Gm reference point

The Gm reference point connects the UE to the IMS. It is used to transport all SIP signalling messages between the UE and the IMS. The IMS counterpart is P-CSCF. Procedures in the Gm reference point can be divided into three main categories: registration, session control and transactions:

• In the registration procedure the UE uses the Gm reference point to send a registration request with an indication of supported security mechanisms to the P-CSCF. During the registration process the UE exchanges the necessary parameters for authenticating both itself and the network, gets implicit registered user identities, negotiates the necessary parameters for a security association with the P-CSCF and possibly starts SIP compression. In addition, the Gm reference point is used to inform the UE if network-initiated de-registration or network-initiated re-authentication occurs.

• Session control procedures contain mechanisms for both mobile-originated sessions and mobile-terminated sessions. In mobile-originated sessions the Gm reference point is used to forward requests from the UE to the P-CSCF. In mobile-terminated sessions the Gm reference point is used to forward request from the P-CSCF to the UE.

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

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