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Scenario

A SIP User Equipment (UE) is registered through an IMS network.

Background Information

• The P-CSCF is analogous to the Access-

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   (A-

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  ). In this IMS model, the AS locating functions are performed by the I-CSCF component of the IMS.
• The P-CSCF/

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  may be located in the home or roaming network.
• The P-CSCF/

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  serves as the initial contact point for users into an IMS network. It determines (via DNS) the I-CSCF address to contact in the home network for the user.
• The P-CSCF/

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  provides a "trust border" between IMS and the subscriber
• The P-CSCF/

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  provides media policing.
• The P-CSCF/

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  interfaces with I-CSCF, S-CSCF.
• The UE is pre configured with the P-CSCF address which is to be contacted.

Description

1. The UE is pre-configured with the P-CSCF address contact.
2. The P-CSCF resolves I-CSCF to forward the registration request (via DNS).
3. The I-CSCF queries HSS to determine which S-CSCF can host the subscriber.
4. The request is delivered to the correct S-CSCF which acts as registrar and invokes AS for features as required.
5. For a roaming subscriber, the flow is similar, except that the P-CSCF in this case protects the border of the visited network and the subscriber is directed to the I-CSCF in the home network.