Transmission fiber dispersion, fiber nonlinear effects, and amplifier noise limit the number of channels and the unregenerated transmission distance of DWDM systems. These factors can be overcome with OOB FEC transponders to enable a 70 percent increase in the number of channels or a 60 percent increase in the transmission distance. Additionally, the OOB FEC allows an improvement in the quality of service (QoS) by guaranteeing a received data channel bit-error rate (BER) of better than 1.0E—15 OOB FEC coding relies on Reed-Solomon algorithms to add redundancy bits to the data stream, enabling the identification and correction of corrupted data bits. These redundant bits take the optical carrier (OC)–192 data rate from 9.953 Gbps to 10.663 Gbps and yield a 7-dB improvement in optical signal-to-noise ratio (OSNR) margin compared to non-FEC transmission. This 7-dB OSNR improvement allows for the improved channel capacity, transmission distance, and QoS.

To further enhance performance, the 10-Gbps OOB FEC transponders utilize optimized threshold crossing control in the receiver side of the transponder to set the decision circuit threshold to the in the received data "eye." When multiple traces of the data stream are superimposed on top of each other, the 0s and 1s form an "eye." The more open the eye, the more reliably the 0s and 1s will be detected and the better the BER. However, amplitude noise from the EDFA amplifiers and electronics, phase noise, dispersion effects, and interference resulting from conversion of phase into amplitude modulation start to close the eye. As the eye closes, the decision circuit that determines if a bit is a 0 or 1 gives fewer bit errors if the decision threshold level can adaptively change to the optimum level. The optical receiver of the OOB FEC line extender modules (LEMs) and receive transponders (RXTs) feature adaptive threshold crossing control driven by the number of errored 0s and 1s determined in the bit stream. The result is improved receiver sensitivity and a resultant improvement in BER performance.