Establishing Sessions in the IMS Using SIP
So far we have covered accessing the IMS and registration with the IMS network. All of
these processes must be completed prior to a session being established. As mentioned
earlier, the IMS uses SIP procedures for session establishment; however, there are a
number of SIP extensions that have been defined specifically for the IMS.
It is also worthy of noting that establishing a session within the IMS is quite a bit
more robust than in most VoIP networks. For example, it is perfectly acceptable to access
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a VoIP network and establish a session without any registration or authentication
(possible but not wise). Much has been added in terms of procedures for IMS networks
in an effort to make the network more secure and reliable. That is not to suggest that
IMS and SIP is perfect, but there have been significant improvements over simple SIP
used in VoIP.
Basic Session Establishment
A session can be any form of communication and is not limited to a voice call. This
includes (but is not limited to) e-mail, text messaging, instant messaging, and even
interactive sessions such as Webinars and gaming sessions. A call may consist of more
than one session if other media types are involved in the call. For example, if video is
used as part of the call, the voice will comprise one session and the video another.
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