The procedures as described in Chapter 11 so far explain the general mechanism used in IMS for establishing multimedia sessions in a mobile environment. As shown there, the procedures for reserving resources over the wireless link and authorization of media add complexity and additional signalling to the SIP flows in IMS when compared with those in IETF.
When specifying 3GPP Release 6 and Release 7, IMS grew towards an open multimedia communication platform that is not only accessed from cellular access networks but in which resources are usually tight. UEs can attach to IMS from fixed networks as well as via Wireless Local Area Network (WLAN) based access technologies.
Also, certain services are not in need of specific resource reservation procedures. A text-based chat service, for example (see Section 11.10.2), has completely different resource and QoS requirements than an audio conversation.
In order to serve the requirements of new access technologies as well as of specific services, alternative session establishment mechanisms were introduced to IMS. Some of them are still in the process of being defined in detail as this book is being written.
Section 11.10.4 shows how session establishment can be performed between an IMS-attached terminal and a SIP terminal outside IMS.
All the examples in this section show the signalling flows directly between the two UEs, the networks inbetween are not depicted, ...