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Summary 353 ITU allows such systems to have some wavelengths that are on a 25 GHz grid; see ITU G.692 for details. That being said, a much more difficult decision is to pick a standard set of wave- lengths for use in 4-, 8-, 16-, and 32-wavelength systems to ensure interoperability. This is because different manufacturers have different optimized channel configu- rations and different upgrade plans to go from a system with a small number of channels to a system with a larger number of channels. As of this writing, ITU is standardizing (ITU G.959) the set of 16 wavelengths starting with 192.1 THz, and spaced 200 GHz apart, for multichannel interfaces between WDM equipment. It is not enough to specify the nominal center frequencies of the channels alone. A maximum deviation must also be specified because of manufacturing tolerances and aging over the system's lifetime. The deviation should not be too large; otherwise, we would get significant penalties due to crosstalk, additional loss, chirp, and the like. The deviation is a function of the interchannel spacing, f . For f 200 GHz, the ITU has specifed that the deviation should be no more than ± f/5 GHz. 5.12.9 Transparency