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At present’s excessive velocity optical hyperlinks would fail if not for this know-how (Half 1)

Written by Vasanta Rao, Technical Advertising Engineer, Transceiver Modules Group (Cisco Optics)

Understanding FEC Half 1: The Fundamentals of FEC

Partly one in all this two-part weblog submit, we overview the fundamentals of forward-error correction (FEC). Partly two, we’ll talk about implementation and the influence of FEC on efficiency.

At present’s insatiable demand for bandwidth is pushing community speeds increased and better. Sadly, the sooner the info charge, the higher the probability of transmission errors. Consequently, forward-error correction (FEC) has turn into a necessary instrument for nearly any community implementation, significantly these working at 100G and above. FEC makes use of a mixture of specialised algorithms and redundant knowledge bits to detect and proper a sure variety of errored bits in every knowledge block. To study extra concerning the specifics, learn on.

What’s FEC?

FEC is a digital course of for detecting and correcting errors in a bitstream by appending redundant bits and error-checking code to the message block earlier than transmission. The error-checking code incorporates ample data on the precise knowledge to allow the FEC decoder on the receiver to reconstruct the unique message.

FEC makes use of n-symbol codewords consisting of a knowledge block that’s ok symbols lengthy and a parity block (the code and redundant bits) that’s n-k symbols lengthy (see determine 1). We determine an FEC code with an ordered pair (n,ok). The kind and most variety of corrupted bits that may be recognized and corrected is decided by the design of the actual error-correcting code (ECC), so completely different ECCs are appropriate for various community implementations and efficiency ranges.

Determine 1: We describe an FEC code utilizing an ordered pair (n, ok) by which ok denotes the size of the info payload and n denotes the size of the whole FEC codeword in symbols.

The selection of FEC code to make use of in a given hyperlink is decided by variety of sources. Sure requirements, corresponding to these created by the IEEE name out particular FEC codes. Module producers should adjust to these necessities with a view to meet requirements. Multi-source agreements (MSAs) may present tips for which FEC code to make use of, however they don’t seem to be essentially binding. In sure circumstances, module producers might elect to make use of a higher-performing FEC code than specified as a transfer to distinguish themselves from the competitors.

When the optical transceiver is operated in a Cisco host, the default FEC is enabled mechanically; nevertheless, there may very well be different FEC codes for particular software protocols that may be supported by the host software program. The person can resolve to allow these, relying on their particular software.

FECs have limitations

A transmission error happens when a bit flips from a 1 to a 0 or from a 0 to a 1. Knowledge corruption can happen within the type of single-bit errors or burst errors (see determine 2). A single bit error is a single, random errored bit in a knowledge block. A burst error is a gaggle of modified bits. The errored bits in a burst error usually are not essentially contiguous, nevertheless.


Determine 2: Single-bit errors (high) happen randomly in a knowledge string. Burst errors (backside) can encompass contiguous bits or remoted bits.

All kinds of ECCs have been developed. Some are solely appropriate to deal with single-bit errors, whereas others work effectively on each. The Reed-Solomon (RS) household of ECCs is most fitted for optical communications. RS codes use algorithms that function on symbols quite than on particular person bits, which provides them the flexibility to detect and proper burst errors. RS codes can appropriate a collection of errors in a block of knowledge to recuperate the unique message block. As a result of they’re so efficient at coping with random and burst errors, RS ECCs have been broadly adopted by optical communications requirements and MSAs such because the IEEE and the 100G Lambda MSA.

Partly two, we’ll talk about the trade-offs concerned in any see what they imply for community efficiency.

For extra particulars on FEC, see our new white paper,  “Understanding FEC and Its Implementation in Cisco Optics.”


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