For example, when the P-CSCF receives an INVITE, it must decide
where the INVITE is to be sent. The P-CSCF acts as the access point into the IMS but
not into individual networks (at least in terms of SIP messaging). The I-CSCF provides
further routing to the proper S-CSCF according to registration procedures.
Like all of the CSCF entities within the IMS, the P-CSCF generates CDRs for all
sessions that pass through it. The P-CSCF also adds headers to request and response
messages before forwarding them onward to the next CSCF. For example, the accessnetwork-
charging-info parameter is added to the P-CHARGING-VECTOR header
prior to sending the request to the S-CSCF. It is the P-CSCF that generates the IMS
Charging Identifier (ICID) used in the charging procedures for correlation of sessions
and charges, since the P-CSCF is the entry point into the IMS.
These headers are added so that entities can exchange charging data within the SIP
message itself, without having to support yet another interface for charging. However,
there is also a charging interface and separate charging overlay that supports the
DIAMETER protocol. This is described in greater detail in Chapter 6.
Headers used for charging are not shared with other networks; the P-CSCF only forwards
these headers to functions within the same network. This prevents other service
providers from learning about subscribers and their service usage.
Pages:
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57