The numerous Modes of a multi-mode Fiber differ in their propagation velocities. As long as they propagate independently of each other, the fiber Bandwidth varies inversely with the fiber length due to multi-mode distortion. As a result of inhomogenejties of the fjber geometry and of the index profile, a gradual Energy exchange occurs between modes with differing velocities. Due to this Mode mixing, the bandwidth of long Multimode fibers is greater than the value obtained by linear extrapolation from measurements on shod fibers.