When a subscriber is establishing a session, the subscriber device will initiate an INVITE
and forward that INVITE to the P-CSCF. The P-CSCF is always the first point of access in
the IMS network. The P-CSCF provides some protection to the network by maintaining
the state of each registered subscriber, and providing access to the IMS only when it is
assured the subscriber is legitimate. The P-CSCF first examines the REQUEST-URI to
determine if the calling party is in the same network as itself. If both are in the same network,
the P-CSCF will then determine the S-CSCF assigned to the calling subscriber and
forward the INVITE to that S-CSCF. The P-CSCF will know the address of the assigned
S-CSCF because it saves this information during the registration process. The P-CSCF
will save this information for the entire duration the subscriber is registered.
The REQUEST-URI contained in the INVITE is critical to routing the INVITE outside
the network as well. The P-CSCF uses the address in the REQUEST-URI (specifically
the domain name) to determine how the INVITE should be routed in the event
the calling party is assigned to an S-CSCF in another network. The domain name will
identify the home network of the calling subscriber, and the location of the S-CSCF assigned
to the calling subscriber.
Establishing a Session in the IMS 131
The INVITE is not sent directly to the S-CSCF in another network.
Pages:
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273