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11.1 General
Table 11.1-1 illustrates the API exposing function APIs. Table 11.1-1: List of API exposing function APIs API Name API Operations Known Consumer(s) Communication Type AEF_Security API Revoke_Authorization CAPIF Core Function Request/ Response Initiate_Authentication API Invoker Request/ Response
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11.2 AEF_Security API
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11.2.1 General
API description: This API allows CAPIF core function to revoke access to service APIs and API invokers to request the authentication parameters necessary for authentication of the API invoker available with the API exposing function.
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11.2.2 Revoke_Authorization operation
API operation name: Revoke_Authorization Description: Revokes API invoker authorization to access service API. Known Consumers: CAPIF core function. Inputs: Refer subclause 8.23.2. Outputs: Refer subclause 8.23.2. See subclause 8.23.4 for the details of usage of this API operation.
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11.2.3 Initiate_Authentication operation
API operation name: Initiate_Authentication Description: Authentication between the API invoker and the AEF prior to service API invocation. Known Consumers: API Invoker. Inputs: Refer subclause 8.14.2. Outputs: Refer subclause 8.14.2. See subclause 8.14.3 for the details of usage of this API operation. Annex A...
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1 Scope
This document defines the stage-2 service description for the IP Multimedia Core Network Subsystem (IMS), which includes the elements necessary to support IP Multimedia (IM) services. ITU‑T Recommendation I.130 [4] describes a three-stage method for characterisation of telecommunication services and ITU‑T Recommendatio...
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2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present document. - References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific. - For a specific reference, subsequent revisions do not apply. -...
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3 Definitions, symbols and abbreviations
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3.1 Definitions
Refer to TS 23.002 [1] for the definitions of some terms used in this document. For the purposes of the present document the terms and definitions given in TR 21.905 [68] and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [68]. F...
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3.2 Symbols
For the purposes of the present document the following symbols apply: Cr Reference Point between an AS and an MRFC for media control. Cx Reference Point between a CSCF and an HSS. Dx Reference Point between an I‑CSCF and an SLF. Gi Reference point between GPRS and an external packet data network. Gm Reference Poin...
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3.3 Abbreviations
For the purposes of the present document, the abbreviations given in TR 21.905 [68] and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905 [68]. 5GS 5G System API Application Program Interface APN Access Point Name ...
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4 IP multimedia subsystem concepts
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4.0 General
The IP Multimedia CN subsystem comprises all CN elements for provision of multimedia services. This includes the collection of signalling and media related network elements as defined in TS 23.002 [1]. IP multimedia services are based on an IETF defined session control capability which, along with multimedia transport ...
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4.1 Relationship to CS domain and the IP-Connectivity Access Network
The IP multimedia subsystem utilizes the IP‑CAN to transport multimedia signalling and bearer traffic. IP‑CANs that maintain the service while the terminal moves, hide these moves from the IP multimedia subsystem. The IP multimedia subsystem is independent of the CS domain although some network elements may be common ...
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4.2 IMS services concepts
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4.2.1 Home-network based services
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4.2.1.1 Support of CAMEL or IN
It shall be possible for an operator to offer access to services based on the CSE or IN Service Environment for its IM CN subsystem subscribers. It should be noted that there is no requirement for any operator to support CAMEL or IN services for their IM CN subsystem subscribers or for inbound roamers. For more inform...
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4.2.1.2 Support of OSA
It shall be possible for an operator to offer access to services based on OSA for its IM CN subsystem subscribers. This shall be supported by an OSA API between the Application Server (AS) and the network. For more information refer to clause 4.2.4.
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4.2.1.3 Dynamic services interactions handling
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4.2.1.3.1 Service information exchanged between Application Servers
To avoid conflicting interactions between services they execute, different ASs involved in the same IMS session (within an operator network or across networks) shall be able to exchange the following service interaction information: - indication of services that have been performed and - optionally, additional indica...
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4.2.1.3.2 Handling by the Application Server
If an AS provides one or more services, the AS may include service interaction information in SIP signalling, identifying the service that it has executed. If an AS provides one or more services which are known to be negatively impacted by the subsequent execution of a service by another AS, the AS may include, in add...
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4.2.1.3.3 Deletion of services interaction information
The service interaction information shall be removed when it is sent to the UE via P-CSCF or to an entity outside the trust domain or when it is not in compliance with service level agreements with other domains.
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4.2.2 Support of numbers in non-international format in the IMS
Phone or telephone numbers which are not in the international format can allow the access of the visited services (local service numbers) and the access of numbers in a local addressing plan. Since numbers in non-international format are widely used in legacy fixed and mobile CS networks the seamless co-operation with ...
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4.2.3 Support of roaming users
The architecture shall be based on the principle that the service control for Home subscribed services for a roaming subscriber is in the Home network, e.g. the Serving‑CSCF is located in the Home network. Figure 4.1: Service Platform in Home Network Figure 4.2: External Service Platform There are two possible s...
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4.2.4 IP multimedia Subsystem Service Control Interface (ISC)
The ISC interface is between the Serving CSCF and the service platform(s). An Application Server (AS) offering value added IM services resides either in the user's home network or in a third party location. The third party could be a network or simply a stand-alone AS. The Serving‑CSCF to AS interface is used to prov...
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4.2.5 The QoS requirements for an IM CN subsystem session
The selection, deployment, initiation and termination of QoS signalling and resource allocation shall consider the following requirements so as to guarantee the QoS requirement associated with an IM CN subsystem session. 1. Independence between QoS signalling and Session Control The selection of QoS signalling and r...
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4.2.6 QoS Requirements for IM CN subsystem signalling
Depending on the bearer establishment mode, the UE or the IP‑CAN shall be able to establish a dedicated signalling IP‑CAN bearer for IM Subsystem related signalling or utilize a general-purpose IP‑CAN bearer for IM subsystem signalling traffic. The use of a dedicated signalling IP‑CAN bearer for IM Subsystem related s...
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4.2.7 Support of SIP forking
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4.2.7.1 SIP Forking
SIP forking is the ability of a SIP proxy server to fork SIP request messages to multiple destinations according to IETF RFC 3261 [12].
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4.2.7.2 Forking within and outside the IM CN Subsystem
The IM CN subsystem shall have the capability to fork requests to multiple destinations; this capability is subject to rules for forking proxies defined in IETF RFC 3261 [12]. - The S‑CSCF shall support the ability for a Public User Identity to be registered from multiple contact addresses, as defined in IETF RFC 3261...
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4.2.7.3 Support for forked requests
UE and MGCF shall be ready to receive responses generated due to a forked request and behave according to the procedures specified in IETF RFC 3261 [12] and in this clause. The UE and MGCF may accept or reject early dialogues from different terminations as described in IETF RFC 3261 [12], for example if the UE is only...
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4.3 Naming and addressing concepts
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4.3.1 Address management
The mechanisms for addressing and routing for access to IM CN subsystem services and issues of general IP address management are discussed in TS 23.221 [7]. When a UE is assigned an IPv6 prefix, it can change the global IPv6 address it is currently using via the mechanism defined in IETF RFC 4941 [16a], or similar mea...
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4.3.2 Void
Figure 4.4: Void
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4.3.3 Identification of users
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4.3.3.0 General
There are various identities that may be associated with a user of IP multimedia services. This clause describes these identities and their use.
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4.3.3.1 Private User Identities
Every IM CN subsystem user shall have one or more Private User Identities. The private identity is assigned by the home network operator and used, for example, for Registration, Authorization, Administration and Accounting purposes. This identity shall take the form of a Network Access Identifier (NAI) as defined in IE...
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4.3.3.2 Public User Identities
Every IM CN subsystem user shall have one or more Public User Identities (see TS 22.228 [8]), including at least one taking the form of a SIP URI (see IETF RFC 3261 [12]). The Public User Identity is used by any user for requesting communications to other users. For example, this might be included on a business card. ...
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4.3.3.3 Routing of SIP signalling within the IP multimedia subsystem
Routing of SIP signalling within the IMS shall use SIP URIs or other (non SIP) AbsoluteURIs. AbsoluteURIs are defined in IETF RFC 3986 [13]. Routing of SIP signalling within the IMS using AbsoluteURI (non SIP) shall only be supported for IMS signalling from IMS user to external networks. E.164 [2] format Public User Id...
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4.3.3.4 Relationship of Private and Public User Identities
The home network operator is responsible for the assignment of the Private User Identities and Public User Identities; other identities that are not defined by the operator may also exist. Figure 4.5: Relationship of the Private User Identity and Public User Identities The IMS Service Profile is a collection of ser...
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4.3.3.5 Relationship of Public User Identities, GRUUs and UEs
Each Public User Identity may have one or more Globally Routable User Agent URIs (GRUUs). There are two types of GRUU, P‑GRUUs and T‑GRUUs which are associated with Public User Identities and are generated and assigned to the UE together during registrations and re-registration in a pair of one P‑GRUU and one T‑GRUU. E...
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4.3.4 Identification of network nodes
The CSCF, BGCF and MGCF nodes shall be identifiable using a valid SIP URI (Host Domain Name or Network Address) on those interfaces supporting the SIP protocol, (e.g. Gm, Mw, Mm and Mg). These SIP URIs would be used when identifying these nodes in header fields of SIP messages. However this does not require that these ...
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4.3.5 E.164 address to SIP URI resolution in an IM CN subsystem
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4.3.5.1 ENUM/DNS translation mechanism
The ENUM/DNS translation mechanism as specified in IETF RFC 3761 [16] can be used by all IMS nodes that require E.164 address to SIP URI resolution. The actual ENUM/DNS database(s) used to perform address translations are outside the scope of 3GPP and are therefore a matter for the network operator. There is no require...
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4.3.5.2 Handling of Tel URIs
The S‑CSCF shall support the ability to translate the E.164 address contained in a Request-URI in the Tel: URI format (as specified in IETF RFC 3966 [15]) to a SIP routable SIP URI using the ENUM/DNS translation mechanism as specified in clause 4.3.5.1. If this translation succeeds, then the session shall be routed acc...
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4.3.5.3 Handling of SIP URIs representing a telephone number
Per network operator policy, the network may attempt to resolve and route a SIP URI representing a telephone number and a domain that does not own the target user using the ENUM/DNS translation mechanism specified in clause 4.3.5.1. The need for address resolution may be triggered by the S‑CSCF and the I‑CSCF or transi...
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4.3.6 Public Service Identities
With the introduction of standardized presence, messaging, conferencing and group service capabilities in IM CN subsystem, there is a need for Public Service Identities (PSIs). These identities are different from the Public User Identities in the respect that they identify services, which are hosted by Application Serv...
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4.4 Signalling concepts
A Single session control between the UE and CSCF: - For Multi-Media type services delivered via the IP‑CAN within this architecture, a single session control protocol shall be used between the user equipment UE and the CSCF (over the Gm reference point). Protocols over the Gm reference point : - The single protocol ...
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4.5 Mobility related concepts
The following procedures are supported by an UE when accessing IMS: - Connect to the IP‑CAN and acquire the necessary IP address, which includes, or is followed by, the P‑CSCF discovery procedure. The mobility related procedures and IP address management principles for the IP‑CAN are described in the relevant IP‑CAN s...
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4.6 Roles of Session Control Functions
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4.6.0 General
The CSCF may take on various roles as used in the IP multimedia subsystem. The following clauses describe these various roles.
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4.6.1 Proxy‑CSCF
The Proxy‑CSCF (P‑CSCF) is the first contact point within the IM CN subsystem. Its address is discovered by UEs using the mechanism described in the clause "Procedures related to Local CSCF Discovery". The P‑CSCF behaves like a Proxy (as defined in IETF RFC 3261 [12] or subsequent versions), i.e. it accepts requests an...
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4.6.2 Interrogating‑CSCF
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4.6.2.0 General
Interrogating‑CSCF (I‑CSCF) is the contact point within an operator's network for all connections destined to a user of that network operator, or a roaming user currently located within that network operator's service area. NOTE- 1: If border control concepts are applied, the contact point within an operator's network...
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4.6.2.1 Void
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4.6.3 Serving‑CSCF
The Serving‑CSCF (S‑CSCF) performs the session control services for the UE. It maintains a session state as needed by the network operator for support of the services. Within an operator's network, different S‑CSCFs may have different functionalities. The functions performed by the S‑CSCF during a session are: For Reg...
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4.6.4 Breakout Gateway Control Function
Based on local configuration, the Breakout Gateway Control Function (BGCF) may be provisioned as the contact point within an operator's network for transit IMS scenarios as described in clause 5.19. Otherwise the BGCF processes requests for routing from an S‑CSCF for the case were the S‑CSCF has determined that the ses...
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4.6.5 Void
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4.7 Multimedia Resource Function
The architecture concerning the Multimedia Resource Function is presented in Figure 4.7 below. Figure 4.7: Architecture of MRF The MRF is split into Multimedia Resource Function Controller (MRFC) and Multimedia Resource Function Processor (MRFP). Tasks of the MRFC are the following: - Control the media stream res...
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4.8 Security Concepts
IM CN Subsystem functional elements provide security, as needed, by security methods defined in TS 33.203 [19] and TS 33.210 [20]. If interacting with external Networks, Security Associations are provided in accordance with operator policy.
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4.9 Charging Concepts
IM CN subsystem functional elements provide support for offline and online charging. This includes support for charging correlation, e.g. between IM CN subsystem and PS domain. The charging architecture, charging principles and charging data for IM CN subsystem are described in TS 32.240 [25] and TS 32.260 [26]. The ch...
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4.10 IMS group management concepts
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4.10.0 General
This clause describes architectural concepts to fulfil the requirements for IMS Group Management described in TS 22.250 [32].
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4.10.1 IMS group administration
The capabilities required for IMS group management are defined in clause 5.4 of TS 22.250 [32]. The Ut reference point is used to manage groups from the UE. This does not preclude the use of other mechanisms for group management, e.g. using OSA or OA&M mechanisms; the details of these other mechanisms are out of scope ...
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4.10.2 Group identifiers
Each group shall be addressable by a globally unique group identifier. The group identifier shall take the form of a Public Service Identifier.
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4.11 Relationship to 3GPP Generic User Profile (GUP)
It shall be possible to apply the mechanisms and format of the 3GPP Generic User Profile (GUP) to IM CN Subsystem user related data. The 3GPP Generic User Profile (GUP) is described in TS 23.240 [31].
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4.12 Network Address Translation traversal in access network
It shall be possible to support the scenario where a NAT(-PT)/NAPT(-PT) residing between the IMS functionality in the UE and the P‑CSCF has to be traversed for IMS communication. This shall include at least the types of NATs that implement address and port dependent mapping together with address and port dependent filt...
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4.13 Identification of IMS communication Services
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4.13.1 General
This clause describes the architectural requirements for the identification of IMS communication services.
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4.13.2 Identification of IMS communication Services
An IMS Communication Service Identifier (ICSI) provides a framework for the identification of IMS communication services utilising the IMS enablers. An IMS communication service is provided via the use of the IMS enablers. At terminals, the use of a communication service identifier is similar to the use of the port con...
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4.13.3 Identification of IMS applications
An IMS application is an application that uses an IMS communication service(s) in order to provide a specific service to the end-user. The IMS application uses specific IMS Communication Service(s) and provides the end user service through the reuse of the SIP communication part of service. The IMS application does not...
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4.14 Border Control concepts
Based on operator preference, border control functions may be applied between two IM CN subsystem networks or between an IM CN subsystem network and other SIP based multimedia network. These functions are provided by the IBCF and include: - Controlling transport plane functions; - Supporting functions to allow establ...
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4.15 IMS in transit network scenarios
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4.15.1 General concepts
IMS generally provides services to end user customers of a network operator by directly supporting multimedia communications services to or from that operator's customers. However IMS may also be used in a number of other configurations where the capabilities of IMS are used to support CS domain customers of an IMS ope...
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4.15.2 IMS transit network configurations
There are at least three general cases in which IMS may be used for transit network support. These could be classified as in the following: a) IMS operator providing transit functionality for its own, non-IMS (CS domain), customers: In this case the operator is serving its own customers, some of which have been migra...
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4.15.3 Providing IMS application services in transit network scenarios
When IMS provides transit functionality to other network operators or enterprise networks, the IMS may also provide IMS applications services to the network operator or enterprise network. Figure 4.15.3-1: IMS application services reference point for transit network scenarios The Transit service invocation shall be...
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4.16 Support of multimedia telephony
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4.16.1 Telephony Application Server
The Telephony Application Server is a SIP AS providing the network support for the multimedia telephony service, TS 22.173 [53]. If specific procedures and message flows include or require media interaction, the TAS and MRFC may be collocated. NOTE: The support of multimedia telephony services may be allocated to one ...
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4.16.2 Identification of multimedia telephony
The multimedia telephony communication service shall be associated with a communication service identifier to allow easy identification of the service. When multimedia telephony is supported in a network, Voice/video calls originating from the PSTN/CS domain shall be marked with the communication service identifier as...
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4.16.3 Session setup principles
When establishment of UE initiated IP‑CAN bearer(s) for the media is required it is recommended to reserve IP‑CAN bearer(s) at the reception of the SDP answer. If the UE has been made aware of the operator policies with regards to allowed media for the multimedia telephony service, then the UE may reserve IP‑CAN bearer...
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4.17 Support of short message service
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4.17.1 IP Short Message Gateway (IP-SM‑GW)
The IP‑SM‑GW acts as an SIP-AS in the IMS domain to provide the protocol interworking for the delivery of the short message between the UE and the Service Centre. All functionalities and interfaces of IP‑SM‑GW are defined in TS 23.204 [56].
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4.18 Support of Number portability
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4.18.1 Number portability
Number portability (NP) allows a user to retain their E.164 number when changing subscriptions from one network operator to another. As such, NP applies to TEL URIs and SIP URIs representing E.164 addresses. NP is subject to regional requirements and is accomplished through the retrieval of ported data from those datab...
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4.19 Support of Preferred Circuit Carrier Access and Per Call Circuit Carrier Selection
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4.19.1 Preferred Circuit Carrier Access and Per Call Circuit Carrier Selection
Preferred Circuit Carrier Access allows the network operator to configure a preferred long distance circuit carrier for a subscriber, set of subscribers or all subscribers on the network. All long distance calls from a subscriber are routed to the long distance circuit carrier when preferred circuit carrier access appl...
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4.20 Support of IMS Service Centralization and Continuity
IMS Service Centralization, defined in TS 23.292 [66] provides communication services such that all services and service control, are based on IMS mechanisms and enablers. It enables IMS services when using CS access as bearer for the media. IMS Service Continuity, defined in TS 23.237 [67] provides Session Transfer m...
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4.21 Support of Overlap Signalling
The support of overlap signalling consists of the functionality for conversion between overlap and en-bloc, as well as the functionality for digit collection. The above mentioned functionalities may be implemented in different network nodes depending on the operator's deployment strategy (e.g. AS, IBCF, MGCF). NOTE 1...
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4.22 Support of Explicit Congestion Notification (ECN)
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4.22.1 General
The ECN profile used to trigger codec rate adaptation for Multimedia Telephony is defined in TS 26.114 [76] and affects the following IMS entities: UE, MGCF/IM-MGW, IBCF/TrGW, IMS-ALG/ IMS-AGW, MRFP/MRFC and the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW. As specified in TS 26.114 [76]: ...
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4.22.2 CS GERAN/UTRAN Interworking at MGCF/IM-MGW
If MGCF/IM-MGW supports Multimedia Telephony compliant ECN, it shall: - support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the MGCF and IM-MGW do not determine whether ECN can be used based on the Radio Access Technology that is used towards the client; the MGCF/ IM-MGW act ...
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4.22.3 Interworking with non-ECN IP network and/or terminal at IBCF/TrGW
An IBCF/TrGW may support Multimedia Telephony using ECN and can be used to enable ECN within the local network when either the remote network cannot be confirmed to properly handle ECN-marked packets or the remote entity does not support or use ECN. In order to support Multimedia Telephony using ECN when interworking ...
d2f3106e4987173630adee044e0dad89
23.228
4.22.4 Interworking with non-3GPP ECN IP terminal at IBCF/TrGW
An IBCF/TrGW supporting Multimedia Telephony compliant ECN can also be used to enable ECN end-to-end if the remote entity uses ECN in a different way than what is described in TS 26.114 [76], e.g. if the remote entity only supports probing for the ECN initiation phase or it needs the ECN feedback. NOTE: For this inter...
d2f3106e4987173630adee044e0dad89
23.228
4.22.5 ECN support at IMS-ALG/IMS-AGW
The P-CSCF shall be able to disallow the negotiation of ECN during SDP offer/answer exchanges if the IMS-AGW does not support transparent forwarding of the ECN bits or if the IMS core network or the access network used towards the Multimedia Telephony terminal do not properly handle ECN-marked packets. An IMS-AGW supp...
d2f3106e4987173630adee044e0dad89
23.228
4.22.6 ECN support at MRFC/MRFP
An MRFC/MRFP may support Multimedia Telephony using ECN and may act as an ECN endpoint to enable ECN with a local ECN-capable terminal within a local network that properly handles ECN-marked packets (see TS 23.333 [73]). This requires that the MRFC/MRFP performs the following: - support SDP ability to negotiate to EC...
d2f3106e4987173630adee044e0dad89
23.228
4.22.7 CS GERAN/UTRAN Interworking at the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW
If the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW supports Multimedia Telephony compliant ECN, it shall support the procedures specified in clause 4.22.2 respectively for the MGCF and IM-MGW.
d2f3106e4987173630adee044e0dad89
23.228
4.23 Support of Load Balancing
d2f3106e4987173630adee044e0dad89
23.228
4.23.1 General
An IMS network may implement functionality for IMS serving network nodes (S‑CSCF, Transit Function) to handle load balancing, i.e. the technique to distribute workload evenly across two or more network nodes implementing the same functions, in order to get optimal resource utilization. For S‑CSCFs, load balancing may ...
d2f3106e4987173630adee044e0dad89
23.228
4.23.2 Registration-based load balancing of S-CSCFs
Load balancing of S‑CSCFs for received initial registration requests may be based on load balancing functionality performed by an I‑CSCF or it may be based on mechanisms outside IMS functional entities (such as DNS). An example of the DNS-based load balancing approach is the use of functionality that collects load info...
d2f3106e4987173630adee044e0dad89
23.228
4.23.3 Registration independent load balancing of Transit Functions
Load balancing of Transit Functions for received service requests may be based on load balancing functionality performed by an IMS functional entity that is the immediate source of the service request to the Transit Function (e.g. an IBCF or an I‑CSCF), or it may be based on mechanisms outside IMS functional entities (...