near effects include unwanted intermodulation between different channels in a wavelength division multiplexed system, the intermodulation will also modulate the refractive index of the channels, other effects include phase shifting in single channel systems (a function of the signal intensity) and scattering and reflected energy effects, not dissimilar to VSWR in radio transmission systems. The more linear the system the higher the ISD. Even assuming an ISD of one bit/s/Hz there is over 100 THz of available bandwidth across the 1000 to 1700 nanometre optical spectrum, which would yield at least 100 T/bit/s/Hz of transmission capacity in a long-haul system, SO capacity does not seem to be an immediate problem. Similarly in the access network the shorter range means that little or no amplification is needed SO the optical signal-to-noise ratio is higher and nonlinear effects are less severe due to the lower launch powers and shorter distances involved. ISD should therefore be higher than in long-haul systems. The final impairment process is dispersion, which is either intermodal, chromatic or polarised. Intermodal dispersion occurs when several modes or angles are allowed to travel along a light-carrying core. It only occurs in multimode fibre. Chromatic dispersion occurs because the refractive index, which dictates the speed at which light travels down the fibre varies with wavelength and will result in pulse spreading in the time domain. This is known as material dispersion. There will also be dispersion known as waveguide dispersion, which is a function of a portion of the light wave at a given wave- length travelling in the cladding as well as the core, both media having a different refractive index. Chromatic dispersion reduces with narrower line widths though device cost increases pro- portionately. Some wavelengths are less susceptible to chromatic dispersion. Additionally, high-speed equalisers can be used to compensate for the effect though these are computation- ally expensive particularly at higher data 