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Error Recovery on a Simplex Data Link: Which Technique Can Work?

Error Recovery on a Simplex Data Link: Which Technique Can Work?

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Nov 3, 2026

A simplex data link provides no return channel for the receiver to signal errors (e.g., ACK/NACK). In contrast, many “backward” techniques require feedback from receiver to sender. Therefore, on a simplex data link, the most plausible error recovery options are those that allow the receiver to correct errors without retransmission requests—i.e., forward error correction (FEC) via error-correcting codes such as Hamming codes.

Specifically, consider the given options:

  • (i) Backward error correction (BEC): typically needs a feedback channel, so it does not fit true simplex.
  • (ii) Automatic Repeat reQuest (ARQ): also needs feedback (ACK/NACK) and retransmission, so it does not fit true simplex.
  • (iii) Use of Hamming codes: works in simplex because correction is done locally at the receiver using redundancy.
  • (iv) Downward error correction (DEC): “DEC” is not a standard widely defined error-control mechanism in classic data-link error control literature; without a clear definition tied to simplex operation, it cannot be reliably selected as a known simplex-compatible recovery method.

This leads to the correct choice: (iii) the use of Hamming codes. 4

Footnotes

  1. Automatic repeat request - Wikipedia - Defines ARQ using acknowledgements and timeouts with retransmission. ↩

  2. Error detection and correction - Wikipedia - Notes ARQ uses acknowledgements/timeouts; retransmits when ACK is not received. ↩

  3. Data Link Layer Designing Issues: Error Control—A ... (PDF) - Contrasts backward error correction (retransmit request) vs forward error correction (auto-recover). ↩

  4. Transmission Errors: Error Detection and Correction (PDF) - Describes Hamming codes as examples of error correction/ FEC at data link layer. ↩

Error Control (Data Link Layer): ARQ vs FEC and Codes

Error-control options and why feedback matters on simplex

At the data link layer, ARQ uses error detection plus retransmission, which inherently relies on a signaling path from receiver back to sender (ACK/NACK or equivalent). ARQ is explicitly described as using acknowledgements and timeouts to decide retransmissions. 2

Meanwhile, FEC (e.g., Hamming codes) does not require retransmission requests; it relies on redundant parity information added by the sender and corrected/decoded by the receiver. Hamming codes are a common example of an FEC technique capable of correcting certain errors at the receiver side. 2

Footnotes

  1. Automatic repeat request - Wikipedia - Defines ARQ using acknowledgements and timeouts with retransmission. ↩

  2. Error detection and correction - Wikipedia - Notes ARQ uses acknowledgements/timeouts; retransmits when ACK is not received. ↩

  3. Transmission Errors: Error Detection and Correction (PDF) - Describes Hamming codes as examples of error correction/ FEC at data link layer. ↩

  4. Data Link Layer Designing Issues: Error Control—A ... (PDF) - Contrasts backward error correction (retransmit request) vs forward error correction (auto-recover). ↩

Determine which option can recover errors on a simplex link

  1. 1
    Step 1

    In simplex, the receiver cannot send ACK/NACK or retransmission requests back to the sender; techniques based on this are incompatible.

  2. 2
    Step 2

    (ii) ARQ relies on acknowledgements/timeouts and retransmission, so it needs feedback. (i) Backward error correction also implies retransmission requests from receiver to sender.

  3. 3
    Step 3

    (iii) Hamming codes are FEC and therefore allow correction without asking the sender to resend.

  4. 4
    Step 4

    (iv) Downward error correction (DEC) is not a standard, clearly defined data-link error-control mechanism in the same way as ARQ/BEC/FEC/Hamming in common references; without a precise definition tied to simplex operation, it is not a justifiable selection.

  5. 5
    Step 5

    Therefore, (iii) the use of Hamming codes is the only well-supported simplex-compatible error recovery technique among the choices.

Compatibility with True Simplex Error Recovery

Feedback-dependent methods generally do not work on simplex; local-correction codes do.

Pro Tip

"When a question says simplex data link, immediately look for mechanisms requiring ACK/NACK or retransmission from sender. If those are required, the method usually cannot be used in true simplex." 2

Footnotes

  1. Automatic repeat request - Wikipedia - Defines ARQ using acknowledgements and timeouts with retransmission. ↩

  2. Error detection and correction - Wikipedia - Notes ARQ uses acknowledgements/timeouts; retransmits when ACK is not received. ↩

Warning on ambiguous terminology

"“Downward error correction (DEC)” is not a commonly standardized term in classic data-link error-control taxonomies alongside ARQ/BEC and FEC/Hamming. Don’t guess it without a precise definition tied to simplex feedback assumptions." 2

Footnotes

  1. Data Link Layer Designing Issues: Error Control—A ... (PDF) - Contrasts backward error correction (retransmit request) vs forward error correction (auto-recover). ↩

  2. Transmission Errors: Error Detection and Correction (PDF) - Describes Hamming codes as examples of error correction/ FEC at data link layer. ↩

Quick decision table

Knowledge Check

Question 1 of 3
Q1Single choice

On a simplex data link, which error recovery technique is possible?

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