tered power attains the value equal to threshold power, the transmitted power becomes almost constant and independent of input signal power. 4.2.10 ITU-T Fiber Standards The International Standardization Organization (the ITU-T) has standardized several types of fibers to allow system designers to rely on well-known set of fiber parameters. Telecommunication Networks TABLE 4.3 Summary of the Main ITU-T and IEC Standards Regarding Transmission Fibers ITU-T G.652 (A) SSMF no water peak G.652 (C) Pure silica core G.654 DS fiber G.653 Dispersion flattened fiber G.655 G.655 G.655 Intermediate dispersion G.566 Besides ITU-T, the most important organization that has done an extensive optical fiber standardization is International Electrotechnical Commission (IEC), whose influence is mainly present in North America. A summary of the main recommendations of ITU-T and IEC for standardization is reported in Table 4.3. The most common fiber in the world, deployed almost by all the carriers is the nondis- persion-shifted fiber (NDSF) or G.652 fiber, also called SSMF. This is a step index fiber optimized for low dispersion in the third windows. Even if SSMFs are the oldest among all the installed fibers, their characteristics are evolving on the push of the increasing speed of transmitted channels. In particular, manufacturing processes are continuously improved to decrease PMD and to control the attenuation in the extreme parts of the spectrum. Due to the relatively high value of the dispersion parameter, SSMFs allow a very good control of FWM and XPM interference in WDM systems. Moreover, a particular class of G.652 fiber exists where the HO content in the fiber glass SO reduced that the HO attenuation peak almost disappears widening the available spectrum. This property is useful especially in coarse wavelength division multiplexing (CWDM) systems, where two of the standard channels are superimposed to the HO atten- uation peak and cannot be used with SSMFs. A few key specifications from ITU-T G.652 are reported in Table 4.