This International Standard specifies the signalling protocol for the support of the Advice Of Charge supplementary services (SS-AOC) at the Q reference point between Private Integrated services Network exchanges (PINX) connected together within a Private Integrated Services Network (PISN). SS-AOC is a set of supplementary services which enable a user to receive information about the charging of its calls that leave the PISN and enter another network. The three AOC supplementary services are: a) Charging information at call set-up time (AOC-S) SS-AOC-S enables the user to receive information about the charging rates at call set-up time and also to receive further information during the call if there is a change of charging rates. b) Charging information during the call (AOC-D) SS-AOC-D enables a user to receive information on the recorded charges for a call during the active phase of the call. c) Charging information at the end of the call (AOC-E) SS-AOC-E enables a user to receive information on the recorded charges for a call when the call is terminated. 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 15049. The signalling protocol for SS-AOC 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-AOC and other supplementary services and ANFs. NOTE — 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 which can interconnect to form a PISN.