IMS Interfaces and Protocols

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IMS Interfaces and Protocols

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1. IMS Interfaces and Protocols Gonzalo.Camarillo@ericsson.com

2. Contents Interfaces The Cx and Dx interfaces DIAMETER The Sh and Si interfaces The Mw and ISC interfaces The Go interface COPS The Mi, Mj, Mk, Mm, Mr, Mg and Mc interfaces The Ut interface

3. Interfaces of IMS

4. Cx Interface The Cx interface is used for communication between the I-CSCF or S-CSCF and the HSS. The Cx interface supports the following features: - The per user allocation of an S-CSCF, at the I-CSCF, based on information provided by the HSS. - Authorisation of the subscriber access to the IMS network (SIP registration). - Procedures related to the passing of routing information from the HSS to the CSCF - Performs Authentication: transfer of security parameters for a subscriber between HSS and CSCF - Enables the S-CSCF to identify the application servers needed for a user - Supports filter control: transfer of filter parameters for the subscriber from HSS to CSCF - Retrieve the subscriber service profile from the HSS. 3GPP has specified the DIAMETER protocol to enable this interface. A Diameter Multimedia Application has been specified. The Diameter command User-Authorization-Request/Answer is used. The service profile defines the service handling for a user at the S-CSCF. The profile itself is a structured data format with the following information elements: Application address: The SIP URI the S-CSCF uses to address the application Filter criteria: This element specifies the conditions for the invocation of the application. The condition is formulated as a filter rule, which can be applied to a SIP request (e.g. request method, presence or absence of header, header contents, etc.). The filter ?language? allows formulating nearly arbitrary complex invocation rules. Application priority: The priority of an application determines the sequence of application execution in case the invocation conditions of multiple applications match a single SIP request received by the S-CSCF. The Cx interface is used for communication between the I-CSCF or S-CSCF and the HSS. The Cx interface supports the following features: - The per user allocation of an S-CSCF, at the I-CSCF, based on information provided by the HSS. - Authorisation of the subscriber access to the IMS network (SIP registration). - Procedures related to the passing of routing information from the HSS to the CSCF - Performs Authentication: transfer of security parameters for a subscriber between HSS and CSCF - Enables the S-CSCF to identify the application servers needed for a user - Supports filter control: transfer of filter parameters for the subscriber from HSS to CSCF - Retrieve the subscriber service profile from the HSS. 3GPP has specified the DIAMETER protocol to enable this interface. A Diameter Multimedia Application has been specified. The Diameter command User-Authorization-Request/Answer is used. The service profile defines the service handling for a user at the S-CSCF. The profile itself is a structured data format with the following information elements: Application address: The SIP URI the S-CSCF uses to address the application Filter criteria: This element specifies the conditions for the invocation of the application. The condition is formulated as a filter rule, which can be applied to a SIP request (e.g. request method, presence or absence of header, header contents, etc.). The filter ?language? allows formulating nearly arbitrary complex invocation rules. Application priority: The priority of an application determines the sequence of application execution in case the invocation conditions of multiple applications match a single SIP request received by the S-CSCF.

5. Dx Interface This interface is used for the communication between the I-CSCF or S-CSCF and the SLF (Subscription Location Function), where the SLF provides the address for the user?s HSS in a multi HSS environment. ? Different mechanisms may be deployed to resolve the user?s HSS: The standardised mechanism is to solve the routing towards the HSS using the SLF and the Dx interface. The CSCF queries the SLF using the user?s identity and the SLR delivers the users HSS address. Alternatively a proxy containing a SLF, able to accept all Diameter Cx requests and route them towards the correct HSS could be used. The standards do not prohibit the use of OA&M or propriety methods to have local databases or look-up tables at the x-CSCF to identify the user?s HSS from the user identity. The protocol used on the Dx interface is the DIAMETER protocol. A Diameter Multimedia Application has been specifiedThis interface is used for the communication between the I-CSCF or S-CSCF and the SLF (Subscription Location Function), where the SLF provides the address for the user?s HSS in a multi HSS environment. ? Different mechanisms may be deployed to resolve the user?s HSS: The standardised mechanism is to solve the routing towards the HSS using the SLF and the Dx interface. The CSCF queries the SLF using the user?s identity and the SLR delivers the users HSS address. Alternatively a proxy containing a SLF, able to accept all Diameter Cx requests and route them towards the correct HSS could be used.The standards do not prohibit the use of OA&M or propriety methods to have local databases or look-up tables at the x-CSCF to identify the user?s HSS from the user identity. The protocol used on the Dx interface is the DIAMETER protocol. A Diameter Multimedia Application has been specified

6. DIAMETER protocol

7. Sh and Si Interfaces Sh interface This is the interface between an Application Server and the HSS allowing application servers to access to HSS. The interface will support: - Request for subscription and service related information from the HSS. - Modify or Add subscription and service related information in the HSS. - Notification about the modification of a subscription in the HSS, by the HSS. - Transfer of transparent data from the HSS to AS. Transparent data refers to data stored at the HSS related to a service, but not understood by the HSS or the Sh interface and only understood by the application servers. The protocol used on the Sh interface is Diameter. Si Interface This is the interface between the HSS and IM-SSF. The IM-SSF can query CAMEL subscription information from the HSS and the Si interface supports CAMEL subscription information including triggers for use by CAMEL based application services. The function of the Si interface is to enable legacy services on an IMS network. We can consider the set IMS-SSF as an ?Application Server? and so the Si interface allows SCP to control the session. The scope of this interface is limited to the current set of legacy services and it is not envisaged that new services will be offered on the Camel platforms. The protocol used on this interface is Diameter. Sh interface This is the interface between an Application Server and the HSS allowing application servers to access to HSS. The interface will support: - Request for subscription and service related information from the HSS. - Modify or Add subscription and service related information in the HSS. - Notification about the modification of a subscription in the HSS, by the HSS. - Transfer of transparent data from the HSS to AS. Transparent data refers to data stored at the HSS related to a service, but not understood by the HSS or the Sh interface and only understood by the application servers. The protocol used on the Sh interface is Diameter.

8. Mw and ISC Interfaces Mw interface This is the interface between the x-CSCF Elements. These CSCFs can be in the same network or in different networks.This interface is defined to enable communication between CSCFs. For example, an I-CSCF could use this interface to direct a mobile terminated call to the intended S-CSCF or during registration and session control. Mw is a SIP based interface so the CSCF can be considered as Proxy Server from the point of view of SIP Signalling. ? ISC Interface ISC Interface stands for IP multimedia Subsystem Service Control and is the interface between S-CSCF and Service Platforms. This interface is used to offer to the user value added services residing in the Application Server. The ISC will allow the session to be controlled by the Application Server and will allow the S-CSCF to delegate the service control to an AS. The following requirements apply to the ISC interface: The ISC interface shall be able to convey charging information. The CSCF provides the AS an ICID (identifier). The AS use that ICID to identify own CDR?s produced for that Session. The protocol on the ISC interface shall allow the S-CSCF to differentiate between SIP requests on Mw, Mm and Mg interfaces and SIP Requests on the ISC interface. ? The ISC interface is based on SIP. Mw interface This is the interface between the x-CSCF Elements. These CSCFs can be in the same network or in different networks.This interface is defined to enable communication between CSCFs. For example, an I-CSCF could use this interface to direct a mobile terminated call to the intended S-CSCF or during registration and session control. Mw is a SIP based interface so the CSCF can be considered as Proxy Server from the point of view of SIP Signalling. ? ISC Interface ISC Interface stands for IP multimedia Subsystem Service Control and is the interface between S-CSCF and Service Platforms. This interface is used to offer to the user value added services residing in the Application Server. The ISC will allow the session to be controlled by the Application Server and will allow the S-CSCF to delegate the service control to an AS. The following requirements apply to the ISC interface: The ISC interface shall be able to convey charging information. The CSCF provides the AS an ICID (identifier). The AS use that ICID to identify own CDR?s produced for that Session. The protocol on the ISC interface shall allow the S-CSCF to differentiate between SIP requests on Mw, Mm and Mg interfaces and SIP Requests on the ISC interface. ? The ISC interface is based on SIP.

9. Go Interface Go interface In IMS, all services will be delivered through a single IMS APN. With IMS, the packet domain becomes service oriented and no longer bearer oriented. It is the network that decides on the appropriate bearer capabilities for services. If the user requests a bearer different to those recommended by the network, the bearer is renegotiated to the appropriate levels or the request rejected. To enable these policy enforcement and management capabilities, during session setup, the P-CSCF will provide the UE with a token. The UE, when requesting a PDP Context, sends this token to the GGSN. The GGSN asks for Authorization from the PDF using this token. The exchange of signalling between the PDF (service control plane) and GGSN (bearer plane) is done on the Go Interface using the COPS Protocol. The GGSN is the policy enforcement point which is managed over the Go interface by the PDF. The set of functionalities that the Go interface shall support are:? - Media authorization request from GGSN - The interface will support the request for authorisation of resources by the GGSN from the PDF. - Media authorization decision from PDF - The interface will support the delivery of resource authorisation from the PDF to the GGSN. - Enable charging coordination between the IMS and GGSN. - Approval of QoS Commit / Removal of QoS Commit / Revoke Authorization for GPRS and IP resources - The PDF may at anytime beyond the initial authorisation of the resources, modify or revoke the authorisation of resources. - Indication of exceptional situations in the access network to the IMS system ? for example, loss of radio coverage. ? The Go interface uses the COPS protocol.Go interface In IMS, all services will be delivered through a single IMS APN. With IMS, the packet domain becomes service oriented and no longer bearer oriented. It is the network that decides on the appropriate bearer capabilities for services. If the user requests a bearer different to those recommended by the network, the bearer is renegotiated to the appropriate levels or the request rejected. To enable these policy enforcement and management capabilities, during session setup, the P-CSCF will provide the UE with a token. The UE, when requesting a PDP Context, sends this token to the GGSN. The GGSN asks for Authorization from the PDF using this token. The exchange of signalling between the PDF (service control plane) and GGSN (bearer plane) is done on the Go Interface using the COPS Protocol. The GGSN is the policy enforcement point which is managed over the Go interface by the PDF. The set of functionalities that the Go interface shall support are:? - Media authorization request from GGSN - The interface will support the request for authorisation of resources by the GGSN from the PDF. - Media authorization decision from PDF - The interface will support the delivery of resource authorisation from the PDF to the GGSN. - Enable charging coordination between the IMS and GGSN. - Approval of QoS Commit / Removal of QoS Commit / Revoke Authorization for GPRS and IP resources - The PDF may at anytime beyond the initial authorisation of the resources, modify or revoke the authorisation of resources. - Indication of exceptional situations in the access network to the IMS system ? for example, loss of radio coverage. ? The Go interface uses the COPS protocol.

10. COPS protocol

11. Mi,Mj,Mk,Mm,Mr, Mg & Mc reference points These interfaces interconnect respectively: Mi (CSCF-BGCF) S-CSCF forwards the session signalling to the BGCF for the purpose of interworking to PSTN networks Mj (MGCF-BGCF) BGCF forwards the session signalling to the MGCF for the purpose of interworking to PSTN networks Mk (BGCF-BGCF) A BGCF forwards the session signalling to another BGCF Mm (CSCF-other Sip Network) This interface is used, for example, to receive a call request from another VoIP call control server or terminal. Signalling messages passing between IP Networks, specifically between CSCFs in different networks, would have to be sent over this interface. Mr (CSCF-MRFC) The CSCF communicates with the MRF over this interface to set up resources for services such as announcements, conference calling and transcoding. Mg (MGCF-CSCF) This interface is used for interworking with legacy networks. All the above interfaces use the SIP Protocol. ? Mc (MGCF-MGW) The Mc interface facilitates the management of media and transport resources on the media gateway by the MGFC. The MGCF initiates interactions to establish, modify and release connections at the MGW. It also instructs the MGW to reserve the resources necessary for a session. The Mc interface uses the H.248 (Megaco) protocol. Mp (MRFC-MRFP). The Mp interface allows an MRFC to control the media stream resources provided by an MRFPThese interfaces interconnect respectively: Mi (CSCF-BGCF) S-CSCF forwards the session signalling to the BGCF for the purpose of interworking to PSTN networks Mj (MGCF-BGCF) BGCF forwards the session signalling to the MGCF for the purpose of interworking to PSTN networks Mk (BGCF-BGCF) A BGCF forwards the session signalling to another BGCF Mm (CSCF-other Sip Network) This interface is used, for example, to receive a call request from another VoIP call control server or terminal. Signalling messages passing between IP Networks, specifically between CSCFs in different networks, would have to be sent over this interface. Mr (CSCF-MRFC) The CSCF communicates with the MRF over this interface to set up resources for services such as announcements, conference calling and transcoding. Mg (MGCF-CSCF) This interface is used for interworking with legacy networks. All the above interfaces use the SIP Protocol. ? Mc (MGCF-MGW) The Mc interface facilitates the management of media and transport resources on the media gateway by the MGFC. The MGCF initiates interactions to establish, modify and release connections at the MGW. It also instructs the MGW to reserve the resources necessary for a session. The Mc interface uses the H.248 (Megaco) protocol. Mp (MRFC-MRFP). The Mp interface allows an MRFC to control the media stream resources provided by an MRFP

12. The Ut interface The Ut interface is defined between an Application Server and the terminal. It is available since Release 6. Used to do some sort of data manipulation, such us: Configuration of resource lists Authorization of watchers Others not related to presence (e.g, configuration of conference participants) The protocol uses HTTP as a transport protocol. On top of HTTP there is an XML based protocol named XCAP (XML Configuration Access Protocol) On top of XCAP there are "usages of XCAP" for each particular usage The Ut interface is defined between an Application Server and the terminal. It is available since Release 6. Used to do some sort of data manipulation, such us: Configuration of resource lists Authorization of watchers Others not related to presence (e.g, configuration of conference participants) The protocol uses HTTP as a transport protocol. On top of HTTP there is an XML based protocol named XCAP (XML Configuration Access Protocol) On top of XCAP there are "usages of XCAP" for each particular usage


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