The paper considers various aspects of obtaining the design availability requirements for long-haul toll fibre-optic transmission systems in Canada, i.e. an availability of 99.98% for a reference length of 6400 km. The paper looks into unavailabilities of various subsystems (e.g. fibre splices, fibre connectors, protection switches etc.) and estimates the mean time between failure (MTBF) requirement for field and office fibre-optic repeaters and its dependence on protection ratio, protection switching lengths, mean down time (MDT) etc. The effect of the degradation of laser lifetime due to environmental conditions is also considered. Both the short-and long-wavelength operation of long-haul toll fibre-optic transmission systems are discussed.