4 Reference Points. Page 1 of 22. IMS Basics, Version 1.1e. T.O.P. BusinessInteractive GmbH

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1 4 Reference Points Page 1 of 22

2 4.1 IMS Reference Points: Overview IMS Reference Points: Gm IMS Reference Points: Mw IMS Reference Points: Cx IMS Reference Points: Dx IMS Reference Points: ISC IMS Reference Points: Sh IMS Reference Points: Si IMS Reference Points: Dh IMS Reference Points: Ut IMS Reference Points: Mm IMS Reference Points: Mg IMS Reference Points: Mi IMS Reference Points: Mj IMS Reference Points: Mk IMS Reference Points: Mr IMS Reference Points: Mp IMS Reference Points: Mn IMS Reference Points: Go IMS Reference Points: Gq Page 2 of 22

3 4.1 IMS Reference Points: Overview Let us now have a look at the interfaces of the various IMS elements to better understand their functionality. Because the network structure is so complex, we will set some limitations for clarity: We will describe interfaces to Charging Functions and Security Gateway separately We will not differentiate between different types of Application Servers nor show the interfaces between Application and User Plane. Page 3 of 22

4 4.2 IMS Reference Points: Gm The Gm interface connects the UE to the IMS. Please note: Whatever RAN is used, Gm traverses it transparently! Gm carries all SIP signaling messages between the UE and the IMS counterpart which is the P-CSCF. 3 main Gm procedures can be identified: Registration / De-registration Session control and Transactions, i.e., message exchange without previous dialog definition between the entities. Page 4 of 22

5 4.3 IMS Reference Points: Mw The Mw interface then takes over the responsibility for the procedures. P-CSCF, I-CSCF and S-CSCF process Registration / De-registration, session control and transaction messages respectively. Page 5 of 22

6 4.4 IMS Reference Points: Cx HSS permanently stores subscriber and service data that is needed by the I-CSCF and the S-CSCF when the user registers or receives sessions. Therefore, there must be a reference point (Cx) between the HSS and these two CSCFs. In contrast to the previously mentioned interfaces Cx uses the Diameter protocol. This is an evolution of the familiar RADIUS protocol (Remote Authentication Dial-In User Service) and will be described later. Cx procedures can be divided into three main categories: Page 6 of 22

7 4.5 IMS Reference Points: Dx When multiple and separately addressable HSSs have been deployed in a network, I-CSCF or S-CSCF will need help to know which HSS to contact. This is the task of the SLF, a fuction integrated to either I-CSCF or S-CSCF. A dedicated Dx interface is defined and is always used in conjunction with the Cx reference point. It also works using the DIAMETER protocol. As the SLF knows which HSS will host the requested subscriber data it informs the requesting network element to re-direct the Cx messages to the appropriate HSS. Page 7 of 22

8 4.6 IMS Reference Points: ISC Let us now involve the Application Plane with an AS that hosts and executes services, e.g., presence, messaging and session forwarding. An interface is needed to send and receive SIP messages between the I-CSCF, S-CSCF and the AS. This reference point is called the IMS Service Control (ISC) interface. The selected protocol is SIP. ISC procedures can be triggered by either the CSCFs or the AS. Page 8 of 22

9 4.7 IMS Reference Points: Sh An AS may need subscriber data or need to know which S-CSCF to address. This information is stored in the HSS. Hence, a reference point between the HSS and the AS must be established: it's the Sh interface, again using the DIAMETER protocol. Sh procedures are divided into two main groups: data handling and subscription notification The HSS maintains a list of ASs per user which are allowed to obtain or store data. Page 9 of 22

10 4.8 IMS Reference Points: Si When the AS provides the service using CAMEL it is the AS-inherent IM-SSF that communicates with the HSS. Instead of Sh interface, the Si reference point is used to transport CAMEL subscription information including triggers from the HSS to the IM-SSF. The protocol used is SS7 Mobile Application Part (MAP). Page 10 of 22

11 4.9 IMS Reference Points: Dh Again we might encounter the problem of multiple HSSs in the network where the AS cannot know which HSS it needs to contact. Once more the SLF will help out being contacted first via the Dh reference point. As Cx and Dx reference points are always used in conjunction Dh and Sh are always used together by DIAMETER messages. To get an HSS address, the AS sends the Sh request which is aim for the HSS to the SLF. On receipt of the HSS address from the SLF, the AS sends the Sh request to the HSS. Page 11 of 22

12 4.10 IMS Reference Points: Ut The Ut reference point links the UE and the AS. It enables users to surely manage and configure their network services-related information hosted on an AS. HTTP is used for Ut message transfer, hence any protocol chosen for an application that makes use of Ut must be based on HTTP. Page 12 of 22

13 4.11 IMS Reference Points: Mm For communicating with other IP multimedia networks, an interface is needed between both networks. The Mm reference point allows I-CSCF to receive a session request from another SIP server or terminal. Similarly, the S-CSCF uses the Mm reference point to forward IMS UE-originated requests to other multimedia networks. Page 13 of 22

14 4.12 IMS Reference Points: Mg The Mg reference point links the circuit-switched edge function, i.e., the Media Gateway Control Function, to the I-CSCF. This interface allows MGCF to forward incoming session signaling from any CS domain to the I-CSCF. MGCF is responsible for converting incoming SS7 ISUP or Bearer Independent Call Control (BICC) signaling to SIP. This is the protocol used on Mg. Page 14 of 22

15 4.13 IMS Reference Points: Mi When the S-CSCF discovers that a session needs to be routed to a CS domain it uses the Mi interface to forward the session to a Breakout Gateway Control Function, this in turn knows how to route the signaling to the destination circuit-switched network. Page 15 of 22

16 4.14 IMS Reference Points: Mj In order to address the destination CS network BGCF forwards the session to MGCF via the Mj reference point using SIP. Again MGCF converts SIP into SS7 ISUP or BICC. Page 16 of 22

17 4.15 IMS Reference Points: Mk If the destination CS network has to be reached via another IMS the Mk interface between the two BGCFs involved takes care of signaling message transfer using SIP. Page 17 of 22

18 4.16 IMS Reference Points: Mr When the S-CSCF needs to activate bearer-related services, e.g., an announcement, it activates the Media Resource Function Controller (MRFC) via the SIP Mr interface. Page 18 of 22

19 4.17 IMS Reference Points: Mp When the MRFC needs to control media streams it addresses the Media Resource Function Processor (MRFP) within the User Plane using ITU s H.248 protocol. This might be the case if connections for conference media must be established or released. Page 19 of 22

20 4.18 IMS Reference Points: Mn H.248 is also used for signaling between MRFC and the IMS Media Gateway for general user plane resource management. Page 20 of 22

21 4.19 IMS Reference Points: Go In order to provide the GGSN with the allowed service characteristics for a particular user session, the Policy Decision Function submits the needed QoS level information via the Go reference point. Charging correlation information can also be added. The protocol used here is the Common Open Policy Service (COPS) protocol. Page 21 of 22

22 4.20 IMS Reference Points: Gq PDF information about session parameters is also needed in the network element that acts as the primary contact point of the UE. That is the P-CSCF. The Gq reference point caters for this task, using the Diameter protocol. Page 22 of 22