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Interfaces description | Interface Name | IMS entities | Description | Protocol | | Cr | MRFC, AS | Used by MRFC to fetch documents (scripts and other resources) from an AS | HTTP over dedicated TCP/SCTP channels | | Cx | I-CSCF, S-CSCF, HSS | Used to communicate between I-CSCF/S-CSCF and HSS | Diameter | | Dh | SIP AS, OSA, SCF, IM-SSF, HSS | Used by AS to find a correct HSS in a multi-HSS environment | Diameter | | Dx | I-CSCF, S-CSCF, SLF | Used by I-CSCF/S-CSCF to find a correct HSS in a multi-HSS environment | Diameter | | Gm | UE, P-CSCF | Used to exchange messages between UE and CSCFs | 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 | | ISC | S-CSCF, I-CSCF, AS | Used to exchange messages between CSCF and AS | SIP | | Ma | I-CSCF -> AS | Used to directly forward SIP requests which are destinated to a Public Service Identity hosted by the AS | SIP | | Mg | MGCF -> I-CSCF | MGCF converts 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 to exchange messages between BGCF and MGCF in the same IMS network | SIP | | Mk | BGCF -> BGCF | Used to exchange messages between BGCFs in different IMS networks | SIP | | Mm | I-CSCF, S-CSCF, external IP network | Used for exchanging messages between IMS and external IP networks | Not specified | | Mn | MGCF, IM-MGW | Allows control of user-plane resources | H.248 | | Mp | MRFC, MRFP | Used to exchange messages between MRFC and MRFP | H.248 | | Mr | S-CSCF, MRFC | Used to exchange messages between S-CSCF and MRFC | SIP | | Mw | P-CSCF, I-CSCF, S-CSCF | Used to exchange messages between CSCFs | SIP | | Rf | P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS | Used to exchange offline charging information with CCF | Diameter | | Ro | AS, MRFC | Used to exchange online charging information with ECF | Diameter | | Sh | SIP AS, OSA SCS, HSS | Used to exchange information between SIP AS/OSA SCS and HSS | Diameter | | Si | IM-SSF, HSS | Used to exchange information between IM-SSF and HSS | MAP | | Sr | MRFC, AS | Used by MRFC to fetch documents (scripts and other resources) from an AS | HTTP | | Ut | UE, AS (SIP AS, OSA SCS, IM-SSF) | Enables UE to manage information related to his services | HTTP(s) |
Security aspects of early IMS systems It is envisaged that security defined in TS 33.203 may not be available for a while especially because of the lack of USIM/ISIM interfaces and prevalence of devices that support IPv4. For this situation, to provide some protection against the most significant threats, 3GPP defines some security mechanisms, which are informally known as "early IMS security", in TR33.978.
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