The modelling of optical fiber reliability has been extensively discussed, but there is little understanding of how the fatigue of deployed fiber due to stress corrosion and environmental corrosion contributes to the reliability of a telecommunications network. There are a number of models that can be used to calculate or predict the lifetime of optical fibers for both pristine conditions and conditions where zero stress ageing occurs (water immersion, high temperature, high humidity and the presence of chemicals). The failure rates for fiber can be combined with the failure rates for the other network components to give a figure for the network reliability which will indicate whether the network will be able to meet its Quality of Service (QoS) targets. A method for calculating fiber failure rates will be presented and the results for various environments will be compared with previous assumptions and with the failure rate due to cable dig-up. The increasing demand for tight bending of fiber in customer premises will be mentioned and its effect on network reliability discussed.