ITU G.6x Recommendations

The ITU has defined a series of recommendations that describe the geometrical properties and transmissive properties of multimode and single-mode fiber-optic cables. The four most important recommendations are listed here:

• ITU G.651    Covers multimode graded-index fiber-optic cable having a 50-micron nominal core diameter and a 125-micron nominal cladding diameter.
  
• ITU G.652    Covers single-mode NDSF (non-dispersion-shifted fiber). This cable constitutes most of the cable that was installed in the 1980s. Transmissions take place in the 1,310-nm range where there is minimal signal dispersion. Dispersion causes signal problems over long distances, as described later. G.652 cable supports the following distances and data rates: 1000 km at 2.5 Gbits/sec, 60 km at 10 Gbits/sec, and 3 km at 40 Gbits/sec.
  
• ITU G.653    Covers single-mode dispersion-shifted optical fiber cable. The cable is designed in a way to "shift" the region where dispersion is minimized to the 1,550-nm wavelength range. At this range attenuation is also minimized, so longer distance cables are possible.
  
• ITU G.655    Covers single-mode NZ-DSF (nonzero dispersion-shifted fiber) cable, which takes advantage of dispersion characteristics that suppress the growth of four-wave mixing, an effect that is harmful to WDM (wavelength division multiplexing) systems. NZ-DSF supports high-power signals and longer distances, as well as closely spaced DWDM (dense WDM) channels at rates of 10 Gbits/sec or higher. Lucent True Wave is an example of this cable. It supports the following distances and data rates: 6000 km at 2.5 Gbits/sec, 400 km at 10 Gbits/sec, and 25 km at 40 Gbits/sec.

G.655 is the latest development in fiber-optic cable. In particular, G.655 is optimized for WDM and long-distance cable runs such as submarine cables. It uses dispersion to advantage. Dispersion can help reduce an effect called four-wave mixing (FWM), which occurs in DWDM systems when three wavelengths mix in such a way to produce a fourth wavelength that overlays and interferes with the original signals.