Specialist FPGA solutions for OTN, SONET/SDH and PDH networks
Saturday 18th May 2013
FEC
Overview
In the Optical Transport Network forward error correction (FEC) is used as a tool to control error's on data links where the transmitter adds systematically generated error-correcting codes (ECC) to each frame.
These extra codes allows the receiver to detect / correct a number of errors and avoid forcing the transmitter to resend the frame. While this feature helps to make the data link more robust and resilient, it also reduces the payload bandwidth (as a percentage of the overall transmitted frame). As a result, FEC is only used in applications where the resending of the frame is either impossible of prohibitive in terms of cost.
FEC solutions are typically described in terms of Net Electrical Coding Gain (NECG*) performance and the percentage of the frame overhead that can be made available for the task.
Aliathon Solution
Aliathon offer's a range of ITU-T standard and proprietary FEC solutions all of which are tightly integrated with our OTN framer technology and fall within the categories shown below;
- Supported Line Rates: OTU1, OTU2, OTU3, OTU4....
- Overhead %: 7% to 20%+....
- NECG Performance: 6.3db to 9.35db (7% OH)....
* FEC performance can be measured in terms of coding gain or the effective improvement in the electrical signal-to-noise ratio (ESNR) at the receiver with the FEC employed. The ITU-T G.975 recommendation, originally developed for submarine fiber-optic transmission, calls for a Reed-Solomon (RS) block code. This RS(255,239) code provides a net electrical coding gain (NECG) of 6.3 dB at 1E-15 corrected bit-error rate (BER) and is currently incorporated into most long-haul fiber transmissions (commonly referred to as Generic FEC or GFEC). As the OTN / ITU-T standard evolves to higher data rates, stronger FEC schemes (commonly referred to as Enhanced FEC or EFEC) have begun to become standardized alongside traditional GFEC (as shown in the table above).
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