Architecture of the IMS 11
There are three entities defined that are responsible for call session control:
?– P-CSCF The Proxy Call Session Control Function
?– I-CSCF The Interrogating Call Session Control Function (I-CSCF)
?– S-CSCF The Serving Call Session Control Function (S-CSCF)
The difference between these entities (shown in Figure 1.4) lies in their individual
purpose and procedures they perform. Each entity acts as a stateful proxy (although
they can also be stateless); therefore, each entity maintains details about all sessions
in progress, as well as registration status of the subscriber device.
As I will discuss in later sections and chapters, dividing these three functions into
separate entities also makes sense for security purposes. There are specific security
functions that have been defined for each of the CSCFs that are best suited for a distributed
architecture.
Proxy CSCF (P-CSCF) The first access point into the IMS is the P-CSCF. The P-CSCF
acts as the access point to the SIP domain from a session control perspective. Bearer
traffic is not passed through this portion of the IMS, as this is a signaling and control
network. The bearer traffic is passed through IP and uses the various access methods
for transport.
What passes through the P-CSCF is SIP. The first communications are to register the
location of a device; location being the IP address of the device in its present location.
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