This Recommendation specifies the electrical@ physical@ procedural and protocol requirements for the V5.1 interface between an Access Network (AN) and the Local Exchange (LE) for the support of the following access types: ?C analogue telephone access; ?C ISDN basic access with a line transmission system conforming to ITU-T G.960 for the case with a NT1 separate from the AN; ?C ISDN basic access with a user-network interface according to ITU-T I.430at the user side of the AN (i.e. the interface at the T reference point); ?C other analogue or digital accesses for semi-permanent connections without associated outband signalling information@ with flexible (provisioned) information channel (bearer channel) allocation but without concentration capability within the AN. This Recommendation does not specify the implementation of the requirements within the AN and does not constrain any implementation alternative as long as the functionality at the V5.1 interface as specified in this Recommendation is met. A complementary Recommendation specifies interface V5.2 which is based on the V5.1 interface. Interface V5.1 shall be upgradable to interface V5.2. The concept and the requirements for the upgrade are defined in Annex F. Annex A provides an overview of the service scenarios and architecture taken as the conceptual basis for the specification of the V5.1 interface. Annex B defines the use of the protocol information elements for the definition of the national PSTN protocols and information flow diagrams to the PSTN protocol specification. Annex I provides the definition of the layer 3 PSTN protocol error detection. Annex C specifies the basic requirements of the management function in the LE and the AN to support correct operation and control of the configuration. Annex D describes the protocol architecture for the ISDN and PSTN user port status control information transfer. Annex E provides an overview of frame formats used in the V5.1 interface. Abbreviations used in this Recommendation are listed in Annex G. Appendix I gives the bibliography.