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4 Reference Points

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4.1 IMS Reference Points: Overview....................................................34.2 IMS Reference Points: Gm............................................................. 44.3 IMS Reference Points: Mw.............................................................5

4.4 IMS Reference Points: Cx ..............................................................64.5 IMS Reference Points: Dx ..............................................................74.6 IMS Reference Points: ISC............................................................. 84.7 IMS Reference Points: Sh ..............................................................94.8 IMS Reference Points: Si .............................................................104.9 IMS Reference Points: Dh............................................................114.10 IMS Reference Points: Ut ...........................................................124.11 IMS Reference Points: Mm.........................................................134.12 IMS Reference Points: Mg..........................................................14

4.13 IMS Reference Points: Mi...........................................................154.14 IMS Reference Points: Mj...........................................................164.15 IMS Reference Points: Mk..........................................................174.16 IMS Reference Points: Mr ..........................................................184.17 IMS Reference Points: Mp..........................................................194.18 IMS Reference Points: Mn..........................................................204.19 IMS Reference Points: Go.......................................................... 214.20 IMS Reference Points: Gq.......................................................... 22

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4.1 IMS Reference Points: Overview

Let us now have a look at the interfaces of the various IMS elements to better understandtheir functionality. Because the network structure is so complex, we will set some limitationsfor 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.

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4.2 IMS Reference Points: Gm

The Gm interface connects the UE to the IMS. Please note: Whatever RAN is used, Gmtraverses it transparently! Gm carries all SIP signaling messages between the UE and theIMS 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.

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4.3 IMS Reference Points: Mw

The Mw interface then takes over the responsibility for the procedures. P-CSCF, I-CSCF andS-CSCF process Registration / De-registration, session control and transaction messagesrespectively.

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4.4 IMS Reference Points: Cx

HSS permanently stores subscriber and service data that is needed by the I-CSCF and theS-CSCF when the user registers or receives sessions. Therefore, there must be a referencepoint (Cx) between the HSS and these two CSCFs. In contrast to the previously mentionedinterfaces Cx uses the Diameter protocol. This is an evolution of the familiar RADIUSprotocol (Remote Authentication Dial-In User Service) and will be described later. Cxprocedures can be divided into three main categories:

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4.5 IMS Reference Points: Dx

When multiple and separately addressable HSSs have been deployed in a network, I-CSCFor S-CSCF will need help to know which HSS to contact. This is the task of the SLF, a fuctionintegrated to either I-CSCF or S-CSCF. A dedicated Dx interface is defined and is alwaysused 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 therequesting network element to re-direct the Cx messages to the appropriate HSS.

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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 receiveSIP messages between the I-CSCF, S-CSCF and the AS. This reference point is called theIMS Service Control (ISC) interface. The selected protocol is SIP. ISC procedures can betriggered by either the CSCFs or the AS.

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4.7 IMS Reference Points: Sh

An AS may need subscriber data or need to know which S-CSCF to address. Thisinformation is stored in the HSS. Hence, a reference point between the HSS and the ASmust be established: it's the Sh interface, again using the DIAMETER protocol. Shprocedures 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.

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4.9 IMS Reference Points: Dh

Again we might encounter the problem of multiple HSSs in the network where the AS cannotknow which HSS it needs to contact. Once more the SLF will help out being contacted firstvia the Dh reference point. As Cx and Dx reference points are always used in conjunction Dhand Sh are always used together by DIAMETER messages. To get an HSS address, the ASsends the Sh request which is aim for the HSS to the SLF. On receipt of the HSS addressfrom the SLF, the AS sends the Sh request to the HSS.

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4.10 IMS Reference Points: Ut

The Ut reference point links the UE and the AS. It enables users to surely manage andconfigure their network services-related information hosted on an AS. HTTP is used for Utmessage transfer, hence any protocol chosen for an application that makes use of Ut mustbe based on HTTP.

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4.11 IMS Reference Points: Mm

For communicating with other IP multimedia networks, an interface is needed between bothnetworks. The Mm reference point allows I-CSCF to receive a session request from anotherSIP server or terminal. Similarly, the S-CSCF uses the Mm reference point to forward IMSUE-originated requests to other multimedia networks.

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4.12 IMS Reference Points: Mg

The Mg reference point links the circuit-switched edge function, i.e., the Media GatewayControl Function, to the I-CSCF. This interface allows MGCF to forward incoming sessionsignaling from any CS domain to the I-CSCF. MGCF is responsible for converting incomingSS7 ISUP or Bearer Independent Call Control (BICC) signaling to SIP. This is the protocolused on Mg.

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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 Miinterface to forward the session to a Breakout Gateway Control Function, this in turn knowshow to route the signaling to the destination circuit-switched network.

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4.14 IMS Reference Points: Mj

In order to address the destination CS network BGCF forwards the session to MGCF via theMj reference point using SIP. Again MGCF converts SIP into SS7 ISUP or BICC.

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4.15 IMS Reference Points: Mk

If the destination CS network has to be reached via another IMS the Mk interface betweenthe two BGCFs involved takes care of signaling message transfer using SIP.

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4.16 IMS Reference Points: Mr

When the S-CSCF needs to activate bearer-related services, e.g., an announcement, itactivates the Media Resource Function Controller (MRFC) via the SIP Mr interface.

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4.17 IMS Reference Points: Mp

When the MRFC needs to control media streams it addresses the Media Resource FunctionProcessor (MRFP) within the User Plane using ITUs H.248 protocol. This might be the caseif connections for conference media must be established or released.

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4.18 IMS Reference Points: Mn

H.248 is also used for signaling between MRFC and the IMS Media Gateway for generaluser plane resource management.

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4.19 IMS Reference Points: Go

In order to provide the GGSN with the allowed service characteristics for a particular usersession, 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 hereis the Common Open Policy Service (COPS) protocol.

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4.20 IMS Reference Points: Gq

PDF information about session parameters is also needed in the network element that actsas the primary contact point of the UE. That is the P-CSCF. The Gq reference point catersfor this task, using the Diameter protocol.