By adding parity bits during transmitter coding, the Forward Error Correction (FEC) technology enables the receiver to correct bit errors in the bit stream by calculating the parity bits. According to the latest ITU-T Recommendations G.709 and G.975, the STM-16 transmission rate or higher rates are now used in the FED technology to ensure the reliability of data transmission and to significantly extend the communication distance.
FEC is used very early in the extra-long-distance submarine cable systems. With the development of terrestrial optical fiber systems and the increase of single-channel transmission rate, the application of FEC becomes a optimal choice for reducing the required device tolerance and system networking costs.
1.2 FEC Classification
l FEC is also called channel coding which is a type of error control coding. It can be classified into out-of-band FEC and in-band FEC.
p In-band FEC
n In-band FEC as defined in the ITU-T Recommendation G.707 uses certain overhead bytes in SDF frames to carry supervisory elements of FEC codes. (Applicable to SDH systems)
n Advantage: Avoid the dispersion limitation by not increasing the line rate, support interface compatibility, and improve error performance by 1 dB to 2 dB.
n Disadvantage: low error-correcting tolerance.
p Out-of-band FEC
n Out-of-band FEC adopts the OTN FEC scheme and is supported by the ITU-T Recommendations G.975/709. (Widely used in WDM systems)
n Advantage: Support high code redundancy, good error-correcting capability, and high code gains of 5 dB to 6 dB, and allow convenient insertion of FEC overheads without limitation of the SDH frame structure.
n Disadvantage: The inserted overheads cause the increase of line rate. Therefore, modification is required on relevant devices.
l In the definition for OTN architecture in the G.709, the FEC overheads belong to the OTN OTUk layer and the Reed Solomon RS (255, 239) FEC codes are used as standard FEC codes. In consideration of later expansion, other codes can also be used.
Currently, the out-of-band FEC serves as the actual FEC code standard. The FEC types in Recommendations G.975 and G.709 are all out-of-band FEC, and can use RS (255, 239) codes. However, the two FEC types are different. Recommendation G.975 recommends direct FEC encoding/decoding with RS codes for SDH signals, and Recommendation G.709 defines the OTN architecture, in which the FEC overheads are defined as a part of OTN OTUk layer and thus become a standard part of the OTN architecture. In the OTN architecture, columns 3825 to 4080 contain FEC codes.
1.3 Standard Codes Of Out-Of-Band FEC
l Interleaved RS coding supports convenient encoding and decoding, and its coding structure is compatible with binary codes. RS (255, 239) (RS-8 for short) can increase line rate by 7.14%.
l The RS (255, 239) is a type of RS (n, k) coding. The maximum number of corrected burst errors in a single block: r = (n – k)/2. The RS (n, k) supports convenient encoding and decoding and its coding structure is compatible with binary codes. In RS (255, 239) (RS-8 for short), k equals 239. The 239 data bits and 16 parity bits form a packet, and the packet code length n equals 255. With RS-8, the maximum number of corrected burst errors r equals 8 and the line rate increases by 7.14%.
1.4 Out-Of-Band FEC-AFEC
ITU-T Recommendation G.975.1 proposes a powerful FEC method for the high-rate DWDM submarine optical communication systems. This method provides a more powerful error-correcting capability, compared with the RS (255, 239) FEC codes recommended by the Recommendation G.975. The use of this method can improve the transmission performance of the high-rate DWDM submarine optical communication systems.
The powerful FEC method uses cascaded codes in which the long codes are composed of short codes. No unified standards have been established for the powerful FEC method. Different companies develop their own high-gain FEC code types, resulting in poor compatibility between FEC codes. Currently, the OTU boards support adjustable FEC and AFEC to facilitate application and interconnection.
1.5 FEC Summary
The WDM products now use only two out-of-band FEC methods. In terms of coding mode, the two methods are FEC and AFEC, namely, RS coding and AFEC coding. At present, many boards support setting of AFEC and FEC. If compatibility is supported, the two methods can be used.
Talking with FEC use in Huawei transmission product, some cards for long distance use FEC coding, SF64 such us SSND00SF64(out of band STM-64 board), SF16 such as SSN1SF16(out of band STM-16 board), they are all Out of band FEC adopted.