3

IMS Concepts

3.1 Overview

This chapter begins with a first-glance description of IP Multimedia Subsystem (IMS) registration and session establishment. It depicts the IMS entities that are involved. The intention is not to show a full-blown solution; rather, it is to give an overview and help the reader to understand the different IMS concepts explained in this chapter. Detailed registration and session establishment flows will be shown and explained later in the book.

Prior to IMS registration the User Equipment (UE) must discover the IMS entity to which it will send a REGISTER request. This concept is called a Proxy-Call Session Control Function (P-CSCF) discovery and is described in Section 3.8. In addition, before a registration process the UE needs to fetch user identities from identity module. Identity module is covered in Section 3.6 and identities are presented in Section 3.5. During the registration a Serving-CSCF (S-CSCF) will be assigned (Section 3.9), authentication will be performed and corresponding security associations will be established (Section 3.21), a user profile (Section 3.12) will be downloaded to the assigned S-CSCF, Session Initiation Protocol (SIP) compression will be initialized (Section 3.18) and implicitly registered public user identities will be delivered (Section 3.3). The concept of sharing a single user identity between multiple terminals is covered in Section 3.7.

Section 3.10 explains how Internet Protocol (IP) policy control is applied when ...

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