In addition to increased speed and bandwidth, 40 Gbit/s technology comes with its own set of issues. Chromatic dispersion, which is caused by individual wavelengths traveling at different speeds in the fiber, is controllable at 10 Gbit/s. Since tolerance at that speed is fairly high, non-optimized compensation techniques can be used; namely, dispersion-compensating fiber (DCF), which acts as a broadband compensator. Although this seldom compensates for all the wavelengths with the same efficiency, at 10 Gbit/s transmission, broadband compensation is good enough (unless there are several spans in a row with several DCFs, each with their residual dispersion adding up). Residual dispersion can easily reach 300 ps/nm without causing any real issues.
In 40 Gbit/s transmission, on the other hand, the limited tolerance for chromatic dispersion leads to rather restrictive requirements when it comes to measuring link dispersion as well as applying compensator tolerances. A way to circumvent this is to use adaptive compensation. When using non-return-to-zero (NRZ) modulation, the chromatic dispersion at 40 Gbit/s is 16 times more stringent than at 10 Gbit/s. The maximum residual dispersion of the 40 Gbit/s application codes is defined at 32 ps/nm. This tolerance has to be shared between the possible variation, during the lifetime of the link, and measurement precision and the tolerance of the dispersion compensator.