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ISO/IEC 15991:2003 specifies the Supplementary Services Call Priority Interruption (SS-CPI) and Call Priority Interruption Protection (SS-CPIP), which are applicable to various basic services supported by Private Integrated Services Networks (PISN). Basic services are specified in ISO/IEC 11574.

NOTE This edition of ISO/IEC 15991 does not apply to the circuit-mode Multiple Rate Bearer Service.

SS-CPI is a supplementary service that 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 is a supplementary service that allows for the protection of calls against interruption from other, priority, calls.

SS-CPI and SS-CPIP are described separately because SS-CPI is a service used by a calling user, and SS-CPIP is a service used by either a calling user or a called user.

Supplementary service specifications are produced in three stages, according to the method described in ETS 300 387. ISO/IEC 15991:2003 contains the stage 1 and stage 2 specifications of SS-CPI. The stage 1 specification (clause 6) specifies the general feature principles and capabilities. The stage 2 specification (clause 7) identifies the Functional Entities involved in the supplementary service and the information flows between them.