The IMS: IP Multimedia Concepts And Services, Second Edition

9.1. The example scenario

This section gives a detailed example of a normal IMS session between two users and all the required prerequisites. It is based on the assumption that both users are attached to the General Packet Radio Service (GPRS), which is used as the example access technology throughout the section.

Tobias, who is a student in France and currently visiting Finland, is calling his sister Theresa, who is working in Hungary and currently on a business trip to Austria (see Table 9.1 and Figure 9.1).

Tobias's home operator is located in France. As he is roaming in Finland, the Finnish operator provides the Proxy Call Session Control Function (P-CSCF), as the home operator and the Finnish operator have signed an IMS roaming agreement. Consequently, the Gateway GPRS Support Node (GGSN) that Tobias is using is also located in Finland.

Table 9.1. Location of CSCFs and GPRS access for the example scenario.

User	Home operator S-CSCF location	P-CSCF location	GPRS access
Tobias	France	Finland	Finland
Theresa	Hungary	Hungary	Austria

Figure 9.1. The example scenario.

Theresa's home operator in Budapest has no IMS roaming agreement with the operator in Austria. Therefore, her terminal gets attached to the P-CSCF in her Hungarian home network, where the GGSN is also located. Theresa's access to the IMS is based on the GPRS-level roaming agreement between the operators of her home network and ...