 SO that D can be written as the sum of a material dispersion term DM, a waveguide dispersion term Dw, and a mixed term DMW, that in typical fibers is much smaller with respect to the other two. Accurate expressions of the three terms are obtained as a result of the propagation modal analysis; here we will introduce approximated expressions for the material and guide terms in order to analyze their behavior with respect to the main fiber parameters. The material term is related to the wavelength dependence of the silica refraction index; thus, it is the same term causing dispersion in bulk silica. It is positive, independently from the structure and design of the fiber. In order to derive an approximated expression of this term, the propagating wave front can be approximated as a plane. The differential delay between two frequency components of the field can be written as a function of the frequency dependent group velocity as (4.14) The expression of the propagation constant in bulk material in the case of a plane wave is simply = W n(w)/c. Substituting this expression in (4.14) and considering that in the near infrared region that is of interest for telecommunication, the angular frequency is on the order of 1015, it results as (4.15) Telecommunication Networks From (4.15) and the dispersion parameter definition, passing from W to a, the following expression is obtained. (4.16) The material dispersion, as it results from Equation 4.16 and the measured behavior of silica refraction index, is zero at a wavelength of 1300 nm, positive at longer wavelength, and negative at shorter once. In any case, DM increases while increasing a in all the near infrared region. The waveguide term is due to the fact that the guided mode travels part in the core and part in the cladding. Considering the case of a step index fiber, if we analyze the distribu- tion of optical power at different frequencies we found that it depends critically on the ratio a/n due to the different effect of the core-cladding interface diffraction on wa