To obtain the actual IP address, my PC will have to query
a Domain Name Server (DNS). Once my PC learns the IP address for the P-CSCF serving
its area, the device can begin registration procedures with the IMS network. No
sessions can be established until registration has been completed.
This is much different than the way VoIP has been implemented today. There is a
registration process in some implementations, but communications with VoIP networks
do not require much in the way of authentication and authorization. In other words,
registration is much simpler in most VoIP networks, only requiring the communication
of the device??™s location so that call routing can be completed.
For a wireless device, the process is somewhat different because the device can be
outside of its own home network (roaming). When this is the case, the device must still
obtain an IP address, as well as learn the address of the P-CSCF serving the area in
which the device is located. The difference is that the device may be locating a P-CSCF
from another service provider rather than its own home network.
As I have discussed in Chapter 1 and 2, the P-CSCF serves as the access point to
the IMS and therefore controls all communications from other networks. The P-CSCF
serves as the gateway into the IMS, communicating to all other SIP devices. All subscribers
must connect first to the P-CSCF prior to accessing the S-CSCF.
Pages:
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237