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4.24 Support of Restoration Procedures
An I‑CSCF that performs recovery of S‑CSCFs for registration requests shall determine failure of an S‑CSCF implicitly (i.e. via a timeout) or explicitly (i.e. via a failure message) and shall reroute the registration request to another S‑CSCF based on the restoration procedures as defined in TS 23.380 [80].
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4.25 Support of Overload Control
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4.25.1 General
Network elements within the IP Multimedia CN subsystem may have to cope with high volume of signalling traffic with significant traffic peaks. An IMS network may therefore implement overload control functionality in IMS network elements to prevent overload situations. For that purpose, two different mechanisms are pro...
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4.25.2 Next-hop monitoring of overload
For IMS entities supporting next-hop monitoring of overload, these IMS entities shall support a mechanism for monitoring overload of neighbour nodes with minimal overhead, to enable scenarios where the overload status needs to be adjusted frequently. IMS entities supporting SIP, e.g. S-CSCF, shall be able to provide o...
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4.25.3 Filter based Overload Control
When filter based Overload Control is deployed, an IMS entity supporting SIP overload control (e.g. originating AS or S-CSCF) may subscribe to traffic filter information of specific IMS SIP server destination which may be subject to overload (e.g. an 800 application overloaded by mass calling to a specific number) and ...
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4.26 Support for Business Trunking
An IMS network may support business trunking for IP‑PBXs in two different modes. - Registration mode, or - Static mode. In both modes, the IP‑PBX can be provisioned as a subscriber in the HSS. In registration mode, the IP‑PBX registers to and receives service from the IMS network in same manner as an ordinary subsc...
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5 IP multimedia subsystem procedures
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5.0 General
This clause documents the main procedures that are used for the provision of services in the IP multimedia subsystem. These procedures are described using text description as well as information flow diagrams. The procedures described in this document are meant to provide a high level description and are not intended t...
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5.1 CSCF related procedures
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5.1.0 Establishing IP-Connectivity Access Network bearer for IM CN Subsystem Related Signalling
Before the UE can request IM services, an appropriate IP‑CAN bearer must be available to carry IM Subsystem related signalling. For a UE using the IMS Local Breakout procedure as shown in Annex M, the IP address of the UE obtained from the local Gateway (i.e. single IP address) is used for both IM Subsystem related si...
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5.1.1 Procedures related to Proxy‑CSCF discovery
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5.1.1.0 General
The Proxy‑CSCF discovery shall be performed using one of the following mechanisms: - As part of the establishment of connectivity towards the IP-Connectivity Access Network, if the IP-Connectivity Access Network provides such means. - Alternatively, the P‑CSCF discovery may be performed after the IP connectivity has ...
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5.1.1.1 DHCP/DNS procedure for P‑CSCF discovery
The DHCP relay agent within the IP-Connectivity Access Network relays DHCP messages between UE and the DHCP server. Figure 5.0a: P‑CSCF discovery using DHCP and DNS 1. Establish an IP-Connectivity Access Network bearer if not already available by using the procedures available in the IP-Connectivity Access Network....
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5.1.1.2 Void
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5.1.2 Procedures related to Serving‑CSCF assignment
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5.1.2.1 Assigning a Serving‑CSCF for a user
When a UE attaches and makes itself available for access to IMS services by explicitly registering in the IMS, a S‑CSCF shall be assigned to serve the UE. The assignment of an S‑CSCF is performed in the I‑CSCF. The following information is needed in the selection of the S‑CSCF: 1. Required capabilities for user servi...
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1 The Cx reference point shall support the transfer of CSCF-UE security parameters from HSS to CSCF.
- This allows the CSCF and the UE to communicate in a trusted and secure way (there is no à priori trust relationship between a UE and a CSCF) - The security parameters can be for example pre-calculated challenge-response pairs, or keys for an authentication algorithm, etc.
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2 The Cx reference point shall support the transfer of service parameters of the subscriber from HSS to CSCF.
- This may include e.g. service parameters, Application Server address, triggers, information on subscribed media etc. The information on subscribed media is provided in the form of a profile identifier; details of the allowed media parameters associated with the profile identifier are configured in the S‑CSCF.
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3 The Cx reference point shall support the transfer of CSCF capability information from HSS to CSCF.
- This may include e.g. supported service set, protocol version numbers etc.
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4 The Cx reference point shall support the transfer of session signalling transport parameters from CSCF to HSS. The HSS stores the signalling transport parameters and they are used for routing mobile terminated sessions to the Serving‑CSCF.
- The parameters may include e.g. IP-address and port number of CSCF, transport protocol etc. The information mentioned in items 1 – 4 above shall be transferred before the CSCF is able to serve the user. It shall also be possible to update this information while the CSCF is serving the user, for example if new servic...
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5.1.2.2 Cancelling the Serving‑CSCF assignment
Cancellation of the assigned Serving CSCF is either: - Initiated from the Serving CSCF itself, e.g. due to timeout of the registration - Performed as a result of an explicit deactivation/de-registration from the IMS. This is triggered by the UE. - Performed due to a request from the HSS over the Cx interface, e.g. d...
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5.1.2.3 Void
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5.1.3 Procedures related to Interrogating‑CSCF
The architecture shall support multiple I‑CSCFs for each operator. A DNS-based mechanism for selecting the I‑CSCF shall be used to allow requests to be forwarded to an I‑CSCF based, for example, on the location or identity of the forwarding node.
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5.1.4 Procedures related to Proxy‑CSCF
The routing of the SIP registration information flows shall not take into account previous registrations (i.e. registration state). The routing of the session information flows (e.g. INVITE) shall take into account the information received during the registration process.
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5.1.5 Subscription Updating Procedures
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5.1.5.0 General
Whenever a modification has occurred in the subscription data that constitutes the data used by the S‑CSCF, the complete subscription data set shall be sent to the S‑CSCF by the HSS. HSS shall use the Push model for downloading the subscription data to the S‑CSCF.
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5.1.5.1 Subscription updating information flow
This clause provides the information flows for subscription data updating procedure. Figure 5.0b: Subscription data updating 1. The HSS sends the Cx-Update_Subscr_Data with the subscription data to the S‑CSCF. 2. The S‑CSCF sends Cx-Update_Subscr_Data Resp to the HSS to acknowledge the sending of Cx-Update_Subscr_...
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5.2 Application level registration procedures
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5.2.0 General
The following clauses address requirements and information flows related to registration in the IP multimedia subsystem. Assumptions that apply to the various information flows are listed as appropriate.
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5.2.1 Requirements considered for registration
The following points are considered as requirements for the purpose of the registration procedures. 1. The architecture shall allow for the Serving‑CSCFs to have different capabilities or access to different capabilities. E.g. a VPN CSCF or CSCFs in different stages of network upgrade. 2. The network operator shall n...
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5.2.2 Registration flows
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5.2.2.1 Requirements to consider for registration
The additional requirement for the registration information flow for this clause is: 1. A Serving‑CSCF is assigned at registration, this does not preclude additional Serving‑CSCFs or change of CSCF at a later date. Procedures for use of additional CSCFs are not standardised in this release.
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5.2.2.2 Assumptions
The following are considered as assumptions for the registration procedures as described in clause 5.3.2.3: 1. IP‑CAN bearer is already established for signalling and a mechanism exists for the first REGISTER message to be forwarded to the proxy. 2. The I‑CSCF shall use a mechanism for determining the Serving‑CSCF ad...
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5.2.2.3 Registration information flow – User not registered
The application level registration can be initiated after the registration to the access is performed and after IP connectivity for the signalling has been gained from the access network. For the purpose of the registration information flows, the user is considered to be always roaming. For user roaming in their home n...
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5.2.2.4 Re-Registration information flow – User currently registered
Periodic application level re-registration is initiated by the UE either to refresh an existing registration or in response to a change in the registration status of the UE. A re-registration procedure can also be initiated when the capabilities of the UE have changed or the IP‑CAN has changed. The UE should perform IM...
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5.2.2.5 Stored information.
Table 5.1 provides an indication of some of the information stored in the indicated nodes during and after the registration process. Note that Table 5.1 is not an exhaustive list of stored information, i.e. there can be additional information stored due to registration. Table 5.1 Information Storage before, during and...
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5.3 Application level de-registration procedures
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5.3.1 Mobile initiated de-registration
When the UE wants to de-register from the IMS then the UE shall perform application level de-registration. De-registration is accomplished by a registration with an expiration time of zero seconds. De-registration follows the same path as defined in clause 5.2.2.3 "Registration Information Flow – User not registered". ...
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5.3.2 Network initiated de-registration
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5.3.2.0 General
If an ungraceful session termination occurs (e.g. flat battery or mobile leaves coverage), when a stateful proxy server (such as the S‑CSCF) is involved in a session, memory leaks and eventually server failure can occur due to hanging state machines. To ensure stable S‑CSCF operation and carrier grade service, a mechan...
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5.3.2.1 Network Initiated Application (SIP) De-registration, Registration Timeout
The following flow shows a network initiated IM CN subsystem terminal application (SIP) de-registration based on a registration timeout. A timer value is provided at initial registration and is refreshed by subsequent re-registrations. The flow assumes that the timer has expired. The locations (home or visited network)...
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5.3.2.2 Network Initiated Application (SIP) De-registration, Administrative
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5.3.2.2.0 General
For different reasons (e.g. subscription termination, lost terminal, etc.) a home network administrative function may determine a need to clear a user's SIP registration. This function initiates the de-registration procedure and may reside in various elements depending on the exact reason for initiating the de-registra...
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5.3.2.2.1 Network Initiated De-registration by HSS, administrative
Figure 5.5: Network initiated application de-registration by HSS, administrative 1. HSS initiates the de-registration, sending a Cx-Deregister (user identity) which may include the reason for the de-registration. 2. Based on the filter criteria, the S‑CSCF shall send de-registration information to the service contr...
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5.3.2.2.2 Network Initiated De-registration by Service Platform
A service platform may determine a need to clear a user's SIP registration. This function initiates the de-registration procedure and resides in a service platform. The following flow shows a service control initiated IMS terminal application (SIP) de-registration. Figure 5.5a: Network initiated application de-regi...
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5.4 Procedures for IP multi-media sessions
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5.4.0 General
Basic IMS sessions between users will always involve two S‑CSCFs (one S‑CSCF for each). The session flow is decomposed into two parts: an origination part between the UE & the S‑CSCF and termination part between the S‑CSCF and the UE, including all network elements in the path. A basic session between a user and a PST...
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5.4.1 Bearer interworking concepts
Voice bearers from the IM CN subsystem need to be connected with the voice bearers of other networks. Elements such as Media Gateway Functions (MGW) are provided to support such bearer interworking. One of the functions of the MGW may be to support transcoding between a codec used by the UE in the IM CN subsystem and t...
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5.4.2 Interworking with Internet
Depending on operator policy, the S‑CSCF may forward the SIP request or response to another SIP server located within an ISP domain outside of the IM CN subsystem. It is possible that the external SIP client does not support one or more of the SIP extensions required for IMS end points to set up IMS sessions (e.g. Pre...
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5.4.3 Interworking with PSTN
The S‑CSCF, possibly in conjunction with an Application Server, shall determine that the session should be forwarded to the PSTN. The S‑CSCF will forward the Invite information flow to the BGCF in the same network. The BGCF selects the network in which the interworking should occur and the selection of the interworkin...
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5.4.4 Requirements for IP multi-media session control
In order for operators to be able to offer a "carrier-grade" IP multimedia service and to require bearers whose features (e.g. Bandwidth) are coherent with the media components negotiated through CSCFs, the following features shall be offered: 1. Both end points of the session shall be able to negotiate (according to ...
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5.4.5 Session Path Information
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5.4.5.1 Session Path Information during Registration and Session Initiation
During registration and session initiation there are SIP mechanisms, which provide the means to determine the session path. After registration the P‑CSCF stores the S‑CSCF name, possibly IBCF names and the S‑CSCF stores the P‑CSCF name and possibly IBCF names (see 4.3.4) as part of the UE related information. There i...
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5.4.5.2 P‑CSCF in the Session Path
All SIP signalling to or from the UE traverses the P‑CSCF.
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5.4.5.3 S‑CSCF in the Session Path
All initial requests to or from the UE traverse the S‑CSCF assigned to the UE. The S‑CSCF uses the "Record-Route" mechanism defined in IETF RFC 3261 [12] to remain in the signalling path for subsequent requests too; in short terms: the S‑CSCF "record-routes". This is considered the default behaviour for all IMS communi...
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5.4.6 End-user preferences and terminal capabilities
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5.4.6.0 General
Due to different capabilities of the originating and terminating terminals, it might not be possible to establish all the media suggested by the originator for a particular session. In addition, the destination user may have different preferences of type of media depending on who is originating and on the situation e.g...
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5.4.6.1 Objectives
The general objectives concerning terminal capabilities and end-user behaviour are listed below. - The capabilities of the terminal have impact on the SDP description in the SIP session flows, since different terminals may support different media types (such as video, audio, application or data) and may have implement...
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5.4.6.2 End-user expectations
From the end-user point of view the following user interactions can be listed: - For outgoing sessions, it is assumed that the user would like to select certain parameters that define the proposed session. This can be pre-configured as preferences or defined on a per session basis. - For incoming sessions, it is assu...
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5.4.6.3 Mechanism for bearer establishment
In order to fulfil the above requirements, it is needed that the destination user can be pre-alerted before the bearer establishment and negotiation and IP-Connectivity Access Network bearer activation has taken place. This gives room for the destination user to choose the media streams and codecs required before an ex...
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5.4.6.4 Session progress indication to the originating UE
The pre-alerting or alerting indications returned to the originating UE shall enable the originating UE to inform the calling user of the session progress prior to the arrival of the incoming media (for example the originating UE may synthesise ringing locally).
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5.4.7 Interaction between QoS and session signalling
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5.4.7.0 General
At IP‑CAN bearer activation the user shall have access to either IP‑CAN services without Policy and Charging Control, or IP‑CAN services with Policy and Charging Control. It is operator choice whether to offer both or only one of these alternatives for accessing the IM Subsystem. When using IP‑CAN without Policy and C...
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5.4.7.1 Authorize QoS Resources
The Authorize QoS Resources procedure is used during an establishment and a modification of a SIP session. The P‑CSCF shall use the SDP contained in the SIP signalling to derive the session information that is relevant for Policy and Charging Control and forwards it to the PCRF/PCF. The PCRF/PCF shall use the received ...
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5.4.7.2 Enabling of Media Flows
The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP‑CAN that the user is now allowed to use the allocated QoS resources for per-session authorizations unless this was done based on Policy and Charging Control at the time of the Resource Reservation procedure. If there is more than o...
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5.4.7.3 Disabling of Media Flows
The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP‑CAN about revoking the user's capacity to use the allocated QoS resources for per-session authorizations. The indication for disabling media flows shall be sent as a separate decision to the PCEF within the IP‑CAN corresponding to ...
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5.4.7.4 Revoke Authorization for IP-Connectivity Access Network and IP Resources
At IP multimedia session release, the UE should deactivate the IP‑CAN bearer(s) used for the IP multimedia session. In various cases the UE will be unable to perform this release itself. The PCRF/PCF provides indication to the PCEF within the IP‑CAN when the resources previous authorized and possibly allocated by the U...
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5.4.7.5 Indication of IP-Connectivity Access Network bearer release
Any release of IP‑CAN bearer(s) that were established based on authorization from the PCRF/PCF shall be reported to the PCRF/PCF by the PCEF within the IP‑CAN. This indication is forwarded to the P‑CSCF and may be used by the P‑CSCF to initiate a session release towards the remote endpoint e.g. if all IP-CAN bearer(s)...
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5.4.7.6 Authorization of IP-Connectivity Access Network bearer modification
When an IP‑CAN bearer is modified by the UE, such that the requested QoS falls outside of the limits that were authorized at IP‑CAN bearer activation (or last modification) or such that new binding information is received, then the PCEF within the IP‑CAN shall verify the authorization of this IP‑CAN bearer modification...
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5.4.7.7 Indication of IP-Connectivity Access Network bearer modification
When an IP‑CAN bearer is modified such that the maximum bit rate (downlink and uplink) is downgraded to 0 kbit/s or changed from 0 kbit/s to a value that falls within the limits that were authorized at IP‑CAN bearer activation (or last modification) then the PCEF within the IP‑CAN shall report this to the PCRF/PCF. Th...
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5.4.7.8 Sharing of Resources for Network Detected Concurrent Sessions
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5.4.7.8.1 Network Detected Concurrent Sessions
The following scenarios for concurrent sessions are subject to resource sharing: - The UE is engaged in a session, puts the session on hold, then initiates a new session to a new UE. - The UE is engaged in a session and receives an incoming session, puts the ongoing session on hold to accept the incoming session. -...
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5.4.7.8.2 Initiating Resource Sharing for Network Detected Concurrent Sessions
If the P-CSCF is configured to apply resource sharing, it may at establishment of a new Rx session or a new Npcf_PolicyAutorization application session context with the PCRF/PCF, indicate that resources may be shared in uplink and/or downlink direction by assigning an uplink and/or downlink tag to each media component ...
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5.4.7.8.3 Void
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5.4.7.9 Priority sharing for concurrent sessions
The P-CSCF may indicate to the PCRF/PCF that the resource allocation for a media flow is allowed to use the same priority as other media flows of the same media type for the UE engaged in multiple sessions by providing a priority sharing indicator and an optional pre-emption control information in addition to the appli...
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5.4.8 QoS-Assured Preconditions
This clause contains concepts for the relation between the resource reservation procedure and the procedure for end-to-end sessions. A precondition is a set of constraints about the session, which are introduced during the session initiation. The recipient of the session generates an answer, but does not alert the use...
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5.4.9 Event and information distribution
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5.4.9.0 General
The S‑CSCF and Application Servers (SIP‑AS, IM-SSF, OSA-SCS) shall be able to send service information messages to endpoints. This shall be done based on a SIP Request/Response information exchange containing the service information and/or a list of URI(s) pointing to the location of information represented in other me...
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5.4.9.1 Subscription to event notifications
The SIP‑event notification mechanism allows a SIP entity to request notification from remote nodes indicating that certain standardised events have occurred. Examples of such of events are changes in presence states, changes in registration states, changes in Subscription authorization policies (see TS 23.141 [36]) and...
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5.4.10 Void
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5.4.11 Signalling Transport Interworking
A Signalling gateway function (S‑GW) is used to interconnect different signalling networks i.e. SCTP/IP based signalling networks and SS7 signalling networks. The signalling gateway function may be implemented as a stand alone entity or inside another entity (see TS 23.002 [1]). The session flows in this specification ...
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5.4.12 Configuration and Routing principles for Public Service Identities
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5.4.12.0 General
Depending on the service nature, different mechanisms may be used for configuration and routing of PSIs according to operator preference. When PSIs are created, the uniqueness of a PSI shall be ensured. Note that only the username part of a PSI is definable within a predefined hostname(s). Whenever possible, routing ...
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5.4.12.1 PSIs on the originating side
The Application Server hosting the PSI may be invoked as an originating Application Server. This can be achieved by modifying the filter information within the subscription information of the users intending to use the service identified by the PSI. The PSI is then made available to these users. The SIP requests are d...
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5.4.12.2 PSIs on the terminating side
The Application Server hosting the PSI may be invoked as a terminating Application Server via information stored in the HSS. Such PSIs are globally routable and can be made available to users within and outside the operator domain and can take the following form: - Distinct PSIs are defined in TS 23.003 [24]. Distinct...
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5.4.12.3 Subdomain based PSIs
Subdomains defined for PSIs allow both operators and users to define specific PSIs within subdomains for specific applications. For this purpose, subdomains can be defined by the operator in the DNS infrastructure. Specific PSIs within a subdomain can be created and deleted by users using the Ut interface to the AS hos...
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5.4.12.4 PSI configuration in the HSS
In order to support configuration of an AS hosting a PSI, the distinct PSIs and/or wildcarded PSI ranges hosted in the AS need to be configured in the HSS. The configuration shall include procedures to allow: - Distinct PSIs and wildcarded PSI ranges to be configured in the HSS via operation and maintenance procedures...
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5.4.12.5 Requests originated by the AS hosting the PSI
The AS hosting the PSI may originate requests with the PSI as the originating party. For such originating requests, the home IMS network shall be capable to perform the following functions: - Network Domain Security, TS 33.210 [20], shall be used where applicable. - Charging requirements such as providing appropriate...
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5.4.13 Transcoding concepts
IMS control plane entities, including the P‑CSCF, Application Servers or (for inter-domain sessions) the IBCF, may check the SDP offer/answer information associated with session requests and responses, to determine the need for transcoding. If such a need is determined to exist, media transcoding resources are reserved...
d2f3106e4987173630adee044e0dad89
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5.5 Serving‑CSCF/MGCF to serving‑CSCF/MGCF procedures
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5.5.0 General
This clause presents the detailed application level flows to define the procedures for Serving‑CSCF/MGCF to Serving‑CSCF/MGCF. In the IM CN subsystem the MGCF is considered as a SIP endpoint. It translates ISUP/BICC messages of the PSTN side to SIP signalling of the IM CN subsystem side and vice-versa. It should also ...
d2f3106e4987173630adee044e0dad89
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5.6 Origination procedures
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5.6.0 General
This clause presents the detailed application level flows to define the Procedures for session originations. The flows presented in the clause assume the use of Policy and Charging Control for the establishment of QoS-Assured Sessions. The session origination procedures specify the signalling path between the UE init...
d2f3106e4987173630adee044e0dad89
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5.6.5 Application Server Origination Procedure
5.6.5.1 (AS-O) Origination at Application Server This origination procedure applies to an Application Server that initiates a session on behalf of a user or a Public Service Identity. If the AS initiates the session on behalf of a user, the user may be an IMS user (i.e. referred to by a Public User Identity) or a non-...
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5.6.5.2 Void
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5.6.5.3 S‑CSCF selection by I‑CSCF for AS Originating call procedures
In figure 5.16c below the AS has no information of the serving S‑CSCF and therefore the AS sends the request to an I‑CSCF as the entry point of the home network of the Public User Identity or the Public Service Identity. The AS finds an I‑CSCF by using the same mechanism as the S‑CSCF uses to find an I‑CSCF of the term...
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5.7 Termination procedures
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5.7.0 General
This clause presents the detailed application level flows to define the Procedures for session terminations. The flows presented in the clause assume the use of Policy and Charging Control for the establishment of QoS-Assured Sessions. The session termination procedures specify the signalling path between the Serving...
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5.8 Procedures related to routing information interrogation
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5.8.0 General
When a mobile terminated session set-up arrives at an I‑CSCF that is authorized to route sessions, the I‑CSCF interrogates the HSS for routing information. The mobile terminated sessions for a user shall be routed to a S‑CSCF. The Cx reference point shall support retrieval of routing information from HSS to I‑CSCF. Th...