1st , 2nd , 3rd Generation FEC Forward Error correction
Introduction
One of the most important parameters while designing and operating the optical core network is OSNR ,
This value gives us indications a bout how long distance can the signal travel and how could we apply ROADM network for our core network , also what kind of amplifiers and Line boards should be used , and amplification values .
But this value (OSNR) could be greatly enhanced through using a very useful technology , it is FEC (Forward Error Correction) , this technology has been developed to enhance as big as 11db in OSNR value , the enhancement value that FEC support called net coding gain (NCG).
FEC First & Second Generation
There is a number of FEC algorithms to choose from that vary in complexity, strength, and performance. One of the most common and standardized first-generation FECs, is Reed-Solomon (255, 239). Reed-Solomon adds slightly less than 7% overhead for the FEC bytes and provides approximately 6 dB net coding gain. In high-speed optical networks, a 6 dB gain is a very significant performance improvement— approximately quadrupling the distance between regenerators. In addition to Reed-Solomon FEC, many vendors offer stronger, second generation FECs as a provisionable parameter on 10G and 40G optical interfaces. These UFEC/EFEC algorithms still use 7% overhead, but implement stronger, more complex encoding and decoding algorithms, which provide an additional 2–3 dB coding gain over Reed-Solomon.
3rd FEC Version HDFEC & SDFEC
The 100G era imposes extra requirements on the optical Transmission network , leading to more sophisticated based on more powerful encoding and decoding algorithms .
HDFEC (Hard Decision FEC) implements the decoding block to make a firm decision based upon the incoming signal, and provides a single bit of information (1 or 0) to the FEC decoder. A signal is received and compared to a threshold; anything above the threshold is a “1” and anything below the threshold is a “0.”
SDFEC (Soft Decision FEC) A soft-decision decoder uses additional data bits to provide a finer, more granular indication of the incoming signal. In other words, the decoder not only determines whether the incoming signal is a “1” or a “0” based on the threshold, but also provides a “confidence” factor in the decision. This provides an indication of how far the signal is from the threshold crossing. These additional “confidence” or “probability” bits are used by the soft-decision FEC decoder, along with the stronger, more complex third-generation FEC coding algorithms, to provide 1–2 dB of additional net coding gain. In practice, a 3-bit confidence estimation normally provides most of the theoretically achievable performance improvement. While 1–2 dB coding gain doesn’t sound like much, it can translate into 20–40% improvement in overall achievable distances, which is a very substantial improvement at 100G.
Drawback of SDFEC , The high BW that SDFEC consumes (25%) and the high complexity in hardware also the large processing delay and power consumption considered drawbacks for this technology, but With the algorithm improvement and application of ASIC chips with more advanced techniques, higher integration, and lower power consumption, SDFEC has greatly optimized performance indicators such as delay and power consumption .
Resources :-
https://info.support.huawei.com/network/ptmngsys/Web/WDMkg/en/65_FEC.html
Some quotation from (https://www.fujitsu.com/us/Images/Soft-Decision-FEC-Benefits-or-100G-wp.pdf)
