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5.8.1 User identity to HSS resolution
This clause describes the resolution mechanism, which 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. This resolution mechanism is implemented usin...
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5.8.2 SLF on register
Figure 5.20: SLF on register (1st case) 1. I‑CSCF receives a REGISTER request and now has to query for the location of the user's subscription data. 2. The I‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the REGISTER request. 3. The SLF looks up its database fo...
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5.8.3 SLF on UE invite
Figure 5.21: SLF on UE invite 1. I‑CSCF receives an INVITE request and now has to query for the location of the user's subscription data. 2. The I‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the INVITE request. If the user identity is an E.164 number in the SI...
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5.8.4 SLF on AS access to HSS
The flow shown below is where the AS queries the SLF to identify the HSS to access. Figure 5.21a: SLF on AS access to HSS 1. An AS sends a DH_SLF_QUERY to the SLF and includes as a parameter the Public User Identity. 2. The SLF looks up its database for the queried Public User Identity. 3. The SLF answers with th...
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5.9 Routing of mid-session signalling
During the signalling exchanges that occur to establish an IM Session, the following elements must ensure future signalling messages related to this session are routed through them: - P‑CSCF serving the originating UE, in order to generate the CDR record in the roaming case and to force release of the resources used f...
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5.10 Session release procedures
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5.10.0 General
This clause provides scenarios showing SIP application session release. Note that these flows have avoided the strict use of specific SIP protocol message names. This is in an attempt to focus on the architectural aspects rather than the protocol. SIP is assumed to be the protocol used in these flows. The session rele...
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5.10.1 Terminal initiated session release
The following flow shows a terminal initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established directly between the two visited networks (the visited networks could be the Home network in either or both cases). Furthermore, the flow also ass...
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14 Void.
15. The SIP OK message is sent to the S‑CSCF. 16. The S‑CSCF of the other party forwards the OK to the S‑CSCF of the releasing. 17. The S‑CSCF of the releasing party forwards the OK to the P‑CSCF of the releasing. 18. The P‑CSCF of the releasing party forwards the OK to the UE.
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5.10.2 PSTN initiated session release
The following flow shows a PSTN terminal initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established to the PSTN from the Home Network (the visited network could be the Home network in this case). Furthermore, this flow assumes that Policy an...
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5.10.3 Network initiated session release
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5.10.3.0 Removal of IP‑CAN bearer used to transport IMS SIP signalling
It is possible that the IP‑CAN removes the IP‑CAN bearer used to transport IMS SIP signalling (e.g. due to overload situations). In this case the UE or network shall initiate a procedure to re-establish (or modify where possible) an IP‑CAN bearer to transport IMS SIP signalling. After the re-establishment of an IP‑CAN...
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5.10.3.1 Network initiated session release - P‑CSCF initiated
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5.10.3.1.0 General
This clause assumes that Policy and Charging Control is applied The following flows show a Network initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established directly between the two visited networks (the visited networks could be the Home ...
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5.10.3.1.1 Network initiated session release - P‑CSCF initiated – after removal of IP-Connectivity Access Network bearer
Figure 5.26: Network initiated session release - P‑CSCF initiated – after removal of IP‑CAN bearer 1. A bearer related to the session is terminated. The P‑CSCF receives an indication via PCRF/PCF of IP‑CAN bearer release. 2. The P‑CSCF instructs PCRF/PCF to remove the authorization for resources related to the rele...
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5.10.3.1.2 Void
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5.10.3.2 Network initiated session release - S‑CSCF Initiated
The following flow shows a network-initiated IM CN subsystem application session release, where the release is initiated by the S‑CSCF. This can occur in various service scenarios, e.g. administrative, or prepaid. The procedures for clearing a session, when initiated by an S‑CSCF, are as shown in the following informa...
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5.11 Procedures to enable enhanced multimedia services
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5.11.1 Session Hold and Resume Procedures
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5.11.1.0 General
This clause gives information flows for the procedures for placing sessions on hold that were previously established by the mechanisms of clauses 5.4, 5.5, 5.6 and 5.7 and resuming the session afterwards. Two cases are presented: mobile-to-mobile (UE-UE) and a UE-initiated hold of a UE-PSTN session. For a multi-media ...
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5.11.1.1 Mobile-to-Mobile Session Hold and Resume Procedures
An IMS session was previously established between an initiating UE and a terminating UE. Each of these UEs has an associated P‑CSCF and a S‑CSCF assigned in their home network. The procedures are independent of whether the P‑CSCFs are located in the home or visited networks. Therefore there is no distinction in this cl...
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5.11.1.2 Mobile-initiated Hold and Resume of a Mobile-PSTN Session
An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network an...
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5a MGCF sends a CPG(hold) in order to express that the call has been placed on hold.
6. MGCF initiates a H.248 interaction with MGW instructing it to stop sending the media stream, but to keep the resources for the session reserved. 7. MGCF acknowledges receipt of the Hold message with a 200-OK final response, send to BGCF. 8. BGCF forwards the 200-OK to the S‑CSCF. 9. S‑CSCF forwards the 200 OK fin...
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5.11.1.3 PSTN-initiated Hold and Resume of a Mobile-PSTN Session
An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network an...
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5.11.2 Procedures for anonymous session establishment
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5.11.2.0 General
This clause gives information flows for the procedures for an anonymous session. However, sessions are not intended to be anonymous to the originating or terminating network operators. The purpose of the mechanism is to give an IMS user the possibility to withhold certain identity information as specified in IETF RFC ...
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5.11.2.1 Signalling requirements for anonymous session establishment
The user shall be able to request that her identity information is not revealed to the terminating party. If the originating user requests the session to be anonymous, the terminating side must not reveal any identity or signalling routing information to the destination endpoint. The terminating network should disting...
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5.11.2.2 Bearer path requirements for anonymous session establishment
Procedures for establishment of an anonymous bearer path are not standardised in this release.
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5.11.3 Procedures for codec and media characteristics flow negotiations
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5.11.3.0 General
This clause gives information flows for: - the procedures for determining the set of negotiated characteristics between the endpoints of a multi-media session, determining the initial media characteristics (including common codecs) to be used for the multi-media session and - the procedures for modifying a session wi...
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5.11.3.1 Codec and media characteristics flow negotiation during initial session establishment
Initial session establishment in the IM CN subsystem must determine a negotiated set of media characteristics (including a common codec or set of common codecs for multi-media sessions) that will be used for the session. This is done through an end-to-end message exchange to determine the complete set of media characte...
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5.11.3.2 Codec or media characteristics flow change within the existing reservation
After the multi-media session is established, it is possible for either endpoint to change the set of media flows or media characteristics (e.g. codecs) for media flows. If the change is within the resources already reserved, then it is only necessary to synchronise the change with the other endpoint. Note that an admi...
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5.11.3.3 Codec or media characteristics flow change requiring new resources and/or authorization
After the multi-media session is established, it is possible for either endpoint to change the set of media flows or media characteristics (e.g. codecs) for media flow(s). If the change requires different resources beyond those previously reserved, then it is necessary to perform the resource reservation and bearer est...
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5.11.3.4 Sample MM session flow - addition of another media
For this end-to-end session flow, we assume the originator is a UE located within the service area of the network operator to whom the UE is subscribed. The UE has already established an IM CN session and is generating an invite to add another media (e.g. video to a voice call) to the already established session. Note ...
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5.11.4 Procedures for providing or blocking identity
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5.11.4.0 General
Identity is composed of a Public User Identity and an optional display name: - The Public User Identity is used by any user for requesting communications to other users (see clause 4.3.3.2). - The display name is the user's name if available, an indication of privacy or unavailability otherwise. The display name is a...
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5.11.4.1 Procedures for providing the authenticated identity of the originating party
Authentication of the subscriber is performed during the registration procedures, as described in clause 5.2.2.3. As a result of the registration procedures, one or several Public User Identity(ies) of the originating party is/are stored in P‑CSCF#1. As part of this procedure, the display name associated with each Publ...
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5.11.4.2 Procedures for blocking the identity of the originating party
Regulatory agencies, as well as subscribers, may require the ability of an originating party to block the display of their identity either permanently or on a session by session basis. This is a function performed by the destination P‑CSCF. In this way, the terminating party is still able to do a session-return, sessio...
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5.11.4.3 Procedures for providing the authenticated identity of the originating party (PSTN origination)
For calls originating from the PSTN, the MGCF extracts information received from the PSTN and inserts an asserted identity into the SIP message. If the incoming information includes the calling name, or the MGCF can obtain the calling name, the MGCF may insert the information into the display name portion of the assert...
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5.11.4.4 Procedures for providing the authenticated identity of the originating party (PSTN termination)
For calls terminating to the PSTN, the MGCF extracts information received in the SIP message and inserts the information into the PSTN signalling. This information must include the privacy setting and may include the display name.
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5.11.5 Session Redirection Procedures
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5.11.5.0 General
This clause gives information flows for the procedures for performing session redirection. The decision to redirect a session to a different destination may be made for different reasons by a number of different functional elements and at different points in the establishment of the session. Three cases of session red...
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5.11.5.1 Session Redirection initiated by S‑CSCF to IMS
One of the functional elements in a basic session flow that may initiate a redirection is the S‑CSCF of the destination user. The user profile information obtained from the HSS by the 'Cx-pull' during registration may contain complex logic and triggers causing session redirection. S‑CSCF#2 sends the SIP INVITE request ...
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5.11.5.2 Session Redirection to PSTN Termination (S‑CSCF #2 forwards INVITE)
The S‑CSCF of the destination user (S‑CSCF#2) may determine that the session is to be redirected to a PSTN Termination; e.g. CS-domain endpoint, or to the PSTN. For session redirection to PSTN termination where the S‑CSCF of the called party (S‑CSCF#2) wishes to remain in the path of SIP signalling, the S‑CSCF forwards...
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5.11.5.3 Session Redirection initiated by S‑CSCF to general endpoint (REDIRECT to originating UE#1)
The S‑CSCF in the scenario above may determine that the session is to be redirected to an endpoint outside the IP MultiMedia System and outside the CS-domain. Examples of these destinations include web pages, email addresses, etc. It recognizes this situation by the redirected URI being other than a sip: URI or tel: UR...
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5.11.5.4 Session Redirection initiated by P‑CSCF
One of the functional elements in a basic session flow that may initiate a redirection is the P‑CSCF of the destination user. In handling of an incoming session setup attempt, the P‑CSCF normally sends the INVITE request to the destination UE and retransmits it as necessary until obtaining an acknowledgement indicating...
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5.11.5.5 Session Redirection initiated by UE
The next functional element in a basic session flow that may initiate a redirection is the UE of the destination user. The UE may implement customer-specific feature processing and base its decision to redirect this session on such things as identity of caller, current sessions in progress, other applications currently...
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5.11.5.6 Session Redirection initiated by originating UE#1 after Bearer Establishment (REDIRECT to originating UE#1)
The UE of the destination user may request the session be redirected after a customer-specified ringing interval. The UE may also implement customer-specific feature processing and base its decision to redirect this session on such things as identity of caller, current sessions in progress, other applications currently...
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5.11.6 Session Transfer Procedures
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5.11.6.0 General
This clause gives information flows for the procedures for performing session transfers. This is presented in two steps: first a basic primitive that can be used by endpoints to cause a multi-media session to be transferred and second the procedures by which this primitive can be used to implement some well-known sessi...
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5.11.6.1 Refer operation
The refer primitive is an information flow indicating a "Refer" operation, which includes a component element "Refer-To" and a component element "Referred-By". The end point receiving a referral may be UE#1 as shown in the example flow in figure 5.42 or it may be any other type of originating entity as defined in claus...
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5.11.6.2 Application to Session Transfer Services
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5.11.6.2.0 General
This clause shows how the Refer primitive given above can be used to provide common session-transfer services.
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5.11.6.2.1 Blind Transfer and Assured Transfer
A Blind Transfer starts with an existing session, established between the Initiator (I) and the Recipient (R). In a typical case, this session was actually initiated by R. In the end it is desired that the Recipient has a session with the Target (T). From the starting configuration, shown in the leftmost diagram, I se...
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5.11.6.2.2 Consultative Transfer
A Consultative Transfer again starts with an existing session, established from the Initiator (I) to the Recipient (R). The Initiator first consults with the Target (T), then decides to transfer the original session to T. From the starting configuration, as shown in the leftmost diagram in the previous clause, I place...
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5.11.6.2.3 Three-way Session
A three-way session starts with an existing session, between the Initiator (I) and party (A). The initiator places this session on hold and establishes a second session with party (B). The initiator then decides to create an ad-hoc conference of all three parties. From the point where the initiator decides to create t...
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5.12 Mobile Terminating call procedures to unregistered Public User Identities
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5.12.0 General
This clause describes information flows for the procedures of Mobile Terminating call flows for unregistered IMS Public User Identities. The detection of an unregistered Public User Identity is done in HSS and if this Public User Identity has services related to unregistered state, a S‑CSCF is selected for the unregist...
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5.12.1 Mobile Terminating call procedures to unregistered Public User Identity that has services related to unregistered state
In Figure 5.43 below the Public User Identity is unregistered for IMS and the Public User Identity has services related to unregistered state. In this case, the HSS responds back to I‑CSCF with an indication that I‑CSCF should select S‑CSCF for this MT call to the unregistered Public User Identity of the user or provid...
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5.12.2 Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state
In the example information flow the Public User Identity of the user is unregistered and the Public User Identity has no services related to unregistered state. This is shown in the following information flow (figure 5.44): Figure 5.44: Mobile Terminating call procedures to unregistered Public User Identity that ha...
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5.13 IMS Emergency Sessions
Emergency sessions via IMS are specified in TS 23.167 [58].
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5.14 Interactions involving the MRFC/MRFP
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5.14.0 General
The MRFC/MRFP are resources of the IMS that provide support for bearer related services such as for example multi-party sessions, announcements to a user or bearer transcoding. This clause describes how the resources of the MRFC/MRFP are used.
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5.14.1 Interactions between the UE and the MRFC
In some cases an operator may wish to make an MRFC available directly to a UE, for example to support ad-hoc multi-party sessions to be initiated by the UE. In this case, the operator advertises the name of one or more MRFCs and a UE will invite an MRFC to a session. The session invitation would need to contain additio...
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5.14.2 Service control based interactions between the MRFC and the AS
The MRFC/MRFP resources may also be used, based on service control in an IMS, for services such as multiparty sessions, announcements or transcoding. In this case an Application Server interacts with an MRFC. Session control messages are exchanged between the AS and the MRFC. There are two approaches for the AS to con...
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5.14.3 Interactions for services using both the Ut interface and MRFC capabilities
Network services hosted on an AS and configurable by the user via the Ut interface may also use the capabilities provided by the MRFC. For this case, the AS either supports MRFC capabilities, or communicates with an MRFC. Communications across the Ut interface between the UE and the AS allow the UE to securely manage ...
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5.14.4 Transcoding services involving the MRFC/MRFP
Network services involving MRFC and MRFP are not limited to conferencing and announcements, but also involve transcoding support for interworking between IMSs or inter-domain sessions and intra-domain sessions between access technologies supported in an IMS (e.g. wireline wireless interworking, or interworking with non...
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5.15 Mobile Terminating session procedure for unknown user
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5.15.0 General
This clause describes information flows Mobile Terminating procedure for an unknown user. The unknown user cases include those where session requests are made towards Public User Identities that are incorrect, un-issued or have been cancelled/deleted. The determination of unknown user is carried out in the HSS and/or t...
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5.15.1 Unknown user determined in the HSS.
In Figure 5.45 the unknown status of the requested party is determined in the HSS. The I‑CSCF requests information on the user to be reached and the HSS responds back to the I‑CSCF with an indication that the user is unknown. The I‑CSCF uses the indication that the user is unknown returned from the HSS to formulate the...
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5.15.2 Unknown user determined in the SLF
In Figure 5.46 the unknown status of the requested party is determined in the SLF. The I‑CSCF requests information on the user to be reached and the SLF responds back to the I‑CSCF with an indication that the user is unknown. The I‑CSCF uses the indication that the user is unknown returned from the SLF to formulate the...
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5.16 IMS messaging concepts and procedures
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5.16.0 General
This clause describes architectural concepts and procedures for providing Messaging in the IM CN Subsystem. The service enablers for Messaging and possible reuse of IMS service enablers within this context as well security and charging expectations, addressing, privacy, content handling and limitations, filtering, medi...
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5.16.1 Immediate Messaging
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5.16.1.0 General
This clause describes architectural concepts and procedures for fulfilling the requirements for Immediate Messaging described in TS 22.340 [29a].
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5.16.1.1 Procedures to enable Immediate Messaging
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5.16.1.1.0 General
IMS users shall be able to exchange immediate messages with each other by using the procedure described in this clause. This procedure shall allow the exchange of any type of multimedia content (subject to possible restrictions based on operator policy and user preferences/intent), for example but not limited to: - Pi...
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5.16.1.1.1 Immediate messaging procedure to registered Public User Identity
Figure 5.47: Immediate Messaging procedure to registered Public User Identity 1. UE#1 generates the multimedia content intended to be sent to UE#2. 2. UE#1 sends the MESSAGE request to P‑CSCF#1 that includes the multimedia content in the message body. 3. P‑CSCF#1 forwards the MESSAGE request to S‑CSCF#1 along the ...
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5.16.1.1.2 Immediate messaging procedure to unregistered Public User Identity
Figure 5.48: Immediate messaging to unregistered Public User Identity, service control invoked 1-5. The same actions apply as for when the Public User Identity is registered, see step 1-5 in clause 5.16.1.1.1. 6. I‑CSCF#2 interacts with the HSS as per the terminating procedures defined for unregistered Public User ...
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5.16.1.2 Immediate messages with multiple recipients
IMS users shall be able to send a single immediate message to multiple recipients, as specified in TS 22.340 [29a]. The following means are supported to achieve this: - A PSI identifying a new group is created in the appropriate Application Server and members are added to this group (e.g. by the user via the Ut interf...
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5.16.2 Session-based Messaging
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5.16.2.0 General
This clause describes architectural concepts and procedures for fulfilling the requirements for Session-based Messaging described in TS 22.340 [29a].
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5.16.2.1 Architectural principles
Session-based IMS messaging communications shall as much as possible use the same basic IMS session delivery mechanisms (e.g. routing, security, service control) as defined in clause 4 and 5 of this document. For session based messaging the session shall include a messaging media component, other media components may a...
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5.16.2.2 Procedures to enable Session based Messaging
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5.16.2.2.0 General
IMS users shall be able to exchange session-based messages with each other by using the procedures described in this clause. These procedures shall allow the exchange of any type of multimedia content (subject to possible restrictions based on operator policy and user preferences/intent), for example but not limited to...
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5.16.2.2.1 Session based messaging procedure to registered Public User Identity
The following procedure shows the establishment of a message session between two registered UEs where the UEs are able to exchange messages end-to-end. The signalling flow is based on the general flow shown in clause 5.7a of this specification. Figure 5.48a: Message session establishment 1-30. These steps are ident...
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5.16.2.2.2 Session based messaging procedure using multiple UEs
Session based messaging between more than two UEs require the establishment of a session based messaging conference. Within session based messaging conferences including multiple UEs (e.g. multiparty chat conferences) an MRFC/MRFP or an IMS AS shall be used to control the media resources. When MRFC/MRFP are used, the...
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5.16.2.2.3 Session based messaging procedure with an intermediate node
The following procedure shows the originating session based messaging involving an intermediate node. An optional ringing response from AS to the UE or vice versa is not shown in the following procedure. Figure 5.48c: Session based messaging with an intermediate node 1. UE#1 sends the SIP INVITE request addressed t...
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5.16.2.2.4 Session based messaging release procedure
The following procedure shows the release of a message session, which was established between two UEs. It is assumed that UE#1 is the session host. Figure 5.48d: Message session release procedure 1–6. UE#1 indicates its intent to terminate the message session by sending a BYE request to UE#2. UE#1 stops sending mes...
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5.16.2.2.5 Session based messaging release procedure with an intermediate node
The following procedure shows the release of a message session, which was established between two UEs via an intermediate node. It is assumed that UE#1 is the session host. Figure 5.48e: Message session release procedure with intermediate node 1–4. UE#1 indicates its intent to terminate the message session by sendi...
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5.17 Refreshing sessions
The active sessions in stateful network elements (e.g. CSCFs, ASs) may need to be refreshed periodically. This allows these stateful elements to detect and free resources that are being used by hanging sessions. This SIP‑level session refreshing mechanism is to be used to allow removing session state from the stateful...
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5.18 Void
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5.19 Support for Transit scenarios in IMS
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5.19.1 General
This clause presents some high level flows to describe the procedures for supporting IMS transit network scenarios. The IMS Transit Functions perform an analysis of the destination address and determine where to route the session. The session may be routed directly to an MGCF, BGCF, or to another IMS entity in the sam...
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5.19.2 Providing IMS application services in transit network scenarios
This clause provides an overview of how IMS application services in transit network scenarios are provided. Figure 5.50c: IMS application services in transit network The procedure for IMS application services in transit network is as follows: 1. The Transit function receives an incoming request from a preceding ne...
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5.20 Procedures for Assigning, Using and Processing GRUUs
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5.20.1 UE
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5.20.1.1 Obtaining a GRUU during registration
A UE shall indicate its support for the GRUU mechanism in the registration request and retain the GRUU set (P‑GRUU and T‑GRUU) in the registration response. The UE may retain some or all of the previous T‑GRUUs obtained during the initial registration or previous re-registrations along with the new T‑GRUU or the UE may...
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5.20.1.2 Using a GRUU
When sending SIP requests from an explicitly or implicitly registered Public User Identity for which a UE obtained P‑GRUU and at least one T‑GRUU, the UE should use the corresponding retained P‑GRUU or a T‑GRUU as a Contact address. When responding to SIP requests where the identification of the called party is a regi...
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5.20.1.3 Using a GRUU while requesting Privacy
When a UE sends a request or response containing a GRUU and it wishes to block the delivery of its Public User Identity to an untrusted destination, the UE shall use a T-GRUU as the Contact address.