Introduction to the IP Multimedia Subsystem (IMS)

This chapter introduces the IP Multimedia Subsystem (IMS), which is the core network of Voice over LTE (VoLTE). It then focuses on IMS architecture, IMS core network elements, and the IMS core network call and signaling flow between different network elements. The chapter covers practical deployment scenarios for IMS‐network‐based real use cases from a live network deployment. It shows the IMS reference architecture including interfaces towards legacy networks and other IP‐based multimedia systems. The chapter describes private and public user identities (user IDs) in IMS, the support of user IDs, implicit and explicit registration,and the dependencies for service subscription. The application server (AS) can provide location‐based services in various roles: multimedia telephony (MMTEL), service centralization and continuity AS (SCC AS), and IM‐service switching function (IM‐SSF) without the need to implement major changes to the backend infrastructure.