With SIP, IMS has a core set of protocols for establishing an IP-based future for mobile telephony. Moreover, with the additional features just described, IMS presents a common environment for billing, roaming and QoS within the IP world. All of the above features needed to build an IMS core and to allow inter-operator working and user roaming were specified in Release 5 of the 3GPP specifications for UMTS. IMS-R5 was a distinctly mobile vision for an ‘all IP’ future. However, as we shall now discuss, IMS and its future lies beyond a purely mobile vision.

14.7.1 Seamless Mobility and Convergence

Any device that can handle IP packets is potentially capable of engaging in a SIP dialogue. It doesn't have to be a telephone device, nor wireless. SIP is access-network agnostic. Therefore, inasmuch as IMS is based on SIP, IMS is potentially access-network agnostic. 3GPP Release 6 of IMS fully embraces this potential by ensuring that IMS has true access independence. The gives rise to the potential of seamless mobility, and network convergence, particularly motivated by Fixed-Mobile Convergence (FMC).

Seamless mobility is the ability for a user to access the IMS network from any type of wireless access technology, as long as it supports IP and is capable of supporting the desired session that results from the SIP dialogue. This doesn't have to be a media session. A SIP dialogue could establish an IM session, a PTT session, or some other ...

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