The IMS is used to simply control all of those connections, and to manage those
connections in such a way that services can be provisioned and charged for accurately
without multiple systems in place to support those functions. Otherwise, there would
be a need for many different protocols and platforms to deliver true multimedia to
subscribers.
In the IMS, any time a subscriber wishes to establish any form of a dialog, they must
create a session. Think of a session as a portion of a dialog between two parties. For
example, in a videoconference, the voice portion of the transmission would be one session,
while the video portion would be a completely different session.
This allows networks to use different media types to deliver the bearer traffic. The
media types and the path of the bearer traffic then become transparent to the user. The
network only needs one simple method of establishing and managing all of these connections
within the network using a common call control layer. The purpose of the IMS,
as we have already said many times, is to simply provide this control layer.
The IMS uses SIP for controlling all sessions within the IMS domain; therefore,
the rules of SIP registration and session establishment apply here. There are some
extensions to SIP that have been defined by 3GPP specifically for use within the IMS
domain, in an effort to make IMS more robust and secure.
Pages:
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233