4.18.10 What Is Forward Error Correction and Automatic Repeat Request?
- What Is Forward Error Correction?
- Why Is It Called Forward Error Correction?
- How Does Forward Error Correction Work?
- What Types of Codes Are Used for FEC?
- What Are the Advantages of FEC?
- What Are the Disadvantages of FEC?
- What Is Automatic Repeat Request?
- Why Is It Called Automatic Repeat Request?
- Why Is Automatic Repeat Request Abbreviated as ARQ?
- How Does ARQ Work?
- What Is an Acknowledgment?
- Why Does ARQ Require Error Detection?
- What Is Stop-and-Wait ARQ?
- What Is Go-Back-N ARQ?
- What Is Selective Repeat ARQ?
- What Are the Advantages of ARQ?
- What Are the Disadvantages of ARQ?
- When Is FEC Preferred?
- When Is ARQ Preferred?
- What Is Hybrid ARQ?
- Why Is Hybrid ARQ So Effective?
- Where Is Hybrid ARQ Used?
- How Do FEC and ARQ Compare?
- Which Is More Efficient?
- Why Are FEC and ARQ Important?
Reliable communication is one of the primary objectives of communications engineering. Whether information is transmitted through a satellite link, mobile-phone network, Wi-Fi system, optical-fiber cable, or deep-space communications channel, the receiver must obtain an accurate representation of the original message despite the presence of noise, interference, fading, distortion, and other impairments.
Error-control coding provides the tools needed to achieve this reliability. Once errors have been detected, however, a communication system must decide how to deal with them. Broadly speaking, there are two approaches:
- Correct the errors automatically at the receiver.
- Request that the corrupted information be transmitted again.
These approaches are known as Forward error correction (FEC) and Automatic repeat request (ARQ). Both techniques are widely used in modern communications systems, and many systems combine them to achieve high reliability and efficiency.
Understanding the strengths and weaknesses of FEC and ARQ is essential for understanding how modern digital communication systems operate.
What Is Forward Error Correction?
Forward Error Correction (FEC) is a technique in which redundant information is added to a message before transmission so that the receiver can detect and correct errors without requiring retransmission.
The transmitter employs an encoder that generates additional parity or check bits. The receiver uses these additional bits to identify and correct errors that occur during transmission.
The key characteristic of FEC is that all information required for correction is transmitted in advance. No feedback from the receiver is required.
Why Is It Called Forward Error Correction?
The term reflects the fact that the correction information is sent forward with the original message.
Rather than waiting for the receiver to report errors, the transmitter proactively provides enough redundancy to permit correction at the receiving end.
This distinguishes FEC from retransmission-based techniques.
How Does Forward Error Correction Work?
A typical FEC system consists of:
- Source data.
- Channel encoder.
- Communication channel.
- Channel decoder.
The encoder adds redundancy. For example: Information: 10110011. Encoded sequence: 10110011 010101. The receiver analyzes the redundancy and reconstructs the original information even if some bits have been corrupted.
The exact process depends on the coding scheme employed.
What Types of Codes Are Used for FEC?
Many coding techniques can provide forward error correction.
Examples include:
- Hamming codes. Simple single-error-correcting codes.
- BCH codes. Powerful multiple-error-correcting block codes.
- Reed–Solomon codes. Excellent burst-error-correcting codes.
- Convolutional codes. Continuous codes decoded using algorithms such as Viterbi decoding.
- Turbo codes. Near-capacity codes employing iterative decoding.
- LDPC codes. Modern capacity-approaching codes used extensively in communications systems.
- Polar codes. Capacity-achieving codes used in 5G systems.
All of these techniques provide some form of FEC.
What Are the Advantages of FEC?
FEC offers several important benefits.
- No retransmission required. Errors can be corrected immediately.
- Low latency. No delay is introduced while waiting for retransmission.
- Continuous operation. Communication proceeds smoothly even when errors occur.
- Essential for one-way links. Broadcast systems and spacecraft communications cannot easily request retransmissions.
- Improved reliability. Powerful coding can dramatically reduce error rates.
These advantages make FEC indispensable in many applications.
What Are the Disadvantages of FEC?
FEC is not without cost.
- Additional redundancy. Extra bits consume bandwidth.
- Increased complexity. Encoding and decoding require processing resources.
- Power consumption. Advanced decoders may require significant computational effort.
- Residual errors. No code can correct unlimited numbers of errors.
System designers must balance these costs against the benefits obtained.
What Is Automatic Repeat Request?
Automatic Repeat Request (ARQ) is a technique in which the receiver requests retransmission whenever errors are detected.
Rather than correcting errors directly, the receiver identifies corrupted data and asks the transmitter to send the information again. ARQ therefore relies on:
- Error detection.
- Feedback.
- Retransmission.
The technique is widely used in computer networks and data communications systems.
Why Is It Called Automatic Repeat Request?
The process is automatic because retransmission requests are generated by the communication protocol rather than by human intervention.
The receiver continuously monitors incoming data and requests repeats whenever necessary. The procedure occurs transparently to the user.
Modern communications standards and textbooks almost universally define ARQ as: a protocol in which the receiver detects errors and automatically requests retransmission of corrupted data.
Why Is Automatic Repeat Request Abbreviated as ARQ?
When the technique was first developed in the 1940s and 1950s, the term used in telecommunications literature was not Automatic Repeat Request but Automatic Repeat reQuest. The capital Q was deliberately used to distinguish the signaling message sent by the receiver from the retransmitted message itself.
In early telegraphy and data communications terminology:
- RQ was a common abbreviation for request.
- ACK meant acknowledgment.
- NAK or NACK meant negative acknowledgment.
- RQ indicated a request for retransmission.
When the process became automated, the term Automatic Repeat reQuest emerged, giving the abbreviation rather than the more obvious ARR.
The abbreviation became firmly established in communications engineering and has remained in use ever since. You may occasionally see older texts describe ARQ as:
- Automatic repeat request.
- Automatic retransmission request.
- Automatic repeat query (rare and largely obsolete).
How Does ARQ Work?
The basic sequence is:
- Transmitter sends data.
- Receiver checks for errors.
- Receiver sends an acknowledgment.
If the data is correct: ACK (acknowledgment) is returned.
If an error is detected: NACK (negative acknowledgment) or a timeout occurs.
The transmitter then retransmits the data.
This process continues until the information is received correctly.
What Is an Acknowledgment?
An acknowledgment is a short message indicating successful reception.
Common forms include:
- ACK. Data received successfully.
- NACK. Error detected; retransmission required.
Acknowledgments allow the transmitter to determine whether the message arrived correctly.
Why Does ARQ Require Error Detection?
ARQ cannot function without a mechanism for identifying errors.
Consequently, ARQ systems almost always employ:
- Parity checks.
- Checksums.
- CRCs.
The receiver first detects the error and then initiates the retransmission process.
Without reliable error detection, ARQ would be ineffective.
What Is Stop-and-Wait ARQ?
The simplest ARQ protocol is: Stop-and-Wait ARQ.
The transmitter:
- Sends one frame.
- Waits for acknowledgment.
- Sends the next frame.
If acknowledgment is not received, retransmission occurs.
The approach is simple but may be inefficient when propagation delays are large.
What Is Go-Back-N ARQ?
To improve efficiency, many systems employ: Go-Back-N ARQ.
Multiple frames are transmitted before acknowledgments arrive.
If an error occurs the erroneous frame and all subsequent frames are retransmitted.
This approach improves channel utilization compared with stop-and-wait operation.
What Is Selective Repeat ARQ?
A more efficient approach is: Selective Repeat ARQ.
Only the specific corrupted frames are retransmitted. Correctly received frames are retained. This approach:
- Improves efficiency.
- Reduces retransmission overhead.
- Is widely used in modern communications systems.
What Are the Advantages of ARQ?
ARQ offers several important benefits.
- High reliability. Errors can be eliminated almost entirely.
- Minimal redundancy. Only error-detection information is required.
- Efficient in good channels. Retransmissions occur infrequently.
- Simplicity. Many ARQ protocols are relatively straightforward to implement.
These characteristics make ARQ attractive for many applications.
What Are the Disadvantages of ARQ?
Several limitations exist.
- Retransmission delay. Additional latency may occur.
- Feedback required. A return channel must be available.
- Reduced throughput. Retransmissions consume capacity.
- Poor performance on long-delay links. Round-trip delays may become significant.
These limitations restrict the applicability of ARQ in certain environments.
When Is FEC Preferred?
FEC is generally preferred when:
- Retransmission is impossible.
- Latency must be minimized.
- Propagation delays are large.
Examples include:
- Satellite communications. Geostationary satellites introduce significant round-trip delay.
- Deep-space communications. Retransmissions may require hours.
- Digital television broadcasting. Receivers cannot request retransmission.
- Streaming media. Real-time delivery is essential.
In such applications FEC provides the most practical solution.
When Is ARQ Preferred?
ARQ is generally preferred when:
- Feedback channels exist.
- Delay is acceptable.
- Channel conditions are relatively good.
Examples include:
- Internet protocols. TCP relies heavily on retransmission.
- Computer networks. Data integrity is often more important than latency.
- File transfers. Complete accuracy is essential.
In these situations, ARQ is highly effective.
What Is Hybrid ARQ?
Modern systems often combine:
- Forward Error Correction.
- Automatic Repeat Request.
This combination is called:
Hybrid ARQ (HARQ)
The receiver first attempts error correction.
If correction succeeds no retransmission is required. If correction fails retransmission is requested.
Hybrid ARQ combines the strengths of both approaches.
Why Is Hybrid ARQ So Effective?
Hybrid ARQ offers:
- Strong error protection.
- Efficient bandwidth utilization.
- Reduced retransmissions.
- Improved throughput.
Consequently, it has become extremely popular in modern wireless systems.
Where Is Hybrid ARQ Used?
Applications include:
- 4G LTE. HARQ is a fundamental component of LTE systems.
- 5G networks. Extensive use of HARQ improves reliability and efficiency.
- Wireless data networks. Numerous wireless systems employ HARQ.
- Satellite systems. Some satellite networks also utilize hybrid techniques.
Its widespread adoption reflects its effectiveness.
How Do FEC and ARQ Compare?
| Feature | FEC | ARQ |
|---|---|---|
| Error correction | Receiver | Retransmission |
| Feedback required | No | Yes |
| Latency | Low | Potentially high |
| Bandwidth overhead | Higher | Lower |
| Complexity | Higher | Lower |
| Long-delay links | Excellent | Poor |
| One-way links | Excellent | Impossible |
Neither approach is universally superior.
The best choice depends on system requirements.
Which Is More Efficient?
The answer depends on channel conditions.
- Good channels. ARQ may be more efficient because retransmissions are rare.
- Noisy channels. FEC may be more efficient because repeated retransmissions consume significant bandwidth.
- Long-delay channels. FEC is often strongly preferred.
Consequently, system designers must evaluate trade-offs carefully.
Why Are FEC and ARQ Important?
Virtually every modern communication system relies on one or both of these techniques. Without them, digital communications would experience unacceptably high error rates and poor reliability.
FEC enables reliable communication over noisy channels without retransmission, while ARQ provides a powerful mechanism for ensuring data integrity through feedback and retransmission. Together they form the foundation of modern error-control systems.
Summary
Forward Error Correction (FEC) and Automatic Repeat Request (ARQ) are the two principal approaches used to achieve reliable digital communication. FEC adds redundancy that allows the receiver to correct errors without retransmission, while ARQ relies on error detection, acknowledgments, and retransmission to recover corrupted data.
FEC is particularly valuable in satellite systems, broadcasting, streaming media, and deep-space communications, while ARQ is widely used in computer networks and data communications systems. Modern technologies such as LTE and 5G often combine both approaches through Hybrid ARQ, achieving high reliability, low latency, and efficient spectrum utilization.
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