se there is more penetration of the cladding and the "effective" refractive index is reduced. This causes another wavelength dependence on the velocity of light through the fiber, and therefore another form of dispersion. Thus the use of semiconductor lasers with very narrow line widths (e.g., <0.5 nm) helps mitigate chromatic dispersion (Ref. 11). Link margin is another factor for tradeoff. We set this decibel value as a line item in the link budget for the following contingencies: Cable reel loss variability Future added splices (due to cable repair or modification) and their insertion loss Component degradation over the life of the system. This is particularly pronounced for LED output. ITU-T sets aside 2-4 dB for this sector (Ref. 13). ITU-T recommends a 3-dB link margin (inconsistent with Ref. 13). Reference 9 uses 6 dB. Ideally, for system reliability, a large margin is desirable. However, to optimize system first cost, the designer would want as low a value as possible. (Comment: Those extra dBs cost money.) The system designer develops a power budget, similar in many respects to the path anal- ysis or link budget of LOS microwave and satellite communication link design. However, there is little variability in a fiber-optic link budget; for example, there is no fading. For a first-cut design, there are two source types, LED and semiconductor laser. Expect a power output of an LED in the range of -10 dBm; and for the semiconductor laser budget 0 dBm, although up to nearly +10 dBm is possible. There are two types of detec- tors, PIN and APD. For long links with high bit rates, the APD may become the choice. We would expect that the longer wavelengths would be used, but 820/850-nm links are still being installed. We must not forget reliability in our equation for choices. For lower bit rates and shorter links, we would give LEDs a hard look. They are cheaper and are much more reliable (MTBF). Example Link Budget Exercise. The desired bit rate is 140 Mbps. What will be the maximum distance achievable without t