The IP Multimedia System (IMS) is being adopted globally as the standard architecture for next-generation voice and multimedia services. As a result, many operators plan to use IMS and its inherent benefits for public switched telephone network (PSTN) migration, with a goal of eventual elimination of the traditional PSTN. Initial IMS deployments utilize a Telecommunication and Internet converged Services and Protocols for Advanced Networking (TISPAN) IMS-based PSTN emulation subsystem (PES) architecture in which H.248 is used as the control protocol in the access node and is converted to Session Initiation Protocol (SIP) in the IMS core. However, as the adoption of IMS increases and standards become more mature, many operators plan to eliminate H.248 interim architectures and evolve to a more efficient and scaleable SIP-based end-to-end solution. There are several architectural choices the operator has to make in planning the migration to IMS. First, a customer premises equipment (CPE)-based, network-based, or mixed approach can be adopted for PSTN migration. Second, for network-based PSTN migration, the PSTN line termination can be standalone in a line access gateway (LAG) or integrated into a next-generation multi-service access node (MSAN). In addition, there are implementation choices related to the type of SIP endpoint and services supported. This paper will address these architectural and implementation options and describe associated service examples.
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