 can be partially attributed to the significantly longer block lengths considered. Due to the period in which this standard has been developed, no soft decision and no block turbo codes have been considered. Even if the optical community has, SO far, standardized FEC at 7% redundancy, this choice is not at all fundamental. The 7% value certainly reflects a reasonable choice of code rate, given the manner in which optical communication systems have been operated to date. Due to the high SN of optical channels, the uncoded symbol error rate for binary signal- ing schemes is small. For the RS (255, 239) code, communication with bit error rates less than 10-12 requires an uncoded error rate (or "FEC threshold") of less than x10-4 Since optical systems have traditionally been operated to achieve such low pre-FEC error rates, a high-rate code was sufficient to provide sizeable coding gains, and 7% redundancy provided a reasonable balance between bit rate and error-correcting capability. From an information theoretic perspective, however, mandating 7% redundancy unnec- essarily restricts the pre-FEC symbol error rate to be small. When multilevel transmission will come into play, that seems a must for transmission at 100 Gbit/s; this implies that the used multilevel constellation has to include a small num- ber of points SO as not to cause too big an SN penalty (see Chapter 9). To increase the spectral efficiency of fiber-optic communication systems (e.g., via mul- tilevel modulation), it is necessary to design the coded modulation scheme using infor- mation theoretic considerations. Maximizing spectral efficiency may, in fact, lead to operating points in which the "raw" error rate requires the use of codes with greater than 7% redundancy. In view of the next-generation systems, this discussion will be opened inside the ITU-T and for sure, if effective proposals will emerge, standardization will include them into the standard. Telecommunication Networks The opportunity to do SO will be standardization of multilevel optic