The IMS: IP Multimedia Concepts And Services, Second Edition

11.1. Overview

This chapter shows an example session between Tobias and his sister Theresa, who are both registered in their home networks and are both currently roaming in different countries (see Section 9.1).

The Session Initiation Protocol (SIP) and Session Description Protocol (SDP) are facilitated by the Internet Protocol Multimedia Subsystem (IMS) to ensure that Tobias and Theresa can talk to each other and see each other on the screens of their mobile phones. In order to achieve this within the wireless environment certain steps have to be taken:

• Tobias's UE needs to construct an INVITE request that includes a registered public user identity of Theresa in order to reach her – Section 11.2.4.

• All SIP messages must traverse the Proxy Call Session Control Functions (P-CSCFs) and the Serving-CSCFs (S-CSCFs) of both users – Section 11.3.

• All SIP messages are sent via the established IP security (IPsec) Security Associations (SAs) between User Equipment (UEs) and their P-CSCFs – Section 11.3.3.1.

• All SIP messages are sent compressed between UEs and their P-CSCFs – Section 11.4.

• The two UEs agree on the media streams that they will exchange. In the example case they will exchange a bidirectional audio stream, so that brother and sister can talk to each other, and a bidirectional media stream, so that they can also see each other – Section 11.5.

• The two UEs agree on a single codec for every media stream that they will exchange – Section 11.5.

• The networks will authorize the media for ...