Diameter in the IMS

                The 3GPP standards body has adopted Diameter as the primary signaling protocol for AAA and mobility management in the IP Multimedia Subsystem (IMS). The IMS is the service delivery environment for real-time multimedia services for the 3rd Generation wireless network and is also an ideal architecture for service convergence across fixed, wireless, and corporate enterprise networks. IMS offers a secure, mediated access to value-added services and network resources inside a managed core IP network while providing open interfaces for third party developers and enterprise-based applications that need access to the trusted carrier.

The following diagram is an example of how Diameter is used in the IMS network.

The Home Subscriber Server (HSS) is the master database within the IMS for maintaining subscriber information including user identification, control information for user authentication and authorization, location information, and user profile data.

The Call Session Control Function (CSCF) performs SIP session management for a user (or SIP client) requesting access to IMS services. SIP signaling is used to register with the Serving CSCF in the home network of the user. The S-CSCF uses the Diameter Cx interface both to request authorization information from the HSS in response to a SIP registration request and to retrieve subscriber information. The Diameter Dx interface is used by the CSCF to locate the HSS serving the subscriber using the Service Location Feature.

The Diameter Sh interface is used by the Application Servers or OSA/Parlay Gateway to retrieve and update subscriber profiles and shared database information such as call screening lists, directories, or presence information. This information is unique to a subscriber but may require shared access by various authorized application platforms.

IIn addition to the Diameter applications shown above, IMS charging is implemented using the Diameter Ro and Rf interfaces. These interfaces support both on-line charging and off-line collection and forwarding of Call Detail Records using a Diameter protocol interface. The Ro interface is a 3GPP extension to the Diameter Credit-Control draft specification currently under development in the IETF.

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