| Interface Name | IMS entities | Description | Protocol | Technical Specification |
|---|---|---|---|---|
| Cr | MRFC, AS | Used by MRFC to fetch documents (e.g. scripts, announcement files, and other resources) from an AS. Also used for media control related commands. | TCP/SCTP channels | |
| Cx | (I-CSCF, S-CSCF), HSS | Used to send subscriber data to the S-CSCF; including Filter criteria and their priority. Also used to furnish CDF and/or OCF addresses. | Diameter | |
| Dh | AS (SIP AS, OSA, IM-SSF) <-> SLF | Used by AS to find the HSS holding the User Profile information in a multi-HSS environment. DH_SLF_QUERY indicates an IMPU and DX_SLF_RESP return the HSS name. | Diameter | |
| Dx | (I-CSCF or S-CSCF) <-> SLF | Used by I-CSCF or S-CSCF to find a correct HSS in a multi-HSS environment. DX_SLF_QUERY indicates an IMPU and DX_SLF_RESP return the HSS name. | Diameter | |
| Gm | UE, P-CSCF | Used to exchange messages between SIP user equipment (UE) or Voip Gateway and P-CSCF | SIP | |
| Go | PDF, GGSN | Allows operators to control QoS in a user plane and exchange charging correlation information between IMS and GPRS network | COPS (Rel5), Diameter (Rel6+) | |
| Gq | P-CSCF, PDF | Used to exchange policy decisions-related information between P-CSCF and PDF | Diameter | |
| Gx | PCEF, PCRF | Used to exchange policy decisions-related information between PCEF and PCRF | Diameter | TS29.211, TS29.212 |
| Gy | PCEF, OCS | Used for online flow based bearer charging. Functionally equivalent to Ro interface | Diameter | TS23.203, TS32.299 |
| ISC | S-CSCF <-> AS | Reference point between S-CSCF and AS. Main functions are to :
|
SIP | |
| Ici | IBCFs | Used to exchange messages between an IBCF and another IBCF belonging to a different IMS network. | SIP | |
| Izi | TrGWs | Used to forward media streams from a TrGW to another TrGW belonging to a different IMS network. | RTP | |
| Ma | I-CSCF <-> AS | Main functions are to:
|
SIP | |
| Mg | MGCF -> I,S-CSCF | ISUP signalling to SIP signalling and forwards SIP signalling to I-CSCF | SIP | |
| Mi | S-CSCF -> BGCF | Used to exchange messages between S-CSCF and BGCF | SIP | |
| Mj | BGCF -> MGCF | Used for the interworking with the PSTN/CS Domain, when the BGCF has determined that a breakout should occur in the same IMS network to send SIP message from BGCF to MGCF | SIP | |
| Mk | BGCF -> BGCF | Used for the interworking with the PSTN/CS Domain, when the BGCF has determined that a breakout should occur in another IMS network to send SIP message from BGCF to the BGCF in the other network | SIP | |
| Mm | I-CSCF, S-CSCF, external IP network | Used for exchanging messages between IMS and external IP networks | SIP | |
| Mn | MGCF, IM-MGW | Allows control of user-plane resources | H.248 | |
| Mp | MRFC, MRFP | Allows an MRFC to control media stream resources provided by an MRFP. | H.248 | |
| Mr Mr‘ |
S-CSCF, MRFC AS, MRFC |
Used to exchange information between S-CSCF and MRFC Used to exchange session controls between AS and MRFC |
Application Server sends SIP message to MRFC to play tone and announcement. This SIP message contains sufficient information to play tone and announcement or provide information to MRFC, so that it can ask more information from Application Server through Cr Interface. | SIP |
| Mw | P-CSCF, I-CSCF, S-CSCF, AGCF | Used to exchange messages between CSCFs. AGCF appears as a P-CSCF to the other CSCFs | SIP | |
| Mx | BGCF/CSCF, IBCF | Used for the interworking with another IMS network, when the BGCF has determined that a breakout should occur in the other IMS network to send SIP message from BGCF to the IBCF in the other network | SIP | |
| P1 | AGCF, A-MGW | Used for call control services by AGCF to control H.248 A-MGW and Residential Gateways | H.248 | |
| P2 | AGCF, CSCF | Reference point between AGCF and CSCF. | SIP | |
| Rc | MRB, AS | Used by the AS to request that media resources be assigned to a call when utilizing MRB In-Line mode or In Query mode | SIP, In Query mode (Not specified) | |
| Rf | P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS | Used to exchange offline charging information with CDF | Diameter | TS32.299 |
| Ro | AS, MRFC, S-CSCF | Used to exchange online charging information with OCF | Diameter | TS32.299 |
| Rx | P-CSCF, PCRF | Used to exchange policy and charging related information between P-CSCF and PCRFReplacement for the Gq reference point. | Diameter | TS29.214 |
| Sh | AS (SIP AS, OSA SCS), HSS | Used to exchange User Profile information (e.g., user related data, group lists, user service related information or user location information or charging function addresses (used when the AS has not received the third party REGISTER for a user)) between an AS (SIP AS or OSA SCS) and HSS. Also allow AS to activate/deactivate filter criteria stored in the HSS on a per subscriber basis | Diameter | |
| Si | IM-SSF, HSS | Transports CAMEL subscription information including triggers for use by CAMEL based application services information. | MAP | |
| Sr | MRFC, AS | Used by MRFC to fetch documents (scripts and other resources) from an AS | HTTP | |
| Ut | UE and SIP AS (SIP AS, OSA SCS, IM-SSF) PES AS and AGCF | Facilitates the management of subscriber information related to services and settings | HTTP(s), XCAP | |
| z | POTS, Analog phones and VoIP Gateways | Conversion of POTS services to SIP messages |