As
the device communicates with other devices, it must first establish a session with the
device. These session establishments are also made first through the P-CSCF.
When a subscriber device is first activated, it will be assigned an IP address by the
serving network. Once the device has been assigned an IP address, it will search out
the local P-CSCF (or whatever P-CSCF has been assigned to serve this part of the network).
The P-CSCF, like all IMS entities, has an address in the form of a SIP Universal
Resource Identifier (URI) (making it easier to route messages to the proper P-CSCF).
Once the device is powered on, it sends its IP address to the Home Subscriber Server
(HSS) and the S-CSCF using a registration process. The P-CSCF plays an important
role in the registration process, as you will learn later. The first role, however, is to
identify the home network from the subscriber??™s domain (found in the URI of the subscriber
address). The domain name of the home network is of course resolved into an
IP address using the Domain Name Server (DNS) function of the network.
The DNS identifies the address of the I-CSCF to be used to access the home network.
The role of the I-CSCF is described in the next section, but for the sake of this discussion,
the I-CSCF provides the gateway access into any network.
Figure 1.4 The IMS core and the CSCF
P-CSCF I-CSCF S-CSCF
12 Chapter 1
It is up to the P-CSCF to determine how to route any SIP messages received by the
subscriber device.
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