In the last post we talked about VoLTE charging on a high level. This time we will zero-in and focus on a role of Telephony Application Server (TAS) in offline charging during a Voice/Video Call. Charging on the TAS is described in the 3GPP 32.260 and 32.275. The basic charging is fairly simple, but we also have to count with Supplementary Services, Roaming Scenarios and rainy-days scenarios.
We already know that the 3GPP specification requires Network Elements like TAS to provide offline billing so that service providers can collect and correlate charging information from a variety of sources as well as to generate CDRs. An IMS system is composed of many entities that can generate charging events which need to be collated in order to generate a bill for the customer. These elements are referred in our terminology as the Charging Trigger Functions (CTF). The CTF function works in connection with TAS call processing by receiving SIP call events, translating SIP messages into Diameter messages with call parameters copied into 3GPP-defined AVP set, and sending Accounting Request (ACR) messages to the external CDF. The CDF acknowledges the receipt of ACR message by sending back an Accounting Answer (ACA) message indicating success or failure of the operation. The communication between CTF (TAS) and CDF happens over the Rf interface, which uses the Base Diameter protocol.