As data rates increase beyond 10G, the requirements for OSNR become stringent. This is because the advanced modulation techniques used at higher bit rates require higher OSNR performance. Low OSNR is directly related to poor BER performance in DWDM networks. FEC or Forward error connection is a method used to achieve coding gain for higher bit rates. It is a method of encoding optical signal with extra error detection and correction overhead bytes enabling optical receivers to detect errors and correct them. Thus FEC can reduce BER and effectively increases distances reachable by high speed signals without regeneration.
Standard FEC and Enhanced FEC ( EFEC) are methods correctly used on 10G and 40G . Both call for adding extra 7% overhead information in traffic rate for the purpose of BER monitoring and correction. The standard FEC can result is 6 db coding gain ( which is infact quadrupling the distance) while Enhanced FEC can give an additional 2 to 3 % coding gain thus increasing distances further. Both Standard FEC and EFEC are called Hard Decision FECs-the decision rules for receiver are based on two levels ( 1 or 0); receiver decides between 1 or 0 depending on whether signal level is above or below a certain threshold level. A new and more effective FEC is used by some vendors at 100G which is called Soft Decision FEC ( SD FEC). SD FEC can additionally give a confidence factor in the decision used by receiver to decide between 1 and 0 meaning how far is the signal level from 1 or 0 this results in an additional coding gain of 1 to 2 db and hence resulting in overall improvement of 20% to 40% for distance reachability on 100G. Most vendors use SD FEC these days on transponders used for long haul applications while the metro transponders are still implemented with hard decision FEC to keep the price levels low. However it should be kept in mind that SD FEC comes at the cost of adding 20% overhead information ( FEC bytes) resulting in slightly higher optical rates.
Without FEC these days, the distance on high speed links would be severly limited. While Hard Decision FEC is very effective in increasing distances on 10G and 40G; Soft Decision FEC can addtionally provide longer distances and few regenerators on 100G.
Keep in mind that the FEC overhead is a part of the G.709 OTUk frame. Client traffic such as SONET or Ethernet is encapsulated within the OTN at full line rate, the DWDM line-side transponder is then overclocked to provide additional bits for the OTN frame. As an example, the OC192 bitrate is 9.976 Gbps. The OTU2 bitrate is 10.709 Gbps. So just for clarity, the additional bps do not detract from the throughput of the client signal, they are accounted for by overclocking the line-side transmission. Also, with regard to the additional OSNR provided by FEC…. In optical network design we generally budget about .25 dBm per (fiber optic) kilometer of loss. So, if G.709 FEC provides an additional 6.2 dBM of gain, you can (roughly) equate that to about 24 extra kilometers of reach.
Thanks Jason for useful comments.
Jason, ‘x dB’ coding gain could be ‘x dB’ of OSNR/Q gain. And hence, it may not translate to extra distance of (x/0.25)Km. In fact, it could be much more than that in reality (depends on the link characteristics though!)
Dear Mr. Jason
Thanks for the information shared.
Could you please explain “The additional bps do not detract from the throughput of the client signal, they are accounted for by overclocking the line-side transmission” in detail” Just trying to get more clarity.
Thanks