IP Multimedia SubsystemArchitecture

2.1 Architectural Requirements2.2 Description of IMS-Related Entities and Functionalities2.3 IMS Reference Points

2.1 Architectural RequirementsThird Generation Partnership Project (3GPP) stage 1 IMS requirements are documented in [3GPP TS 22.228]

2.1.1 IP Connectivity2.1.2 Access Independence2.1.3 Ensuring Quality of Service for IP multimedia services2.1.4 IP Policy Control for Ensuring Correct Usage of Media Resources2.1.5 Secure Communication

2.1.6 Charging Arrangements2.1.7 Support of Roaming2.1.8 Interworking with other Networks2.1.9 Service Control Model2.1.10 Service Development2.1.11 Layered Design

2.1.1 IP connectivityA fundamental requirement is that a client has to have IP connectivity to access IMS servicesIn addition, it is required that IPv6 is used [3GPP TS 23.221]

The IMS connectivity options when a user is roamingIP connectivity can be obtained either from the home network or the visited network[IMS roaming]UE has obtained an IP address from a visited networkRAN, SGSN and GGSN are located in the visited network when a user is roaming in the visited network

[GPRS roaming]UE has obtained an IP address from the home networkRAN and SGSN are located in the visited network when a user is roaming in the visited network

2.1.2 Access independenceIMS is designed to be access-independentIMS services can be provided over any IP connectivity networkse.g., GPRS, WLAN, broadband access x-Digital Subscriber Line3GPP uses the term "IP connectivity access network to refer to the collection of network entities and interfaces that provides the underlying IP transport connectivity between UE and IMS entities

2.1.3 Ensuring quality of service for IP multimedia servicesOn the public Internetdelays tend to be high and variablepackets arrive out of ordersome packets are lost or discardedNo longer be the case with IMSthe underlying access and transport networks together with IMS provide end-to-end quality of service (QoS)

Via IMSUE negotiates its capabilities and expresses its QoS requirements during a Session Initiation Protocol (SIP) session set-up or session modification procedure

UE is able to negotiate such parameters asmedia typedirection of trafficmedia type bit ratepacket sizepacket transport frequencyusage of RTP payload for media typesbandwidth adaptation

After negotiating the parameters at the application levelUEs reserve suitable resources from the access networkWhen end-to-end QoS is createdUEs encode and packetize individual media types with an appropriate protocol (e.g., RTP)send these media packets to the access and transport network by using a transport layer protocol (e.g., TCP or UDP) over IP

2.1.4 IP policy control for ensuring correct usage of media resourcesIP policy controlthe capability to authorize and control the usage of bearer traffic based on the signaling parameters at the IMS sessionThis requires interaction between the IP connectivity access network and the IMS

The means of setting up interaction can be divided into three different categories [3GPP TS 22.228, 23.207, 23.228]the policy control element is able to verify that values negotiated in SIP signaling are used when activating bearers for media trafficthis allows an operator to verify that its bearer resources are not misused e.g., the source and destination IP address and bandwidth in the bearer level are exactly the same as used in SIP session establishment

the policy control element is able to enforce when media traffic between end points of a SIP session start or stopthis makes it possible to prevent the use of the bearer until the session establishment is completedallows traffic to start/stop in synchronization with the start/stop of charging for a session in IMS

the policy control element is able to receive notifications when the IP connectivity access network service has either modified, suspended or released the bearer(s) of a user associated with a sessionthis allows IMS to release ongoing session because, for instance, the user is no longer in the coverage area

2.1.5 Secure communicationIMS provides at least a similar level of security as the corresponding GPRS and circuit-switched networksfor exampleIMS ensures that users are authenticated before they can start using servicesusers are able to request privacy when engaged in a session

2.1.6 Charging arrangementsIMS architecture allows different charging models to be usedthe capability to charge just the calling party or to charge both the calling party and the called party based on used resources in the transport level

As IMS sessions may include multiple media components (e.g., audio and video)it is required that the IMS provides a means for charging per media componentIt is also required that different IMS networks are able to exchange information on the charging to be applied to a current session [3GPP TS 22.101, TR 23.815]

IMS architecture supports both online and offline charging capabilitiesonline charginga charging process in which the charging information can affect in real time the service rendered and therefore directly interacts with session/service control

in practice, an operator could check the user's account before allowing the user to engage a session and to stop a session when all credits are consumedprepaid services are applications that need online charging capabilities

offline charginga charging process in which the charging information does not affect in real time the service renderedthis is the traditional model in which the charging information is collected over a particular period and, at the end of the period, the operator posts a bill to the customer

2.1.7 Support of roamingRoaminguse services even though the user is not geographically located in the service area of the home networkTypes of roaming to access the IMSGPRS roamingIMS roamingIMS circuit-switched (CS) roaming

GPRS roamingthe visited network provides RAN and SGSNthe home network provides GGSN and IMSIMS roamingthe visited network provides IP connectivity (e.g., RAN, SGSN, GGSN) and the IMS entry point (i.e., P-CSCF)the home network provides the rest of the IMS functionalities

IMS circuit-switched (CS) roamingroaming between the IMS and the CS CN domain refers to inter-domain roaming between IMS and CSwhen a user is not registered or reachable in one domain a session can be routed to the other domainboth the CS CN domain and the IMS domain have their own services and cannot be used from another domainsome services are similar and available in both domains (e.g., Voice over IP in IMS and speech telephony in CS CN)

2.1.8 Interworking with other networksTo be a new, successful communication network technology and architecture, the IMS has to be able to connect to as many users as possibleIMS supports communication with PSTN, ISDN, mobile and Internet usersadditionally, it is able to support sessions with Internet applications that have been developed outside the 3GPP community [3GPP TS 22.228]

2.1.9 Service control modelIn 2G mobile networks the visited service control is in usewhen a user is roaming, an entity (visited mobile service switching centre) in the visited network provides services and controls the traffic for the userIn the early days of Release 5 both visited and home service control models were supportedsupporting two models would have required that every problem have more than one solutionit would reduce the number of optimal architecture solutions, as simple solutions may not fit both models

supporting both models would have meant additional extensions for IETF protocols and increased the work involved in registration and session flowsIn the latter days of Release 5 the visited service control was dropped because it was a complex solution and did not provide any noticeable added value compared with the home service control

service development is slower as both the visited and home network would need to support similar services, otherwise roaming users would experience service degradationsin addition, the number of interoperator reference points increase, which requires complicated solutions (e.g., in terms of security and charging)

therefore, the home service control was selectedthe entity that has access to the subscriber database and interacts directly with service platforms is always located at the user's home network

2.1.10 Service development3GPP is standardizing service capabilities and not the services themselves [3GPP TS 22.101]IMS architecture should include a service framework that provides the necessary capabilities to support the following services within the IMSspeech, video, multimedia, messaging, file sharing, data transfer, gaming and basic supplementary services

2.1.11 Layered design3GPP uses a layered approach to architectural designtransport and bearer services are separated from the IMS signaling network and session management servicesfurther services are run on top of the IMS signaling networkfigure 2.3 shows the design

Signaling planeTransport plane

BGCFBreakout Gateway Control FunctionCSCFCall Session Control FunctionIM-MGWIP Multimedia-Media Gateway FunctionMGCFMedia Gateway Control FunctionMRFCMultimedia Resource Function ControllerMRFPMedia Resource Function ProcessorSEGSecurity GatewaySGWSignaling Gateway

The layered approach aims at a minimum dependency between layersA benefit is that it facilitates the addition of new access networks to the system later onWLAN access to the IMS, in 3GPP Release 6other accesses may follow (e.g., fixed broadband)

The layered approach increases the importance of the application layerwhen applications are isolated and common functionalities can be provided by the underlying IMS network the same applications can run on UE using diverse access types

2.2 Description of IMS-related entities and functionalitiesSix main categories of entitiessession management and routing family (CSCFs)databases (HSS, SLF)interworking elements (BGCF, MGCF, IM-MGW, SGW)services (application server, MRFC, MRFP)support entities (THIG, SEG, PDF)charging

3GPP standards describe reference points between entitiesfunctionalities supported at the reference points (e.g., how does CSCF obtain user data from HSS)

2.2.1 Proxy-CSCF2.2.2 Policy Decision Function2.2.3 Interrogating-CSCF2.2.4 Serving-CSCF2.2.5 Home Subscriber Server2.2.6 Subscription Locator Function2.2.7 Multimedia Resource Function Controller2.2.8 Multimedia Resource Function Processor

2.2.9 Application server2.2.10 Breakout Gateway Control Function2.2.11 Media Gateway Control Function2.2.12 IP Multimedia Subsystem-Media Gateway Function2.2.13 Signaling gateway2.2.14 Security gateway2.2.15 Charging entities2.2.16 GPRS service entities

UMTS/GPRS/WLAN Access to IMS

IMS decomposes the networking infrastructure into separate functions with standardized interfaces between themEach interface is specified as a "reference pointwhich defines both the protocol over the interface and the functions between which it operates

3GPP architecture is split into three main planes or layersService (or Application) PlaneControl (or Signaling) PlaneUser (or Transport) Plane

Application planeprovides an infrastructure for the provision and management of servicesdefines standard interfaces to common functionality including configuration storage, identity management, user status (such as presence and location), which is held by the Home Subscriber Server (HSS)billing services, provided by a Charging Gateway Function (CGF) (not shown)control of voice and video calls and messaging, provided by the control plane

Control planesits between the application and transport planesroutes the call signaling, tells the transport plane what traffic to allow, and generates billing information for the use of the networkthe core of this plane is Call Session Control Function (CSCF), which comprises the following functionsProxy-CSCF (P-CSCF)Interrogating-CSCF (I-CSCF)Serving-CSCF (S-CSCF)

Control Plane also controls User Plane traffic through Resource and Admission Control Subsystem (RACS), which consists ofPolicy Decision Function (PDF)which implements local policy on resource usage, for example to prevent overload of particular access linksAccess-RAC Function (A-RACF)which controls QoS within the access network

User planeprovides a core QoS-enabled IPv6 network with access from UE over mobile, WiFi and broadband networksaccess into the core network is through Border Gateways (GGSN/PDG (Packet Data Gateway)/BAS (Broadband Access Server))these enforce policy provided by the IMS core, controlling traffic flows between the access and core networks

Border Gateways (GGSN/PDG/BAS)

within the User PlaneInterconnect Border Control Function (I-BCF) controls transport level security and tells the RACS what resources are required for a call I-BGF, A-BGF Border Gateway Functions provide media relay for hiding endpoint addresses with managed pinholes to prevent bandwidth theftimplement NAPT (Network Address Port Translation) and NAT (Network Address Translation)/Firewall traversal for media flows

2.2.1 Proxy-CSCFProxy-Call Session Control Function (P-CSCF)the first contact point for users within the IMSall SIP signaling traffic from or to the UE go via the P-CSCFP-CSCF behaves like a proxy [RFC3261]validates the requestforwards it to selected destinationsprocesses and forwards the response

P-CSCF may behave as a user agent (UA) [RFC3261] release sessions in abnormal conditions e.g., when a bearer loss is detected according to service-based local policygenerate independent SIP transactionsThere can be one or many P-CSCFs within an operator's network

Functions performed by P-CSCF [3GPP TS 23.228, TS 24.229]forward SIP REGISTER requests to the Interrogating-CSCF (I-CSCF) based on a home domain name provided by the UE in the requestforward SIP requests and responses received by the UE to Serving-CSCF (S-CSCF)e.g., check to see if the user identity is valid

forward SIP requests and responses to UEe.g., compress the messagedetect emergency session establishment requestsRelease 5, the P-CSCF returns a SIP error message, 380, indicating that the UE should try the CS CN insteadRelease 6, the P-CSCF will select an S-CSCF to handle an emergency session

send accounting-related information to the Charging Collection Function (CCF)provide integrity protection of SIP signaling and maintain a security association between the UE and the P-CSCFintegrity protection is provided by means of Internet Protocol Security (IPsec) Encapsulating Security Payload (ESP)Release 6 is able to provide confidentiality protection as well

decompress and compress SIP messages from the UEP-CSCF supports compression based on three RFCs[RFC3320] Signaling Compression (SigComp)

[RFC3485]Session Initiation Protocol (SIP) and Session Description Protocol (SDP) Static Dictionary for Signaling Compression (SigComp)[RFC3486]compressing the Session Initiation Protocol (SIP)

subscribe a registration event package at the user's registrar (S-CSCF)for downloading registered public user identitiesfor getting notifications on network-initiated de-registration events

execute media policingcheck the content of the Session Description Protocol (SDP) payload and check whether it contains media types or codecswhen the proposed SDP does not fit the operator's policy, the P-CSCF rejects the request and sends a SIP error message, 488, to the UEan operator may want to use this feature for roaming users due to bandwidth restrictions

maintain session timersRelease 5 does not provide a means for a statefull proxy to know the status of sessionsRelease 6 uses session timers to allow the P-CSCF to detect and free resources used up by hanging sessions

interact with the Policy Decision Function (PDF)PDF is responsible for implementing the Service Based Local Policy (SBLP)Release 5, the PDF is a logical entity of the P-CSCFRelease 6, the PDF is a stand-alone function

2.2.2 Policy Decision FunctionPolicy Decision Function (PDF)makes policy decisions based on session and media-related information obtained from the P-CSCFacts as a policy decision point for SBLP control

The following policy decision point functionalities for SBLP are identifiedstore session and media-related information IP addressesport numbersbandwidths, etc.

generate an authorization token that identifies the PDF and the sessionprovide an authorization decision according to the stored session and media-related information on receiving a bearer authorization request from the GGSNupdate the authorization decision at session modifications which changes session and media-related informationthe capability to revoke the authorization decision at any time

the capability to enable the usage of an authorized bearer e.g., Packet Data Protocol (PDP) contextthe capability to prevent the usage of an authorized bearer (e.g., PDP context) while maintaining the authorizationinform the P-CSCF when the bearer (e.g., PDP context) is lost or modifiedpass an IMS-charging identifier to the GGSN and to pass a GPRS-charging identifier to the P-CSCF

2.2.3 Interrogating-CSCFInterrogating-CSCF (I-CSCF)a contact point within an operator's network for all connections destined to a subscriber of the network operator there may be multiple I-CSCFs within an operator's network

Functions performed by I-CSCFcontact the HSS to obtain the name of the S-CSCF that is serving a userassign an S-CSCF based on received capabilities from the HSSforward SIP requests or responses to the S-CSCFsend accounting-related information to the CCF (Charging Collection Function)

provide a hiding functionalityI-CSCF may contain a Topology Hiding Inter-network Gateway (THIG) functionalityTHIG could be used to hide the configuration, capacity and topology of the network from outside an operator's network

2.2.4 Serving-CSCFServing-CSCF (S-CSCF)the brain of the IMSlocated in the home networkperforms session control and registration services for UEswhile UE is engaged in a session the S-CSCF maintains a session state and interacts with service platforms and charging functions for support of the services

There may be multiple S-CSCFs, and S-CSCFs may have different functionalities within an operator's networkFunctions performed by S-CSCFhandle registration requests by acting as a registrar [RFC3261]S-CSCF knows the UE's IP address and which P-CSCF the UE is using as an IMS entry point

authenticate users by means of the IMS Authentication and Key Agreement (AKA) schemaIMS AKA achieves mutual authentication between UE and home networkdownload user information and service-related data from the HSS during registration or when handling a request to an unregistered user

route mobile-terminating traffic to P-CSCFroute mobile originated traffic to I-CSCFBreakout Gateway Control Function (BGCF)Application Server (AS)perform session controlS-CSCF can act as a proxy server and UA [RFC3261] (SIP: Session Initiation Protocol)

interact with service platformsdecide when a request or response needs to be routed to a specific AS for further processingtranslate an E.164 telephone number (international public telecommunication numbering) to a SIP Universal Resource Identifier (URI) using a domain name system (DNS) translation mechanism [Draft-ietf-enum-rfc2916bis]this translation is needed because routing of SIP signaling in IMS uses only SIP URIs

supervise registration timers and de-register users select an emergency centre when the operator supports IMS emergency sessionsexecute media policingcheck the content of the SDP payload (describe multimedia sessions) and check whether it contains media types or codecswhen the proposed SDP does not fit the operator's policy or user's subscription, the S-CSCF rejects the request and sends a SIP error message, 488

maintain session timersRelease 5 does not provide the means for a statefull proxy to know the status of sessions Release 6 uses session timers to allow the S-CSCF to detect and free resources used up by hanging sessionssend accounting-related information to Charging Collection Function (CCF) for offline charging purposesto Online Charging System (OCS) for online charging purposes

2.2.5 Home Subscriber ServerHome Subscriber Server (HSS)the main data storage for all subscriber and service-related data of the IMSThe main data stored in HSS [3GPP TS 23.002]user identitiesprivate user identitythe user identity that is assigned by the home network operator and is used for such purposes as registration and authorization

public user identitiesthe identity that other users can use for requesting communication with the end userregistration informationaccess parametersused to set up sessions and include parameters like user authentication, roaming authorization and allocated S-CSCF namesservice-triggering informationenables SIP service execution

HSS also provides user-specific requirements for S-CSCF capabilitiesused by I-CSCF to select the most suitable S-CSCF for a userIn addition to functions related to IMS functionalityHSS also contains the subset of Home Location Register and Authentication Center (HLR/AUC) functionality required by PS and CS domainFigure 2.4 shows the structure of HSS

HLR provides support for PS domain entities, such as SGSN and GGSN enables subscriber access to PS domain servicesHLR provides support for CS domain entities, such as MSC/MSC servers enables subscriber access to CS domain services supports roaming to GSM/UMTS CS domain networks

AUC stores a secret key for each mobile subscriberused to generate dynamic security data for each mobile subscriberdata are used for mutual authentication of the International Mobile Subscriber Identity (IMSI) and the networksecurity data are also used to provide integrity protection and ciphering of the communication over the radio path between the UE and the network

There may be more than one HSS in a home network depending on the number of mobile subscribersthe capacity of the equipmentthe organization of the networkThere are multiple reference points between the HSS and other network entities

2.2.6 Subscription Locator FunctionSubscription Locator Function (SLF)enables I-CSCF, S-CSCF and AS to find the address of the HSS that holds the subscriber data for a given user identity when multiple and separately addressable HSSs have been deployed by the network operator

2.2.7 Multimedia Resource Function ControllerMultimedia Resource Function Controller (MRFC)used to support bearer-related servicese.g. conferencing, announcements to a userinterprets SIP signaling received via S-CSCFuses MEdia GAteway COntrol Protocol (MEGACO) instructions to control the Multimedia Resource Function Processor (MRFP)send accounting information toCCF (Charging Collection Function)OCS (Online Charging System)

2.2.8 Multimedia Resource Function ProcessorMultimedia Resource Function Processor (MRFP) provides user-plane resources that are requested and instructed by the MRFCMRFP performs the following functionsmix incoming media streams (e.g., for multiple parties)transmit media stream source (for multimedia announcements)process media stream (e.g., audio transcoding, media analysis) [3GPP TS 23.228, TS 23.002]

2.2.9 Application serverApplication servers (ASs)not pure IMS entitiesASs are functions on top of IMSASs are entities providing value-added multimedia services in the IMS

Main functions of ASprocess an incoming SIP session received from the IMSoriginate SIP requestssend accounting information to the CCF and the OCS

Offered services are not limited purely to SIP-based services since an operator is able to offer access to services based on the Customized Applications for Mobile network Enhanced Logic (CAMEL) Service Environment (CSE) GSMthe Open Service Architecture (OSA) for its IMS subscribers [3GPP TS 23.228] UMTS

Therefore, "AS" is the term used generically to capture the behavior of SIP ASOSA Service Capability Server (SCS) [for UMTS core network]CAMEL IP Multimedia Service Switching Function (IM-SSF) [for GSM core network]

CAMEL (Customised Applications for Mobile networks Enhanced Logic)a set of GSM standards designed to work on a GSM core networkallow an operator to define services over and above standard GSM servicesCAMEL architecture is based on the Intelligent Network (IN) standards

OSA (Open Service Architecture)a framework which aims at building various kinds of services on the top of UMTS core NetworkOSA will provide APIs to access the network functions like authentication and authorization of the user

the APIs are guaranteed to be secure, independent of vendor specific solutions and also independent of programming languagevarious services like VPN, conferencing and many more unknown services can be implemented with the help of these APIs

Using the OSA an operator may utilize such service capability features as [3GPP TS 29.198]call controluser interactionuser statusdata session controlterminal capabilitiesaccount managementcharging and policy management

An additional benefit of the OSA frameworkused as a standardized mechanism for providing third-party ASs in a secure manner to the IMS sincethe OSA itself contains initial access, authentication, authorization, registration and discovery featuresNotethe S-CSCF does not provide authentication and security functionality for secure direct third-party access to the IMS

Figure 2.5SIP ASa SIP-based server that hosts a wide range of value-added multimedia servicesused to provide presence, messaging and conferencing servicesOSA SCSuses an OSA application program interface (OSA API) to communicate with an actual OSA application server

IM-SSF (IP Multimedia Service Switching Function)used in the IMS architecture to support legacy services that are developed in the CAMEL Service Environment (CSE)it hosts CAMEL network features and interworks with the CAMEL Application Part (CAP) interface

An AS may be dedicated to a single service and a user may have more than one servicethere may be one or more ASs per subscriberThere may be one or more ASs involved in a single session, e.g.one AS to control terminating traffic to a user based on user preferences e.g., redirecting all incoming multimedia sessions to an answer machine between 5p.m. and 7 a.m.

another AS to adapt the content of instant messages according to the capabilities of the UEe.g. screen size, number of colors, etc.

2.2.10 Breakout Gateway Control FunctionBreakout Gateway Control Function (BGCF)used to choose where a breakout to the CS domain occursif the breakout happens in the same networkthen BGCF selects a Media Gateway Control Function (MGCF) to handle a sessionif the breakout happens in another networkthen BGCF forwards a session to another BGCF in a selected network [3GPP TS 23.228]

BGCF is able to report account information to the CCF (Charging Collection Function) and collect statistical information

2.2.11 Media Gateway Control FunctionMedia Gateway Control Function (MGCF)a gateway that enables communication between IMS and CS usersall incoming call control signaling from CS users is destined to the MGCFperforms protocol conversion between ISDN User Part (ISUP) or Bearer Independent Call Control (BICC) and SIP protocolsforwards the session to IMS

all IMS-originated sessions toward CS users traverses through MGCFMGCF is able to report account information to the CCF

2.2.12 IP Multimedia Subsystem-Media Gateway FunctionIMS Multimedia Gateway Function (IMS-MGW) provides the user-plane link between CS networks (PSTN, GSM) and the IMScontrolled by MGCFprovide tones and announcements to CS users

2.2.13 Signaling gatewaySignaling gateway (SGW)used to interconnect different signaling networks, e.g.SCTP/IP-based signaling networksSS7 signaling networksSGW performs signaling conversion (both ways) at the transport level between the Signaling System No. 7 (SS7)-based transport of signalingthe IP-based transport of signaling

SGW does not interpret application layer (e.g., BICC, ISUP) messagesFigure 2.6 shows the signalling conversion in the SGW

2.2.14 Security gatewayTo protect control-plane traffic between security domainstraffic will pass through a security gateway (SEG) before entering or leaving the security domainSecurity domainrefers to a network that is managed by a single administrative authority

SEGplaced at the border of the security domainSEG enforces the security policy of a security domain toward other SEGs in the destination security domainNetwork operator may have more than one SEG in its networkto avoid a single point of failure or for performance reasons

2.2.15 Charging entitiesDifferent charging entities and corresponding reference points will be described separately in Section 3.10

2.2.16 GPRS service entities2.2.16.1 Serving GPRS Support Node2.2.16.2 Gateway GPRS Support Node

UMTS/GPRS/WLAN Access to IMS

2.2.16.1 Serving GPRS Support NodeSGSN links the RAN to the packet core networkperforms both control and traffic-handling functions for the PS domainThe control part contains two main functionsmobility managementdeals with the location and state of the UE and authenticates both the subscriber and the UE

session managementdeals with connection admission control and any changes in the existing data connectionstraffic handling is part of session managementSGSN acts as a gateway for user data tunnelingrelays user traffic between UE and GGSNensures that connections receive the appropriate QoSIn addition, the SGSN generates charging information

2.2.16.2 Gateway GPRS Support NodeGGSNprovides interworking with external packet data networksthe prime function of GGSNconnect the UE to external data networks, where IP-based applications and services resideGGSN routes IP packets containing SIP signaling from UE to P-CSCF and vice versa

When the UE activates a bearer (PDP context) toward an access point (IMS)the GGSN allocates a dynamic IP address to the UEthis allocated IP address is used whenIMS registrationUE initiates a session as a contact address of the UEAdditionally, the GGSN polices and supervises the PDP context usage for IMS media traffic and generates charging information

2.3 IMS reference points2.3.1 Gm reference point2.3.2 Mw reference point2.3.3 IMS Service Control reference point2.3.4 Cx reference point2.3.5 Dx reference point2.3.6 Sh reference point2.3.7 Si reference point2.3.8 Dh reference point2.3.9 Mm reference point

2.3.10 Mg reference point2.3.11 Mi reference point2.3.12 Mj reference point2.3.13 Mk reference point2.3.14 Ut reference point2.3.15 Mr reference point2.3.16 Mp reference point2.3.17 Go reference point2.3.18 Gq reference point

Figure 2.7 shows the IMS architectureFigure 2.7 does not showcharging-related functions or reference points (see Section 3.10 for more details)different types of ASs (see Section 2.2.9 for more details)the user-plane connections between different IMS networks and the ASthe SEG at the Mm, Mk, Mw reference points

2.3.1 Gm reference point

NameInvolved entitiesPurposeProtocol

GmUE, P-CSCFThis reference point is used to exchange messages between UE and CSCFsSIP

Gm reference point connects the UE to IMSused to transport all SIP signaling messages between UE and IMSthe IMS counterpart is P-CSCFProcedures in Gm reference pointregistrationsession controltransactions

Registration procedureUE uses the Gm reference point to send a registration request with an indication of supported security mechanisms to the P-CSCFduring the registration process the UE exchanges the necessary parameters for authenticating both itself and the networkgets implicit registered user identities

negotiates the necessary parameters for a security association with the P-CSCF and possibly starts SIP compressioninform the UE if network-initiated de-registration or network-initiated re-authentication occurs

Session control proceduresmobile-originated sessionsthe Gm reference point is used to forward requests from UE to P-CSCFmobile-terminated sessionsthe Gm reference point is used to forward request from P-CSCF to UETransaction proceduressend stand-alone requests (e.g., MESSAGE)receive all responses (e.g., 200 OK) to that request

2.3.2 Mw reference point

NameInvolved entitiesPurposeProtocol

Mw

P-CSCF, I-CSCF, S-CSCFThis reference point is used to exchange messages between CSCFsSIP

Mwa SIP-based reference point between different CSCFsProcedures in the Mw reference pointregistrationsession controltransaction

Registration procedureP-CSCF uses Mw reference point to forward a registration request from UE to I-CSCFI-CSCF then uses the Mw reference point to pass the request to the S-CSCFfinally, the response from the S-CSCF traveres back via the Mw reference point

In addition, S-CSCF uses the Mw reference point in network-initiated de-registration procedures to inform the UE about network-initiated deregistration and network-initiated re-authentication to inform the P-CSCF that it should release resources regarding a particular user

Session control proceduresmobile-originated sessionsMw reference point is used to forward requests both from the P-CSCF to the S-CSCF and from the S-CSCF to the I-CSCFmobile-terminated sessionsMw reference point is used to forward requests both from the I-CSCF to the S-CSCF and from the S-CSCF to the P-CSCF

Mw reference point is also used for network-initiated session releases, e.g., the P-CSCF could initiate a session release toward the S-CSCF if it receives an indication from the PDF that media bearer(s) are lostcharging-related information is conveyed via the Mw reference point

Transaction procedurespass a stand-alone request (e.g., MESSAGE)receive all responses (e.g., 200 OK) to that request via the Mw reference point

2.3.3 IMS Service Control reference point

NameInvolved entitiesPurposeProtocol

ISC

S-CSCF, I-CSCF, ASThis reference point is used to exchange messages between CSCF and ASSIP

IMS Service Control (ISC) reference point is used to send and receive SIP messages between CSCF and ASISC procedures are divided into two main categoriesroute the initial SIP request to an ASwhen S-CSCF receives an initial SIP request it will analyze itbased on the analysis the S-CSCF may decide to route the request to an AS for further processingAS may terminate, redirect or proxy the requestAS-initiated SIP requests

2.3.4 Cx reference point

NameInvolved entitiesPurposeProtocol

Cx

I-CSCF, S-CSCF, HSS This reference point is used to communicate between I-CSCF/ S-CSCF and HSSDiameter

Subscriber and service data are permanently stored in the HSSThese centralized data need to be utilized by I-CSCF and S-CSCF when user registers or receives sessionsThe Cx reference point lies in between HSS and CSCF and is used for the above purposeThe selected protocol is Diameter (Chapter 15)

DiameterDiameter is an authentication, authorization and accounting (AAA) protocol developed by IETFDiameter is used to provide AAA services for a range of access technologiesDiameter is loosely based on the Remote Authentication Dial In User Service (RADIUS) [RFC2865], which has previously been used to provide AAA services, at least for dial-up and terminal server access environments

The procedures can be divided into three main categorieslocation managementuser data handlinguser authentication

2.3.4.1 Location management proceduresLocation management procedures can be further divided into two groupsregistration and de-registrationlocation retrieval

2.3.4.1.1 Registration and de-registration procedures between I-CSCF and HSSWhen the I-CSCF receives a SIP REGISTER request from the P-CSCF via the Mw reference pointit will invoke a user registration status query though User-Authorization-Request (UAR) command

UAR command containsPrivate User Identitythe identity to uniquely identify the user from a network perspectiveit identifies subscription and correct authentication data (Section 3.4.1.1)Public User Identitythe identity to be registered (Section 3.4.1.2)

Visited Network Identifieridentifies the visited IMS network in the case of IMS roamingbased on this identifier the HSS is able to enforce roaming restrictions

Routing Informationcontains the address of the HSS if I-CSCF is aware of it if the I-CSCF does not know the address of the HSSthen it contains the destination realm (the SLF (Subscription Locator Function) is used to resolve a correct HSS)

Type of Authorizationthree possible values for the type of authorization information element are definedREGISTRATIONit is included when the expires value in the REGISTER request does not equal zero

REGISTRATION_CAPABILITIESit is included when the expires value in the REGISTER request is not equal to zero, and the I-CSCF explicitly queries S-CSCF capabilities (e.g., when a previously given S-CSCF is not responding)DE-REGISTRATIONit is included when the expires value in the REGISTER request is equal to zero

After receiving the UAR (User-Authorization-Request) command the HSS sends a User-Authorization-Answer (UAA) commandUAA command containsResultinforms the outcome of the UAR commandS-CSCF Name and/or S-CSCF Capabilities depending on the user's current registration status

S-CSCF capabilities are returned if the user does not have an S-CSCF name assigned yet in the HSS, or if the I-CSCF explicitly requests S-CSCF capabilitiesOtherwise, the S-CSCF name is returned

2.3.4.1.2 Registration and de-registration procedures between S-CSCF and HSSAfter I-CSCF finds an S-CSCF that will serve the userI-CSCF forwards a SIP REGISTER request to the S-CSCFwhen the S-CSCF receives this request, it uses a Server-Assignment-Request (SAR) command to communicate with HSS

the SAR command is used to inform the HSS about which S-CSCF will be serving the user when the expires value is not equal to zeroif the expires value equals zero, the SAR command is used to inform that the S-CSCF is no longer serving a user

The SAR command containsPrivate User Identitysee the UAR commandPublic User Identitythe identity to be registered/de-registered (Section 3.4.1.2)Routing Informationcontains the address of the HSS if the S-CSCF is aware of itif the S-CSCF does not know the address of the HSS, then it contains the destination realm

S-CSCF Namecontains the SIP URI (Uniform Resource Identifier) of the S-CSCFServer Assignment Typecontains information about why this operation is executed e.g., due to registration, re-registration, de-registration that is user-initiated or S-CSCF initiated and authentication failure

User Data Already Availableindicates to the HSS whether or not the S-CSCF has already the part of the user profile that it needs for serving the userUser Data Request Typetells whether the S-CSCF wants to download a complete, registered or unregistered profile

After receiving the SAR command the HSS will respond with a Server-Assignment-Answer (SAA) commandSAA command containsResultinforms the outcome of the SAR commandUser Profilebased on the set values of Server Assignment Type and User Data Already Available in the SAR command the User Profile is sent

Charging Information (optional)contains the addresses of the charging functionsIt is the HSS that starts network-initiated de-registration by using Registration-Termination-Request (RTR) command

RTR command containsPrivate User Identitythe identity to uniquely identify the user from a network perspectiveit identifies the subscription and the correct authentication data (Section 3.4.1.1)Public User Identityone or more identities to be deregistered (Section 3.4.1.2)

Routing Informationcontains the name of the S-CSCF that is serving the userReason for de-registrationcontains a reason code that determines S-CSCF behavior and optionally includes a textual message to be shown to the userRTR command is acknowledged by a Registration-Termination-Answer (RTA) commandwhich simply indicates the result of the operation

2.3.4.1.3 Location retrieval proceduresLocation retrieval procedure makes use of a Location- Info-Request (LIR) commandLIR command containsPublic User Identitycontains the identity from the request URI field of a SIP method

Routing Informationcontains the address of the HSS if the I-CSCF is aware of it if the I-CSCF does not know the address of the HSS, then it contains the destination realm

HSS responds with a Location-Info-Answer (LIA) commandLIA command containsResultinform the outcome of the LIR commandThe S-CSCF Name or S-CSCF Capabilitiesthe latter are returned if the user does not have the S-CSCF name assignedotherwise the SIP URI of the S-CSCF is returned

2.3.4.2 User data-handling proceduresDuring the registration processuser and service-related data will be downloaded from the HSS to the S-CSCF via the Cx reference point using SAR and SAA commandsTo update the data in the S-CSCFthe HSS initiates a Push-Profile-Request (PPR) command

PPR command containsPrivate User Identitythe identity to uniquely identify the user from a network perspective (Section 3.4.1.1)Routing Informationcontains the name of the S-CSCF that is serving the userUser Datacontains the updated user profile (Section 3.11)

PPR command is acknowledged by a Push-Profile-Answer (PPA) commandwhich simply indicates the result of the operation

2.3.4.3 Authentication proceduresIMS user authentication relies on a pre-configured shared secretShared secrets and sequence numbers are stored in IP Multimedia Services Identity Module (ISIM) in the UE, andHSS in the networkWhen the S-CSCF needs to authenticate a user it sends a Multimedia-Auth-Request (MAR) command to the HSS

Shared secretan authentication method used to establish trust between computers in a VPN that utilizes a password, also termed pre-shared authentication keys, for establishing trust

MAR command containsPrivate User Identitythe identity to uniquely identify the user from a network perspectiveit identifies subscription and correct authentication data (Section 3.4.1.1)Public User Identitythe identity to be registered (Section 3.4.1.2)

S-CSCF Namecontains the SIP URI of the S-CSCFRouting Informationcontains the address of the HSS if the I-CSCF is aware of itif the I-CSCF does not know the address of the HSS, then it contains the destination realm

Number of Authentication Itemsinformation about how many authentication vectors the S-CSCF wants to download at oncemultiple authentication vectors can be downloaded (e.g., if an operator wants to re-authenticate all re-registrations)Authentication Dataincludes authentication scheme and authentication information in case of synchronization failure

HSS responds with a Multimedia-Auth-Answer (MAA) commandMAA command containsResultinforms the outcome of the MAR commandPrivate User Identitythe identity to uniquely identify the user from a network perspectiveit identifies subscription and correct authentication data (Section 3.4.1.1)

Public User Identitythe identity to be registered (Section 3.4.1.2)Number of Authentication Itemscontains the authentication vectorsAuthentication Dataan authentication vector, which is comprised of Authentication Scheme (e.g., Digest-AKAvl-MD5)Authentication Information (authentication challenge RAND and the token AUTN)

Authorization Information (expected response, or XRES)Integrity KeyConfidentiality Key (optional)an Item Numberindicates the order in which the authentication vectors are to be consumed when multiple vectors are returned

2.3.5 Dx reference point

NameInvolved entitiesPurposeProtocol

Dx

I-CSCF, S-CSCF, SLFThis reference point is used by I-CSCF/S-CSCF to find a correct HSS in a multi-HSS environmentDiameter

When multiple and separately addressable HSSs have been deployed in a networkneither I-CSCF nor S-CSCF know which HSS they need to contactThey need to contact the SLF (Subscription Locator Function) firstFor this purpose the Dx reference point has been introducedDx reference point is always used in conjunction with the Cx reference point

The protocol used in this reference point is based on DIAMETERTo get an HSS addressthe I-CSCF or the S-CSCF sends to the SLF the Cx requests aimed for the HSSOn receipt of the HSS address from the SLFI-CSCF or the S-CSCF will send the Cx requests to the HSS

Figure 2.8 shows how the SLF is used to find a correct HSS when the I-CSCF receives an INVITE request and three HSSs have been deployed

LIRLocation-Info-Request

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