IMS IP Multimedia Subsystem PDF

IMS IP Multimedia Subsystem ANTONIO JAVIER GARCÍA SÁNCHEZ IMS. SUMMARY Introduction Convergence Examples Historical Perspective Requirements Architecture IMS. INTRODUCTION IMS. INTRODUCTION Fixed and Mobile Networks: Convergence Redefinition of th...

IMS IP Multimedia Subsystem ANTONIO JAVIER GARCÍA SÁNCHEZ IMS. SUMMARY Introduction Convergence Examples Historical Perspective Requirements Architecture IMS. INTRODUCTION IMS. INTRODUCTION Fixed and Mobile Networks: Convergence Redefinition of the application concept Peer-to-peer entities In order to communicate, IP-based applications must be a mechanism to reach the correspondent A priori, the IP connectivity capacity is offered only in isolated and single-service provider environments in the Internet As a consecuence, interworking between service providers is not satisfied feature, becoming a crucial requirement IP Multimedia Subsystem (IMS) allows applications in IP-enabled devices to establish peer-to-peer and peer-to-content connections easily and securely. IMS. INTRODUCTION DEFINITION of IMS: IMS is a global, access-independent and standard-based IP connectivity and service control architecture that enables various types of multimedia services to end-users using common Internet- based protocols. True integration of voice and data services increases productivity and overall effectiveness, while the development of innovative applications integrating voice, data and multimedia will create demands for new services, such as presence, multimedia chat, push to talk and conferencing. The skill to combine mobility and the IP network will be crucial to service success in the future  Trends in different regions and countries are different, but on a IMS. INTRODUCTION global level operators are facing increasing competition and declining prices for voice traffic Mobile voice traffic is growing rapidly and substituting that of voice traffic over fixed lines Key enabling technologies, such Convergence of as smart phones, wireline and Networks wireless broadband and IMS for seamless service over different Convergence of access types are readily Services available End users now expect high Convergence of quality with reliable mobility and are using the Internet more as devices the penetration of broadband grows rapidly. Now, Voice over IP (VoIP) is starting to substitute Combined, this means that PSTN. operators are looking for long- term evolutionary strategies towards converged networks, with service integration and interoperability across domains and devices. IMS. CONVERGENCE OF NETWORKS Network convergence simplifies the end user experience and dissolves the barriers and complexities that separate today’s network islands. The same services are available across all networks and, in an ideal world, appear and perform in exactly the same way, making usage easy, transparent and intuitive. Concept of ALL-IP network using IMS as unifiying platform Native IP Access: “IP-based access connection using the SIP protocol between the device and the converged core network” IMS. CONVERGENCE OF DEVICES Typically, a device was only used for a single purpose and the support for its other functions is limited. E.g. PSTN phones, mobile phones are good examples. Consumers use these devices for a single purpose. When they change tasks they change device and access network. This means service islands. It is required to unify devices that can access services in a similar and easy way. Smart Phones allows multimedia services in a mobile enviroment IMS. CONVERGENCE OF SERVICES Multimedia services, such as Presence, Push-to-talk, messaging, interactive applications, data or video sharing plus streaming, browsing and downloading, are being delivered over fixed and mobile packet networks To launch new services and applications quickly, operators can use IMS to eliminate the complexity of different service platforms in the network Standards based Service Delivery Framework (SDF) E.g. IP-DSLAMS: DSL+POTS under IP networks E.g. Smart-Phones: cellular+WLAN The goal is that the user experience doesn’t change: the same voice and multimedia services are used in the same way IMS. EXAMPLES Tickets to movie Video birthday message Football IMS. EXAMPLES IMS. HISTORICAL PERSPECTIVE 3GPP was founded by standardization bodies from Europe, Japan, South Korea, the USA and China to specify a 3G mobile system comprising Wideband Code Division Multiple Access (WCDMA) and Time Division/Code Division Multiple Access (TD-CDMA) radio access and an evolved GSM core network 3GPP Release 99. 3GPP developed the services, system architecture, WCDMA and TD- CDMA radio accesses, and the common core network WCDMA radio access was the most significant enhancement to the GSM-based 3G system. In addition to WCDMA, UMTS Terrestrial Radio Access Network (UTRAN) introduced the Iu interface. Open Service Architecture (OSA) IMS. HISTORICAL PERSPECTIVE 3GPP Release 4 Include the concept ALL-IP New functionalities: the Mobile Switching Centre (MSC) Server–Media Gateway (MGW) concept, IP transport of core network protocols, Location Services (LCS) enhancements for UTRAN and multimedia messaging, etc. 3GPP Releases 5 and 6 Release 5 introduced the IMS as part of 3GPP specifications. The IMS is supposed to be a standardized access-independent IP-based architecture that interworks with existing voice and data networks for both fixed (e.g., PSTN, ISDN, Internet) and mobile users (e.g., GSM, CDMA). The IMS architecture makes it possible to establish peer-to-peer IP communications with all types of clients with the requisite quality of services. In addition to session management, the IMS architecture also addresses functionalities that are necessary for completing service delivery (e.g., registration, security, charging, bearer control, roaming). The IMS will form the heart of the IP core network. It defines a finite architecture for SIP-based IP multimedia service machinery such as a description of how elements are connected, selected protocols and procedures; a sharply optimization for the mobile communication environment in order to design user authentication and authorization based on mobile identities. Also, Release 5 defines rules at the user network interface for compressing SIP messages and security and policy control mechanisms that allow radio loss and recovery detection. Release 6 fixes the shortcomings in Release 5 IMS and also contains novel features. R6 introduced standardized enhancements for services such as routing and signalling modifications IMS. HISTORICAL PERSPECTIVE 3GPP Release 7 It introduces two more access technologies: Data Over Cable Service Interface Specification (DOCSIS) and xDSL2 New features such as IMS multimedia telephony including supplementary services, SMS over any IP access or Voice Call Continuity 3GPP Release 8 It introduces a number of novel IMS features such as multimedia session continuity which would improve the voice call continuity feature to enable, for instance, continuity of multimedia media streams when IP access is changed IMS. REQUIREMENTS There is a set of basic requirements which guides the way in which the IMS architecture has been created and how it should evolve in the future. They are the following ones: IP Multimedia Sessions IP Connectivity Ensuring Quality of Service for IP Multimedia Services IP Policy Control for assuring correct usage of media resources Secure Communication Charging Arrangements Support of Roaming Interworking with other networks Service Control Model Layered Design and access independence IMS. REQUIREMENTS IP Multimedia Sessions IMS users are able to mix and match a variety of IP-based services in any way they choose during a single communication session. Existing communication networks are able to offer voice, video and messaging type of services using circuit-switched bearers. End users’ service offerings should not decline when users move to the packet- switched domain and start using the IMS. The IMS will take communication to the next level by offering enriched communication. IMS. REQUIREMENTS IP Connectivity A fundamental requirement is that a device has to have IP connectivity to access it. Peer-to-peer applications require end-to-end reachability and this connectivity is most easily attained with IP version 6 (IPv6) because IPv6 does not have address shortage. 3GPP has arranged matters so that the IMS exclusively supports IPv6 [3GPP TS 23.221]. Early IMS implementations and deployments may use IP version 4 (IPv4). IMS. REQUIREMENTS Ensuring Quality of Service for IP Multimedia Services On the public Internet, delays tend to be high and variable, packets arrive out of order and some packets are lost or discarded. This will no longer be the case with the IMS. The underlying access and transport networks together with the IMS to provide end-to-end Quality of Service (QoS). Via the IMS, the UE negotiates its capabilities and expresses its QoS requirements during a Session Initiation Protocol (SIP) session setup or session modification procedure. The UE is able to negotiate such parameters as: media type bit rate, packet size, packet transport frequency; usage of RTP payload for media types; bandwidth adaptation. After negotiating the parameters at the application level, UEs reserve suitable resources from the access network if not already available (typical case in mobile access). When end-to-end QoS is created, the UEs encode and packetize individual media types with an appropriate protocol (e.g., RTP) and send these media packets to the access and transport network by using a transport layer protocol (e.g., TCP or UDP) over IP. IMS. REQUIREMENTS IP Policy Control for assuring correct usage of media resources IP policy control means the capability to authorize and control the usage of bearer traffic intended for IMS media, based on the signalling parameters at the IMS session. This requires interaction between the IP connectivity access network and the IMS. The policy control element is able to verify that values negotiated in SIP signalling are used when activating bearers for media traffic The policy control element is able to enforce when media traffic between the end points of a SIP session start or stop 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 session. IMS. REQUIREMENTS Secure Communications The IMS has its own authentication and authorization mechanisms between the UE and the IMS network in addition to access network procedures (e.g., GPRS network). The integrity and optional confidentiality of the SIP messages is provided between the UE and the IMS network and between IMS network entities regardless of the underlaying core network (e.g., RAN and GPRS). The IMS provides at least a similar level of security as,for example, the IMS ensures that users are authenticated before they can start using services, and users are able to request privacy when engaged in a session. IMS. REQUIREMENTS Charging Arrangements The IMS architecture allows different charging models to be used: To charge just the calling party To charge both the calling party and the called party based on used resources in the transport level. In the latter case the calling party could be charged entirely on an IMS-level session. 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 component. The IMS architecture supports both online and offline charging capabilities. IMS. REQUIREMENTS Charging IMS. REQUIREMENTS Support of Roaming It is important to get access to their services regardless of their geographical location. The roaming feature makes possible to use services even though the user is not geographically located in the service area of the home network. Differences between the GPRS roaming and IMS roaming IMS. REQUIREMENTS Interworking with other networks Successful communication network technology and architecture the IMS has to be able to connect to as many users as possible. The IMS supports communication with PSTN, ISDN, mobile and Internet users. It will be possible to support sessions with Internet applications that have been developed outside the 3GPP community IMS. REQUIREMENTS Service Control Model When a user is roaming, an entity in the visited network provides services and controls the traffic for the user. The same solution for home network IMS. REQUIREMENTS Layered Design and access independence 3GPP has decided to use a layered approach to architectural design. This means that transport and bearer services are separated from the IMS signalling network and session management services. Further services are run on top of the IMS signalling network. The layered approach aims at a minimum dependence between layers. The layered approach increases the importance of the application layer as services are designed to work independently of the access network and the IMS is equipped to bridge the gap between them. Whether the subscriber is using a mobile phone or a PC client to communicate, the same presence and group list functions in IMS will be used. Different services have different requirements. These include: bandwidth; latency; processing power in the device. IMS solves this shortcoming. IMS. REQUIREMENTS Layered Design and access independence IMS. ARCHITECTURE IMS. ARCHITECTURE. DESCRIPTION OF ENTITIES The entities can be divided into six categories: session management and routing family (CSCFs) databases (HSS, SLF) services (application server, MRFC, MRFP) interworking functions (BGCF, MGCF, IMS-MGW, SGW) support functions (PCRF, SEG, IBCF, TrGW, LRF) charging IMS. ARCHITECTURE. SESSION MANAGEMENT IMS. ENTITIES. SESSION MANAGEMENT Call Session Control Functions (CSCF) Four different kinds of Call Session Control Functions (CSCF): Proxy-CSCF (P- CSCF), Serving-CSCF (S-CSCF), Interrogating-CSCF (I-CSCF) and Emergency- CSCF (E-CSCF). Each kind of CSCF has its own special tasks Common to P-CSCF, S-CSCF and I-CSCF is that they all play a role during registration and session establishment and form the SIP routing machinery. Moreover, all functions are able to send charging data to an offline charging function. There are some common functions that P-CSCF and S-CSCF are able to perform. Both entities are able to release sessions on behalf of the user (e.g., when S-CSCF detects a hanging session or P-CSCF receives a notification that a media bearer is lost) and are able to check that the content of the SIP request or response conforms operator’s policy and user’s subscription IMS. ENTITIES. SESSION MANAGEMENT.CSCF Proxy Call Session Control Function (P-CSCF) It is the first contact point for users within the IMS. It means that all SIP signalling traffic from the UE will be sent to the P-CSCF. Similarly, all terminating SIP signalling from the network is sent from the P-CSCF to the UE. There are four specific tasks assigned for the P-CSCF: SIP compression, IPSec security association, Interaction with Policy and Charging Rules Function (PCRF) Emergency session detection. IMS. ENTITIES. SESSION MANAGEMENT.CSCF Interrogating Call Session Control Function (I-CSCF) It is a contact point within an operator’s network for all connections destined to a subscriber of that network operator There are three unique tasks assigned for the I-CSCF: Obtaining the name of the next hop (either S-CSCF or application server) from the Home Subscriber Server (HSS) Assigning an S-CSCF based on received capabilities from the HSS Routing incoming requests further to an assigned S-CSCF or the application server IMS. ENTITIES. SESSION MANAGEMENT.CSCF Serving Call Session Control Function (S-CSCF) It is the focal point of the IMS as it is responsible for handling registration processes, making routing decisions and maintaining session states and storing the service profile(s). IMS. ENTITIES. SESSION MANAGEMENT.CSCF Emerging Call Session Control Function (E-CSCF) E-CSCF is a dedicated functionality to handle IMS emergency requests such as sessions towards police, fire brigade and ambulance. It selects an emergency centre also known as a Public Safety Answering Point where an emergency request should be delivered For instance, a selection criterion is a calling user’s location and possible type of emergency (e.g. police, coast guard). Once the appropriate emergency centre is selected the E-CSCF routes the request to the emergency centre. IMS. ENTITIES. SESSION MANAGEMENT.CSCF S-CSCF routing and basic IMS session setup IMS. ARCHITECTURE. DATABASES IMS. ENTITIES. DATABASES. HSS The HSS (Home Subscriber Server) is the main data storage for all subscriber and service-related data of the IMS. It includes the user identities, registration information, access parameters and service-triggering information User identities consist of two types: private and public user identities. IMS access parameters are used to set up sessions and include parameters like user authentication, roaming authorization and allocated S-CSCF names. Service-triggering information enables SIP service execution. The HSS also provides user-specific requirements for S-CSCF capabilities. This information is used by the I-CSCF to select the most suitable S-CSCF for a user. In addition to functions related to IMS functionality, the HSS contains the subset of Home Location Register and Authentication Center (HLR/AUC) functionality required by the Packet-Switched (PS) domain and the Circuit-Switched (CS) domain. IMS. ENTITIES. DATABASES. HLR FUNCTIONALITY It takes part of the HSS HLR functionality is required to provide support to PS and CS domain entities. This enables subscriber access to PS and CS domain services. The AUC stores a secret key for each mobile subscriber, which is used to generate dynamic security data for each mobile subscriber. Security data are also used to provide integrity protection and ciphering of the communication over the radio path between the UE and the network. IMS. ENTITIES. DATABASES. SLF It is a mechanism that enables the I-CSCF, the S-CSCF and the 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 IMS. ARCHITECTURE. SERVICE FUNCTIONS IMS. ENTITIES. SERVICE FUNCTIONS Three functions are categorized as IMS service-related functions: Multimedia Resource Function Controller (MRFC) Multimedia Resource Function Processor (MRFP) Application Server (AS) IMS. ENTITIES. SERVICE FUNCTIONS.AS ASs are not pure IMS entities because they are functions on top of IMS. However, ASs are described as part of IMS functions so ASs are entities that provide value-added multimedia services in the IMS, such as presence and Push to Talk Over Cellular. An AS resides in the user’s home network or in a third-party location. The third party here means a network or a standalone AS. The main functions of the AS are: The possibility to process and impact an incoming SIP session received from the IMS. The capability to originate SIP requests. The capability to send accounting information to the charging functions. IMS. ENTITIES. SERVICE FUNCTIONS.AS The services offered 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) and the Open Service Architecture (OSA) for its IMS subscribers. Therefore, AS is the term used generically to capture the behaviour of the SIP AS, OSA Service Capability Server (SCS) and CAMEL IP Multimedia Service Switching Function (IM-SSF). IMS. ENTITIES. SERVICE FUNCTIONS.AS.OSA An operator may utilize such service capability features as call control, user interaction, user status, data session control, terminal capabilities, account management, charging and policy management for developing services. An additional benefit of the OSA framework is that it can be used as a standardized mechanism for providing third-party ASs in a secure manner to the IMS, as the OSA itself contains initial access, authentication, authorization, registration and discovery features IMS. ENTITIES. SERVICE FUNCTIONS.AS.IM-SSF/SIP AS IM-SSF was introduced in the IMS architecture to support legacy services that are developed SIP AS is a SIP-based server that hosts a wide range of value- added multimedia services. A SIP AS could be used to provide presence, messaging, Push to talk Over Cellular and conferencing services. IMS. ENTITIES. SERVICE FUNCTIONS.AS.IM-SSF/SIP AS Figure shows how different functions are connected. From the perspective of the S-CSCF SIP AS, the OSA service capability server and the IM-SSF exhibit the same reference point behaviour. IMS. ENTITIES. SERVICE FUNCTIONS. MRFC AND MRFP MRFC and MRFP together provide mechanisms for bearer- related services such as conferencing, announcements to a user or bearer transcoding in the IMS architecture. The MRFC is tasked to handle SIP communication to and from the S-CSCF and to control the MRFP. The MRFP performs the following functions: Mixing of incoming media streams (e.g., for multiple parties) Media stream source (for multimedia announcements) Media stream processing (e.g., audio transcoding, media analysis) IMS. ENTITIES. SERVICE FUNCTIONS. COMPARATIVES AS VERSUS MRFC AND MRFP Currently, the role of MRFC in the IMS architecture is minor, as in IMS conferencing work and in IMS Multimedia Telephony service the MRFC is co-located with an AS. IMS. ARCHITECTURE. INTERNETWORKING FUNCTIONS  IMS. ENTITIES. INTERWORKING FUNCTIONS. IMS introduces four interworking functions, which are needed to enable voice and video interworking between IMS and the CS CN: Breakout Gateway Control Function (BGCF) Media Gateway Control Function (MGCF) Signalling Gateway (SGW) IMS Media Gateway (IMS-MGW).  IMS. ENTITIES. INTERWORKING FUNCTIONS. BGCF For breaking out the S-CSCF sends a SIP session request to the Breakout Gateway Control Function (BGCF) BGCF further chooses where a breakout to the CS domain occurs. The outcome of a selection process can be either a breakout in the same network in which the BGCF is located or another network. If the breakout happens in the same network, then the BGCF selects a Media Gateway Control Function (MGCF) to handle the session further. If the breakout takes place in another network, then the BGCF forwards the session to another BGCF in a selected network.  IMS. ENTITIES. INTERWORKING FUNCTIONS. MGCF/SGW/IMS-MGW When a SIP session request hits the MGCF it performs protocol conversion between SIP protocols and the ISDN User Part (ISUP), or the Bearer Independent Call Control (BICC) and sends a converted request via the Signalling Gateway (SGW) to the CS CN. The SGW performs signalling conversion (both ways) at the transport level between the IP-based transport of signalling (i.e., between Sigtran SCTP/IP and SS7 MTP) and the Signalling System No. 7 (SS7) based transport of signalling. The IMS-MGW provides the user-plane link between CS CN networks and the IMS. It determinates the bearer channels from the CS network and media streams from the backbone network (e.g., RTP streams in an IP network or AAL2/ATM connections in an ATM backbone), executes the conversion between these terminations and performs transcoding and signal processing for the user plane when needed. In addition, the IMS-MGW is able to provide tones and announcements to CS users. And viceversa IMS. ARCHITECTURE. SUPPORT FUNCTIONS  IMS. ENTITIES. SUPPORT FUNCTIONS Policy and Charging Rules Function (PCRF) It is responsible for making policy and charging control decisions based on session and media-related information obtained from the P-CSCF. Note that Session establishment in the IMS involves an end-to-end message exchange using SIP and SDP. During the message exchange UEs negotiate a set of media characteristics (e.g., common codec(s)). If an operator applies the policy and charging control, then the P- CSCF will forward the relevant SDP information to the PCRF together with an indication of the originator. The PCRF generates charging rules and authorizes the IP flows of the chosen media components by mapping from SDP parameters to authorized IP QoS parameters for transfer to the access network – e.g., GGSN in case of UMTS/GPRS access (GPRS access is assumed hereafter) – via the Gx reference point. On receiving the SDP context activation or modification, the GGSN asks for authorization information from the PCRF. Based on available information in the PCRF it makes an authorization decision which will be enforced in the GGSN. In addition to a bearer authorization decision the PCRF receives reports on transport plane events e.g. when the bearer is lost or when the bearer is released. Based on this information the PCRF is able to inform the P-CSCF about the occurred event. This allows the P-CSCF to effect charging, and it may even start releasing an IMS session on behalf of the user.  IMS. ENTITIES. SUPPORT FUNCTIONS Interconnection Border Control Function and Transition Gateway(IBCF) It provides specific functions in order to perform interconnection between two operator domains. It enables communication between IPv6 and IPv4 IMS applications, network topology hiding, controlling transport plane functions, screening of SIP signalling information, etc. IBCF is tasked to bridge these two domains by acting as an Application Level Gateway (ALG). ALG takes care of modifying SIP and SDP information in such a way that UEs using different (IPv6 and IPv4) IP version can communicate with each other. The ALG functionality inside the IBCF controls Transition Gateway (TrGW) which is responsible for providing IP version interworking in transport plane (i.e. modifying IP packets transporting actual IMS application media such as RTP).  IMS. ENTITIES. SUPPORT FUNCTIONS Interconnection Border Control Function and Transition Gateway(IBCF) Network topology hiding functionality could be used to hide the configuration, capacity and topology of the network from outside an operator’s network. If an operator wants to use hiding functionality then the operator must place an IBCF in the routing path when receiving requests or responses from other IMS networks. To this end, the IBCF performs the encryption and decryption of all headers which reveal topology information about the operator’s IMS network. An operator may use IBCF to support its local policy. An operator may use IBCF as the entry/exit point for its network and IBCF can be used to screen signalling information (i.e. omit or modify some received SIP headers prior to forwarding SIP messages further to other networks).  IMS. ENTITIES. SUPPORT FUNCTIONS Security Gateway(SEG) It has the function of protecting control-plane traffic between security domains. The security domain refers to a network that is managed by a single administrative authority which usually coincides with operator borders The SEG is placed at the border of the security domain and it enforces the security policy of a security domain toward other SEGs in the destination security domain In the IMS all traffic within the IMS is routed via SEGs The protection covers features as confidentiality as well as data integrity and authentication are mandated  IMS. ENTITIES. SUPPORT FUNCTIONS Location Retrieval Function (LRF) It assists E-CSCF in handling IMS emergency sessions by delivering location information of the UE that has initiated an IMS emergency session and/or address of Public Safety Answering Point (PSAP) where the session should be sent To resolve appropriate PSAP it may contain Routing Determination Function (RDF) which is used to map the user’s location to address of PSAP  IMS. OTHER ENTITIES. GPRS Serving GPRS Support Node (SGSN) It links the RAN to the packet core network. It is responsible for performing both control and traffic-handling functions for the PS domain. The control part contains two main functions: mobility management and session management. Mobility management deals with the location and state of the UE and authenticates both the subscriber and the UE. The control part of session management deals with connection admission control and any changes in the existing data connections. It also supervises 3G network services and resources. Traffic handling is the part of session management that is executed. The SGSN acts as a gateway for relaying user traffic between the UE and the GGSN. As a part of this function, the SGSN also ensures that connections receive the appropriate QoS.  IMS. OTHER ENTITIES. GPRS Gateway GPRS Support Node (GGSN) The Gateway GPRS Support Node (GGSN) provides interworking with external packet data networks So, the prime function of the GGSN is to connect the UE to external data networks, where IP-based applications and services reside. The external data network could be the IMS or the Internet, for instance. In other words, the GGSN routes IP packets containing SIP signalling from the UE to the P-CSCF and vice versa The interworking service provided is realized as access points that relate to the different networks the subscriber wants to connect IMS. ARCHITECTURE. REFERENCE POINTS  IMS. REFERENCE POINTS Gm Reference Point The Gm reference point connects the UE to the IMS. It is used to transport all SIP signalling messages between the UE and the IMS. The IMS counterpart is the P-CSCF. Procedures in the Gm reference point can be divided into three main categories: registration,session control and transactions: In the registration procedure the UE uses the Gm reference point to send a registration request with an indication of supported security mechanisms to the P-CSCF Session control procedures contain mechanisms for both mobile-originated sessions and mobile-terminated sessions. In mobile-originated sessions the Gm reference point is used to forward requests from the UE to the P-CSCF. In mobile-terminated sessions the Gm reference point is used to forward requests from the P-CSCF to the UE. Transaction procedures are used to send standalone requests  IMS. REFERENCE POINTS MW Reference Point A SIP based reference point between different CSCFs is needed. This reference point is called Mw. Procedures in the Mw reference point can be divided into three main categories: registration,session control and transactions: In the registration procedure the P-CSCF uses the Mw reference point to forward a registration request from the UE to the I-CSCF. The I-CSCF then uses the Mw reference point to pass the request to the S-CSCF. Finally, the response from the S- CSCF traverses back via the Mw reference point. Session control procedures contain mechanisms for both mobile-originated sessions and mobile-terminated sessions. In mobile-originated sessions the Mw reference point is used to forward requests both from the P-CSCF to the S-CSCF and from the S-CSCF to the I-CSCF. In mobile-terminated sessions the Mw 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. Transaction procedures are used to send standalone requests IMS. REFERENCE POINTS IMS Service Control (ICS) Application Servers are entities that host and execute services, such as presence, messaging and multimedia telephony. Therefore, there has to be a reference point for sending and receiving SIP messages between S- CSCF and Application Server. This reference point is called the IMS Service Control (ISC) reference point and the selected protocol is SIP. After successful IMS registration the S-CSCF downloads user profile from HSS that contains among other things information when a request has to be sent to AS. Once the S-CSCF receives an initial SIP request it will analyze it. Based on the analysis the S-CSCF may decide to route the request to an AS for further processing. The AS may terminate, redirect or proxy the request from the S-CSCF. IMS. REFERENCE POINTS MA reference point The ISC reference allows communication towards Application Servers when there is need to execute service control in S-CSCF. Not all services require this therefore an optimized routing mechanism to bypass S-CSCF was introduced in Release 7. The Ma reference point is used to convey information directly between I-CSCF and AS. Typically this reference point is used when a target identity in the SIP request is IMS public service identity or AS initiates something on behalf of the service. IMS. REFERENCE POINTS Cx reference point Subscriber and service data are permanently stored in the HSS. These centralized data need to be utilized by the I-CSCF and the S-CSCF when the user registers or receives sessions. Therefore, there has to be a reference point between the HSS and the CSCF. This reference point is called the Cx reference point and the selected protocol is Diameter. The procedures can be divided into three main categories: location management, user data handling and user authentication. IMS. REFERENCE POINTS Dx reference point When multiple and separately addressable HSSs have been deployed in a network, neither the I-CSCF nor the S-CSCF know which HSS they need to contact. However, they need to contact the SLF first. For this purpose the Dx reference point has been introduced. The Dx reference point is always used in conjunction with the Cx reference point. The protocol used in this reference point is based on Diameter. Its functionality is implemented by means of the routing mechanism provided by an enhanced Diameter redirect agent. To get an HSS address the I-CSCF or the S-CSCF sends to the SLF the Cx requests aimed for the HSS. On receipt of the HSS address from the SLF, the I-CSCF or the S-CSCF will send the Cx requests to the HSS. IMS. REFERENCE POINTS Dx reference point IMS. REFERENCE POINTS Sh reference point An AS (SIP AS or OSA SCS) may need data (related to particular identity of user or related to public service identity) or need to know to which S- CSCF to send a SIP request. This type of information is stored in the HSS. Therefore, there has to be a reference point between the HSS and the AS. This reference point is called the Sh reference point and the protocol is Diameter. Procedures are divided into two main categories: data handling and subscription/notification. IMS. REFERENCE POINTS Dh reference point When multiple and separately addressable HSSs have been deployed in the network, the AS cannot know which HSS it needs to contact. However, the AS needs to contact the SLF first. For this purpose the Dh reference point was introduced in Release 6. In Release 5 the correct HSS is discovered by using proprietary means. The Dh reference point is always used in conjunction with the Sh reference point. The protocol used in this reference point is based on Diameter. Its functionality is implemented by means of the routing mechanism provided by an enhanced Diameter re-direct agent. To get an HSS address, the AS sends to the SLF the Sh request aimed for the HSS. On receipt of the HSS address from the SLF, the AS will send the Sh request to the HSS. IMS. REFERENCE POINTS Si reference point When the AS is a CAMEL AS (IM-SSF) it uses the Si reference point to communicate with the HSS. The Si reference point is used to transport CAMEL subscription information including triggers from the HSS to the IM- SSF. The used protocol is Mobile Application Part (MAP). IMS. REFERENCE POINTS Mi reference point When the S-CSCF or E-CSCF discover that a session needs to be routed to the CS domain it uses the Mi reference point to forward the session to BGCF. The protocol used for the Mi reference point is SIP. IMS. REFERENCE POINTS Mj reference point When BGCF receives session signalling via the Mi reference point it selects the CS domain in which breakout is to occur. If breakout occurs in the same network, then it forwards the session to MGCF via the Mj reference point. The protocol used for the Mj reference point is SIP. IMS. REFERENCE POINTS Mk reference point When BGCF receives session signalling via the Mi reference point it selects the CS domain in which breakout is to occur. If the breakout occurs in another network, then it forwards the session to BGCF in the other network via the Mk reference point. The protocol used for the Mk reference point is SIP. IMS. REFERENCE POINTS Mg reference point The Mg reference point links the CS edge function, MGCF, to IMS (namely, to the I-CSCF). This reference point allows MGCF to forward incoming session signalling from the CS domain to the I-CSCF. The protocol used for the Mg reference point is SIP. IMS. REFERENCE POINTS Mm reference point For communicating with other multimedia IP networks, a reference point between the IMS and other multimedia IP networks is needed. The Mm reference point allows the 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. At the time of writing, a detailed specification of the Mm reference point has not been provided. However, it is very likely that the protocol would be SIP. IMS. REFERENCE POINTS Mr reference point When the S-CSCF needs to activate bearer-related services it passes SIP signalling to the MRFC via the Mr reference point. The functionality of the Mr reference point is not fully standardized: for example, it is not specified how the S-CSCF informs the MRFC to play a specific announcement. The used protocol in the Mr reference point is SIP. IMS. REFERENCE POINTS Mp reference point Media server architecture in IMS consists of two entities MRFC and MRFP. These two entities are connected via the Mp reference point. Over this reference point the MRFC is able to ask MRFP to do the following things: play tone to user or number of users; play announcement to user or number of users e.g. ‘person you try to reach is currently out of coverage or not able to receive multimedia communication’; generate speech output from text or annotated text input; record audio or multimedia stream(s) and store it into a file. The function can be used in some services, such as the voice mail box service, conference service, etc; collect and report dialed DTMF digits e.g. to get PIN code for voice mail box; perform automatic speech recognition and report the results; play synchronized audio and video media streams to the user. The function can be used in the services, such as multimedia announcement, multimedia mail box service, etc; provide conferencing transport plane capabilities for audio and multimedia conferencing service; transcoding of audio and video streams. IMS. REFERENCE POINTS Mn reference point The Mn interface is the control reference point between the MGCF and IMS-MGW. The Mn interface controls the user plane between IP access and IMS-MGW (Mb reference point). Also, it controls the user plane between CS access and IMS- MGW. The Mn interface is based on H.248. The Mn interface introduces new H.248 procedures for handling IP access end termination and also some additional procedures for CS end termination handling. The H.248 is primarily used to perform the following tasks: creating or releasing connection between IMS and CS user; connecting or releasing tones to end-point; connecting or releasing announcements to end-point e.g. ‘subscriber you try to reach is speaking on phone at the moment please hold’; sending or receiving DTMF tones e.g. PIN code to verify yourself to the banking service; transcoding of audio and video streams. IMS. REFERENCE POINTS Gx reference point It is in operators’ interests to ensure that IMS session information is correctly taken in use in transport level and IMS entities gets information about main transport level events. For this purpose Diameter based Gx reference point between PCRF and access gateway (e.g. GGSN) was developed. The main procedures supported over this reference point are: delivering gating control ‘Firewall’ instructions i.e. how incoming and outgoing packets should be treated in access gateway; passing instructions what kind of QoS treatment should be applied for particular IP flows; reporting traffic plane event (e.g. bearer is released or lost) as well as the reporting of events related to the resources in the access gateway; capability for exchange of IMS charging identifier and access charging identifier; distribution of primary and secondary addresses of offline and online charging entity addresses to the access gateway; activation of online and offline charging in access gateway (enabled/disabled); rating group information (e.g. 0.1euros per minute); IMS. REFERENCE POINTS Rx reference point When policy and charging control is used in the network the P-CSCF sends information obtained from SIP/SDP session setup signalling to the PCRF via the Rx reference point. This information enables the PCRF to form authorized IP QoS data (e.g. maximum bandwidth and QoS class) and charging rules (e.g. 0.1euros/minute) that will be delivered to the access gateway via the Gx reference point. The P-CSCF is tasked to send policy information to the PCRF about every SIP message that includes an SDP payload. This ensures that the PCRF passes the proper information to perform policy and charging control for all possible IMS session setup scenarios IMS. REFERENCE POINTS Rx reference point When policy and charging control is used in the network the P-CSCF sends information obtained from SIP/SDP session setup signalling to the PCRF via the Rx reference point. This information enables the PCRF to form authorized IP QoS data (e.g. maximum bandwidth and QoS class) and charging rules (e.g. 0.1euros/minute) that will be delivered to the access gateway via the Gx reference point. The P-CSCF is tasked to send policy information to the PCRF about every SIP message that includes an SDP payload. This ensures that the PCRF passes the proper information to perform policy and charging control for all possible IMS session setup scenarios IMS. REFERENCE POINTS Charging reference points Charging-related reference points Rx and Gx IMS. REFERENCE POINTS Mx and Ix reference points The Mx reference point is targeted to enable communication between CSCFs/BGCF and Interconnection Border Control Function (IBCF) to use capabilities of IBCF such as IP version interworking and network topology hiding functionality. IBCF is further expected to control TrGW via the Ix reference point. IMS. REFERENCE POINTS Ml reference point This reference point is used for IMS emergency sessions. E-CSCF uses it when it needs to verify UE provided location information or to obtain location information or to obtain routing information to emergency centres from LRF