# Advice of charge services overview

This section describes how to use the NMS ISDN advice of charge (AOC) services, available within the ETS 300 variant.

AOC services provide users with a way of tracking the costs of a specific call, in real time. Three AOC services are available, each of which causes AOC information to be returned at a different point in the call:

- AOC at start of call (AOC-S). With AOC-S, the user side is notified of the cost of a call when the call is started. This information can take the form of a flat rate currency cost, the cost on a time basis, or the cost based on pre-arranged item numbers.

- AOC during the call (AOC-D). AOC-D provides the user with information about the cost of the call during the call. For example, a subtotal of the cost could be sent to the user on an interval basis.

- AOC at the end of the call (AOC-E). AOC-E provides the user side with the total cost of the call, at the time the call is cleared (or later).

Invocation of the AOC service is performed by the originating node, on a call-by-call basis. Once AOC is invoked, the originating node receives charging information using supplementary service data structures.

Alternatively, as a subscription option, the interface can be configured to provide AOC on all calls. In this case, the originating user side does not need to invoke the service. However, the NAI
must be configured locally with the AOC subscription options (see Configuring the NAI for AOC subscription services).

Note: The basic call signaling over the network is affected by AOC services. In call clearing cases, the call clearing procedure is delayed until AOC signaling is completed.

AOC services are not available under Q.SIG.