This standard defines the Layer 3 protocol for signalling for the support of circuit mode bearer services (used either on their own or in support of teleservices) at the Q reference point between private Telecommunication Network Exchanges (PTNX) connected together within a Private Telecommunication Network (PTN). The Q reference point is defined in ENV 41004. Service specifications are produced in three stages and according to the method specified in ENV 41005. The definition of signalling protocols is stage 3 of the method. Stage 1 and 2 specifications of the basic circuit mode bearer services are to be found in ETS 300 171. The protocol defined in this standard satisfies the requirements identified by the stage 1 and 2 specifications in ETS 300 171.