 of its substantially improved performance, graded index multimode fiber is the clear choice when one wants to exploit the advantages of the multimode fiber with low intermodal dispersion. Multimode fibers have been fabricated in various core-cladding dimensions addressing specific requirements. For communication purposes two core-cladding sizes have been standardized. The two fibers differ in the core size, with the first being 50 um (ITU-I G standard G.651) and the second being 62.5 um. The cladding for both fibers is 125 um. At the moment Multimode Fibers there are no standards for the 62.5-um fiber. The major advantage of the 50/125 um fiber is the higher bandwidth it offers compared to that of 62.5/125 um at 850 nm. An advantage of the 62.5/125 um is that more power can be coupled into the fiber from an optical device such as a Light Emitting Diode (LED) or laser because of the larger NA (NA = 0.275). One of the important questions related to the performance of a fiber is that of the band- width. In general time and frequency domain techniques can be used to establish the performance, but the industry decided to adopt the bandwidth X length product as the performance index of a multimode-fiber bandwidth. A simple equation relating the bandwidth, BW, and the length of the fiber is (5.31) where the subscripts L and S correspond to the bandwidth and length for long and short lengths of fiber, and the exponent s is the length ratio of interest. Usually r = 1, but there has long been debate about the value of s because it reflects the conditions under which the measurements are taken. The value is calculated from the solution of equation 5.31- that is, (5.32) In the rest of this section the value of y= 1 will be assumed. The first-order estimate of the system length that a fiber can support is calculated from the given bandwidth X length product. For example, for a bandwidth X length product of 1000 MHz X km the system length for a 2.4 Gbit/s system is 1000MHzxkm Bitrate When this approach is used to estimate the