As traditional communication networks move to IP-based protocols, SIP based applications are rapidly introduced bringing evolution to telecommunication services but also some new challenges. SIP is accepted by 3GPP as a signaling protocol in IMS, IP-based architecture which is expected to take over a main role from legacy SS7 networks. Services in IMS are provided by application servers and, while the number of services grows, it is essential to provide consistent and reliable charging mechanisms. This article aims to show challenges in implementing a SIP-based application server based on experiences of our team that worked on solution for application server which provides prepaid front-end functions, providing call control of prepaid sessions, allowing session based charging with unit reservation as well as event based charging, and other supporting prepaid functionalities such as self-care services for end-user. Application server can be used to charge various prepaid services but in this work we primarily analyzed aspects of charging voice calls and usage of voice supplementary services. Several issues observed during the implementation and possible solutions are discussed.
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