-Wait ARQ, as it avoids waiting for a timeout period when an error is detected immediately at the receiver.

References:

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