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1/221 02-FGB 101 327 Uen A – December 2006 11

Each role is described below.

The CSCF interfaces the MM for sending Charge Data Output (CDO), the

ENUM DNS for NNA purposes, and the MN-OSS for surveillance and

configuration.

5.1.1.2 P-CSCF

The P-CSCF is the first point of contact for the User Agent (UA). The P-

CSCF may modify (e. g. modification of private headers) an outgoing

request according to a set of provisioned rules defined by the network

operator, e.g. address analysis and message modification. The P-CSCF

provides the following capabilities:

• Forwarding of the SIP REGISTER request received from the UA

to the I-CSCF.

• Storage of contact addresses of the user equipment, as a part of

the registration process.

• Forwarding of SIP messages received from the UA to the S-

CSCF, whose address the P-CSCF has received as a result of the

registration procedure.

• Forwarding of SIP requests or responses from the S/I-CSCF to

the UA.

5.1.1.3 I-CSCF

The I-CSCF is the contact point inside an operator’s network for all

connections destined to a subscriber of that network operator. It may hide

the inner topology of the home network from other networks and locates the

S-CSCF where a subscriber is located through interaction with the HSS.

Multiple I-CSCFs can exist concurrently in an operator’s network.

The I-CSCF provides the following capabilities:

• Dynamic allocation of S-CSCF instance to a user performing SIP

registration.

• Locating the S-CSCF where a subscriber is located through

interaction with the HSS (for terminating calls).

• Routing of SIP requests received from another network to the

S-CSCF.

• Forwarding of the SIP request or response to the S-CSCF.

• ENUM resolution for routing sessions from PSTN to the S-CSCF

(normally not used since the MGC does an ENUM query for calls

from PSTN).

5.1.1.4 S-CSCF

The S-CSCF performs the session control services for the UA. This

includes routing of originating sessions to I-CSCF or external networks and

routing of terminating sessions to P-CSCF. The S-CSCF supports

establishment, modification, and release of IP multimedia sessions using

the SIP/SDP (Session Description Protocol) protocol suite. It also provides

indirect service invocation through the IMS Service Control (ISC) interface

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