The IMS: IP Multimedia Concepts And Services, Second Edition

10.1. Overview

Session Initiation Protocol (SIP) registration is performed in order to bind the Internet Protocol (IP) address that is currently used by the user and the user's public user identity, which is a SIP URI (uniform resource identifier). If Tobias wants to call Theresa, he will send a SIP INVITE request to her address – i.e., sip:[e-mail]; he does not need to be aware of which terminal Theresa is using. The INVITE then gets routed to Theresa's registrar, which is located in home2.hu. This registrar became aware of Theresa's current terminal address during her registration and will replace the address sip:[e-mail] with the registered contact, which is an IP address. Afterwards, the request can be routed to Theresa's terminal.

Therefore, even for non-IMS cases, Theresa needs to be registered at a SIP registrar so that her current terminal address can be discovered. The IP Multimedia Subsystem (IMS) couples more functionality with SIP registration procedures, which makes it necessary for Tobias to register as well, before he can call his sister.

The following procedures are performed during Tobias's IMS registration (see Figure 10.1):

• The dedicated signalling Packet Data Protocol (PDP) context is established between Tobias's User Equipment (UE) and the Gateway GPRS Support Node (GGSN) in the case of General Packet Radio Service (GPRS) – Section 10.2.

• The UE discovers the address of the Proxy Call Session Control Function (P-CSCF), which it uses as a SIP ...