This International Standard specifies the signalling protocol for the support of the Call Priority Interruption (SS-CPI) and Call Priority Interruption Protection (SS-CPIP) supplementary services at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). NOTE 1 — This edition of this International Standard does not apply to calls using the circuit-mode multiple rate bearer service. SS-CPI allows a call request for a priority call to proceed successfully in the case that there is no user information channel available. This is accomplished by force releasing an established call of lower priority. SS-CPIP allows for the protection of calls against interruption from priority calls. The Q reference point is defined in ISO/IEC 11579-1. 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 15991. The signalling protocol for SS-CPI(P) 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 SS-CPI(P) and other supplementary services and ANFs. NOTE 2 — Additional interactions that have no impact on the signalling protocol at the Q reference point can be found in the relevant stage 1 specifications. This International Standard is applicable to PINXs that can interconnect to form a PISN.