This International Standard defines the mapping functions in exchanges of Private Integrated Services Networks (PISNs) required for the utilization of scenarios in which 64 kbit/s circuit mode connections are sub-multiplexed into 4 x 16 kbit/s channels for carrying inter-PINX signalling and user information. NOTE 1 — This International Standard has been prepared to meet the specific needs of an application for a particular user organisation. However, it may also be applicable elsewhere. The requirements contained in this International Standard will be incorporated in any future standard specifying a more generic approach to bearer conditioning. In order to connect a Private Integrated Services Network Exchange (PINX) to another PINX, mapping functions are required to adapt the specific interfaces at the C reference point to the application at the Q reference point. As such, mapping functions provide for physical adaptation to the interface at the C reference point. Mapping functions also provide for the mapping of user channels and signalling information at the Q reference point to the appropriate channels or timeslots at the C reference point. The C and Q reference points are defined in ISO/IEC 11579-1. The type of interface at the C reference point covered by this International Standard is the 64 kbit/s Unrestricted Digital Leased Line (D64U) Terminal Equipment Interface, in accordance with ITU-T Rec. G.703. At the Q reference point the mapping provides a 16 kbit/s service for user channels to support the transfer of unrestricted digital information and to support the transfer of speech, and a packet mode service for the signalling channel. The applied mapping is a static mapping, i.e. there is a fixed relationship between user and signalling channels at the Q reference point and the interface at the C reference point. Management functions relating to failure management are outside the scope of this International Standard. This International Standard is applicable to PINXs that can be interconnected to form a Private Integrated Services Network (PISN) and that support signalling protocols at the Q reference point.