An Example of IMS Registration

11.1 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:theresa@home2.hu; 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:theresa@home2.hu with the registered contact, which is an IP address. Afterwards, the request can be routed to Theresa's terminal.

Therefore, even for nonIMS 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 11.1):

  • The UE gets configured with general IMS related configuration parameters. This configuration happens possibly only once per UE and does not need to be repeated for every registration – Section 11.2;
  • A dedicated signalling Packet Data Protocol (PDP) context is established between Tobias's User ...

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