SDH optical transmission medium of the transmission network is, of course, fiber, the single-mode optical fiber with a large bandwidth, easy to upgrade and expansion and low cost advantages, international has agreed that digital synchronous optical line system using only single mode fiber as transmission medium. There are 3 transmission "windows" in the optical fiber transmission, which is suitable for the transmission wavelength range, 850nm, 1310nm, 1550nm. In which the 850nm window is only used for multi-mode transmission, only 1310nm and 1550nm are used for the single mode transmission window.
The distance of the optical signal transmission in the optical fiber is affected by the dispersion and loss, and the dispersion will cause the transmission of the digital pulse in the optical fiber, which can reduce the signal quality. When the code interference between the transmission performance degradation to a certain extent (for example 10-3), transmission system will not work the, the loss of the optical signals in the optical fiber transmission with power transmission distance decreased, when the optical power is reduced to a certain degree, transmission system cannot work.
In order to extend the transmission distance of the system, it is mainly to reduce the dispersion and loss. 1310nm optical transmission window is called the 0 dispersion window, the optical signal transmission dispersion in this window is minimal, the 1550nm window is called the minimum loss window, the attenuation of the optical signal transmission in this window is minimized.