This standard specifies the signalling protocol for the support of identification supplementary services at an interface at the S reference point between a Terminal Equipment (TE) and a Private Telecommunication Network (PTN). The Protocol specified in this standard is an extension of the layer 3 protocol for the support of circuit mode basic services@ as specified in ETS 300 192. The identification supplementary services concerned are Supplementary Service Calling Line Identification Presentation (SS-CLIP)@ Supplementary Service Connected Line Identification Presentation (SS-COLP) and Supplementary Service Calling / Connected Line Identification Restriction (SS-CLIR). Service specifications are produced in three stages@ according to the method specified in ENV 41005. This standard satisfies the requirements identified by the stage 1 and stage 2 specifications in ETS 300 173. This standard is applicable to user accesses of PTNXs and to TEs which are intended for connection to PTNs. The impact on the protocol of interactions between the supplementary services specified in this standard and other supplementary services is outside the scope of this standard.