3GPP Release 5 IP Multimedia Subsystem (IMS) Architecture

1. IMS Entities

• HSS – Home Subscriber Server
• CSCF – Call Session Control Function
• P-CSCF – Proxy CSCF
• S-CSCF – Serving CSCF
• I-CSCF – Interrogating CSCF
• MGCF – Media Gateway Control Function
• IMS-MGW – IP Multimedia Subsystem – Media Gateway Function
• MRFC – Multimedia Resource Function Controller
• MRFP – Multimedia Resource Function Processor
• SLF – Subscription Location Function
• BGCF – Breakout Gateway Control Function
• AS – Application Server

2. IMS Reference Points

• Cx reference point: Supports information transfer between CSCF and HSS.
• Gm reference point: Supports communication between the MT and the IMS.
• Mn reference point: Communication interface between the MGCF and IMS-MGW. Equivalent to the Mc interface in the CS domain.
• Mg reference point: Allows the MGCF to forward incoming session signalling (from the PSTN) to the CSCF for the purpose of interworking with PSTN networks. Uses SIP for signalling.
• Mr reference point: Supports information transfer between CSCF and MRFC. Uses SIP for signalling.
• Mp reference point: Allows an MRFC to control media stream resources provided by an MRF. Similar to the Mc interface in the CS domain.
• Mw reference point: Allows the communication and forwarding of SIP signalling messaging between CSCFs.
• Go reference point: Allows the Policy Decision Function (PDF) to apply policy decisions to bearer usage in the GGSN. The Policy Decision Function (PDF) is a logical entity in the P-CSCF.
• Mi reference point: Allows the Serving CSCF to forward the session signalling to the BGCF for the purpose of interworking with PSTN networks. Uses SIP for signalling.
• Mj reference point: Allows the BGCF to forward session signalling to the MGCF for the purpose of interworking with PSTN networks. Uses SIP for signalling.
• Mk reference point: Allows the BGCF to forward session signalling to another BGCF. Uses SIP for signalling.
• Dx reference point: The CSCF and SLF interface is used to retrieve the address of the HSS which holds the subscription date for a given user. Not required in a single HSS environment.
• Mb reference point: Used to access IPv6 network services for user data transport.
• ISC reference point: The interface between the CSCF and application servers for access to IMS services.
• Sh reference point: Used for communication from the SIP or OSA application server to the HSS.
• Si reference point: Used for communication from the CAMEL application server to the HSS.