ommon RS codes are designated RS[m, k], in which we use our previous definitions to show that the overhead is (m k)/k. Resulting from its adoption in the ITU-T G.975 standard, the most widely used specific code is RS[255, 239], which has a rate of 239/255, an overhead of 6.7 percent, and a redundancy of 16/255. If, as an example, this code is applied to an STM-16 channel, which, according to Table 6.1, is normally at 2.48832 Gbit/s, the datarate in the optical fiber is raised to ~2.66 Gbit/s. RS[255, 239] can correct up to 8 bit errors from every 255. If the system designer's aim is to achieve a BER of 10-13 after the application of RS[255, 239], the coding gain is 5.8 dB. So far this discussion might seem rather abstract. Therefore, we provide a concrete illustration. Let us assume that the system in Figure 6.30 operates with STM-16 channels and is subject to bad amplifier noise. We require the BER after error correction to be no worse than 10-13, and we employ an RS[255, 239] code to achieve this. We can use equation 6.39 to specify the coding gain, together with equation 6.21 to convert between Q value and BER. By substituting RFEC = 239/255 and ncoding = 5.8 dB, the required performance can obtained at the system's output on the far right of Figure 6.30, provided that the BER of the signals incident on the detectors is no worse than 1.4 x 10 -4. Given that operation is at ~2.5 Gbit/s per channel, the mean time between errors without FEC is only 3 us. However, it is extended to 1 hour and 7 minutes after FEC is applied. Chapter 6 Optical Fiber Transmission The RS[255, 239] code has served optical communications well, but higher-datarate and longer-span systems, such as national and subsea operations at 10 Gbit/s and above, are very demanding. Systems vendors and operators are therefore exploring the use of more powerful FEC. The most common approach uses "concatenated codes," where two or more shorter codes together provide superior coding gains. If this is done, the overhead can be in the range of 10 to 20 pe