THE IMS: IP Multimedia Concepts and Services, 3rd Edition

12

An Example IMS Multimedia Telephony Session

12.1 Overview

This chapter shows an example of a IMS Multimedia Telephony 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 10.1).

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 12.2.4;
• All SIP messages must traverse the Proxy Call Session Control Functions (P-CSCFs) and the Serving-CSCFs (S-CSCFs) of both users – Section 0;
• All SIP messages are sent via the established IP security (IPsec) Security Associations (SAs) between User Equipment (UEs) and their P-CSCFs – Section 12.3.3.1;
• All SIP messages are sent compressed between UEs and their P-CSCFs – Section 12.4;
• 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 12.5;
• The two UEs agree on a single codec for every media stream ...