Mw reference point

The Gm reference point links the UE to the IMS (namely, to P-CSCF). Next, a SIP-based reference point between different CSCFs is needed. This reference point is called Mw. The procedures in the Mw reference point can be divided into three main categories: registration, session control and transaction:

• In the registration procedure the P-CSCF uses the Mw reference point to forward a registration request from the UE to the I-CSCF. The I-CSCF then uses the Mw reference point to pass the request to the S-CSCF. Finally, the response from the S-CSCF traverves back via the Mw reference point. In addition, the S-CSCF uses the Mw reference point in network-initiated de-registration procedures to inform the UE about network-initiated de-registration and network-initiated re-authentication to inform the P-CSCF that it should release resources regarding a particular user.

• Session control procedures contain mechanisms for both mobile-originated sessions and mobile-terminated sessions. In mobile-originated sessions the Mw reference point is used to forward requests both from the P-CSCF to the S-CSCF and from the S-CSCF to the I-CSCF. In mobile-terminated sessions the Mw reference point is used to forward requests both from the I-CSCF to the S-CSCF and from the S-CSCF to the P-CSCF. This reference point is also used for network-initiated session releases: for example, the P-CSCF could initiate a session release toward the S-CSCF if it receives an indication from the PDF that media bearer(s) are lost. In addition, charging-related information is conveyed via the Mw reference point.

• Transaction procedures are used to pass a stand-alone request (e.g., MESSAGE) and to receive all responses (e.g., 200 OK) to that request via the Mw reference point. As already stated, the difference between transaction procedures and session control procedures is that a dialog is not created.

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