"The present document is applicable to international constant bit rate digital paths at or above 64 kbit/s supported by digital networks. International constant bit rate digital paths may be based on the Plesiochronous Digital Hierarchy (PDH)@ the Synchronous Digital Hierarchy (SDH) or some other transport network such as cell-based. The present document is generic in that it defines parameters and requirements independent of the physical transport network providing the paths. Two types of paths are considered; paths between International Switching Centres (ISCs) which consist of an international portion only and paths between Customer Premises (CP) which consist of national and international portions. These paths are referred to as ""type a"" and ""type b"" respectively. Both the national and international portions are made up of Path Elements (PEs). For the national portion of paths of ""type b""@ the present document specifies availability parameters and requirements for the portion as a whole. Subdivision of requirements to the respective PEs making up the national portion is under the responsibility of the network operator. For the international portion of paths of both types@ the present document specifies availability parameters and requirements for the PEs making up the international portion. NOTE: The international measurement point is located on the international side of the ISC. The end-to-end availability performance of an international digital path can be calculated from the arrangement of the constituent PEs and their associated requirements. Annex A gives guidance on evaluating the end-to-end unavailability of a path@ taking into account the unavailability of each PE. In some countries the network may be subdivided into parts which are under the responsibility of different network operators. The partitioning of the requirements between these parts is outside the scope of the present document. Similarly@ application of the present document to national paths which do not cross International Borders (IBs) is outside the scope of the present document."