What is Forward error correction (FEC)?

A method in which data are sent encoded with an error-correcting code that adds redundant bits at the sender, and errors are corrected at the receiver alone. Because the receiver never asks for retransmission, it is used for broadcasting, which has no return channel, for satellite and space communication with long round-trip delays, and on high-speed links that cannot tolerate delay. Typical codes include Hamming codes, BCH codes, Reed–Solomon codes and LDPC codes. Compared with ARQ (automatic repeat request), which detects errors and has the data sent again, it always costs transmission efficiency for the redundant bits, but its delay is constant. It is also used on high-speed serial links such as PCI Express 6.0, which uses PAM4.

Japanese name: 前方誤り訂正(FEC)

Forward error correction (FEC) appears in 2 practice questions across 2 topics, including Information Theory and Coding Basics, I/O Interfaces & Buses.

Field: Information Theory and Coding Basics

Try it: questions about Forward error correction (FEC)

Which of the following correctly describes ARQ (automatic repeat request) and FEC (forward error correction), methods for dealing with errors in communication?

AARQ corrects errors at the receiver alone, so no channel is needed to send notices back to the sender
BFEC asks for retransmission when it detects an error, so delay tends to grow when there are many errors
CFEC corrects errors at the receiver using redundant bits, so it can also be used for one-way broadcasting

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Answer: C. FEC corrects errors at the receiver using redundant bits, so it can also be used for one-way broadcasting

ARQ detects errors with a CRC or similar, tells the sender that the data could not be received and has them sent again; it needs a return channel and is delayed by the retransmissions. FEC sends redundant bits along with the data and corrects errors at the receiver alone, so it suits one-way broadcasting, communication with space probes where a round trip takes a long time, and communication that cannot tolerate delay. Real systems also often combine the two, correcting most errors with FEC and retransmitting the remaining ones with ARQ.

Why does forward error correction (FEC) become more important with PAM4 than with NRZ?

APAM4 makes clock recovery impossible, so every bit is wrong
BFitting four levels into the same swing shrinks the spacing to about one third, so noise causes more bit errors
CPAM4 signals are encrypted and decryption errors must be fixed

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Answer: B. Fitting four levels into the same swing shrinks the spacing to about one third, so noise causes more bit errors

With the same voltage swing, two-level NRZ has the full swing between 0 and 1, but four-level PAM4 leaves only about one third of it between adjacent levels. The noise margin drops by roughly 9.5 dB, so bit errors become more likely. PCI Express 6.0 therefore adds a lightweight error-correcting code so the receiver can fix small errors before use.

Forward error correction (FEC) in the explanations

FEC sends redundant bits along with the data and corrects errors at the receiver alone, so it suits one-way broadcasting, communication with space probes where a round trip takes a long time, and communication that cannot tolerate delay.
Which of the following correctly describes ARQ (automatic r…

Questions that mention Forward error correction (FEC) (2)

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