gy for Telecommunications: Integrated Optics and Microelectronics Since this redundancy has to be placed within the transmitted frame, it is natural to compare the requirement of the most performing codes that are suitable for optical trans- mission application with the ITU-T frame standard for optical transmission. Currently, ITU-T has standardized the use of codes with 7% redundancy (exactly 16 bit redundancy and 255 bits overall length), as recommended in ITU-T G.975 and its successor ITU-T G.975.1. A 7% code is applied directly to blocks of binary information bits, produc- ing an output sequence, whose length is expanded by a factor of 255/239, and the resulting bit stream is transmitted using a binary modulation format. That is, redundancy is accom- modated by sending more symbols. The research on transmission systems, however, is going toward a different direction. As we will see in Chapter 9, in order to transmit speeds higher than 40 Gbit/s binary modulation is not suitable and multilevel coding is a very attractive solution. When multi- level coding is used, redundancy can be either transmitted by increasing the symbol rate or by increasing the number of symbols. For example, if a 16-level transmission is used, redundancy can be accommodated by passing to 32 levels. This means that one redundant bit is transmitted every three message bits. As far as specific codes are concerned, the RS (255, 239) code was the first to see wide- spread application in fiber-optic communication systems, due to its high rate, good perfor- mance, and efficient hard-decision decoding algorithm. As the speed of electronics increased, it became practical to consider more powerful error-correcting codes. The ITU-T G.975.1 standard describes several candidate codes pro- viding better correction performances, all with a 16/255 structure in order to be accom- modated into the Optical Transport Network (OTN) frame in place of the RS (255, 239). In all the considered cases, hard-decision decoding is applied, and improved perfor- mance