This International Standard specifies the signalling protocol for the support of the Call Identification and Call Linkage Additional Network Feature (ANF-CIDL) at the Q reference point between Private Integrated Network Services Exchanges (PINXs) connected together within a Private Integrated Services Network (PISN). ANF-CIDL is an additional network feature which allows the assignment of a Global Call Identification (GID) to identify a call end-to-end over the call route (i.e. between the two end PINXs). As an option, a Thread Identification (TID) may be assigned to different calls which are logically linked together due to the operation of other supplementary services and/or ANFs. Additionally a Leg Identification (LID) may be assigned, to identify the different legs of a call. NOTE 1— This ANF has been developed to support the use of CSTA (ISO/IEC 18051) in a networked environment, i.e. in a PISN. Use of this ANF for other applications is not precluded. The Q reference point is defined in ISO/IEC 11579-1. Supplementary Service specifications are produced in three stages and according to the method specified in ETS 300 387. This International Standard contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the stage 1 and stage 2 specifications in ISO/IEC 21888. The signalling protocol for ANF-CIDL operates on top of the signalling protocol for basic circuit switched call control, as specified in ISO/IEC 11572, and uses certain aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582. This International Standard also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between Call Identification and Call Linkage and other supplementary services and ANFs. This International Standard is applicable to PINXs which can be interconnected to form a PISN.