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Content for  TS 23.228  Word version:  19.2.0

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AE Support of UE-Satellite-UE communication in IMS  AE.1 General  AE.2 Architecture and functional entities  AE.2.1 Architecture  AE.2.2 Functional entities  AE.3 Optimized media routing activation  AE.4 Interaction with IMS AGW  AE.5 Procedures for optimizing media routing 
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AE (Normative)  Support of UE-Satellite-UE communication in IMS |R19|  p. 441

AE.1  General  p. 441

This Annex describes IMS architecture enhancements to support UE-Satellite-UE communication in IMS, i.e. optimized media routing via IMS user plane on-board satellite.
In this Release of the specification, this feature is supported only for IMS voice/video service and for UEs belonging to the same PLMN and in the non-roaming scenario.
The term and the 5GS architecture for UE-Satellite-UE communication is defined in TS 23.501. The optimized media routing described in this annex refers to a routing of media between UEs under the coverage of the same or of different serving satellites, using the 5GS network as the IP-CAN and with the media not transiting through any network elements on the ground. The ground fallback routing refers to the case with the media transiting through the ground segment.
The P-CSCFs serving the MO UE and the MT UE negotiate with each other to determine whether or not to activate UE-satellite-UE communication, that is, whether it is possible to apply media routing that only relies on IMS user plane on-board satellite.
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AE.2  Architecture and functional entities  p. 441

AE.2.1  Architecture  p. 441

AE.2.1.1  Support of IMS satellite media plane optimization with IMS-AGW deployed on satellite(s)  p. 441

To support IMS satellite media plane optimization, as depicted in Figure AE.2.1.1-1, the IMS-AGW may be deployed on the satellite(s) that host the gNB and UPF (UL CL/BP and L-PSA) of the 5GC.
Reproduction of 3GPP TS 23.228, Fig. AE.2.1.1-1: Reference architecture of IMS satellite media plane optimization
Figure AE.2.1.1-1: Reference architecture of IMS satellite media plane optimization
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The above Figure depicts a reference architecture for IMS satellite media plane optimization with following assumption:
  • For the IMS related PDU Session, the IP address allocated to the UE corresponds to a PSA UPF located on the ground. Thus, the IP address of the UE is not changed when serving satellite is changed.
  • UE(s) register onto IMS with this IP address and don't need to reregister when there is change of serving satellite.
  • The architecture deployment assumes, ISL(s) can be set-up within the same satellite constellation or across different constellations depending on satellite operator's deployments (SLA). The set of ISL(s) builds up an IP network which is out of 3GPP scope.
  • The routing between Onboard AGWs across satellite via ISL is assumed to be based on IP routing and the Onboard AGWs in satellite need to have non-conflicting IP address during resource allocation. It is up to the deployment to manage IP routing across ISL links and is out of 3GPP scope.
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AE.2.2  Functional entities  p. 442

AE.2.2.1  P-CSCF  p. 442

The P-CSCF used for this feature is enhanced to support the following functionalities:
  • the P-CSCF determines the activation of optimized media routing and interacts with 5GS, as described in clause AE.3.
  • the P-CSCF interacts with IMS AGW, as described in clause AE.4.
  • the P-CSCF requests IMS AS used for this feature to initiate SIP re-INVITE to complete IMS AGW relocation, as described in clause AE.4.
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AE.2.2.2  IMS AS  p. 442

The IMS AS used for this feature is enhanced to support the following functionalities:
  • the IMS AS, requested by P-CSCF used for this feature, initiates SIP re-INVITE to complete IMS AGW relocation, as described in clause AE.4.

AE.3  Optimized media routing activation  p. 442

AE.3.1  General  p. 442

If UE-Satellite-UE communication in IMS is supported and if enabled subject to operator policy, the following behaviour described in clause AE.3 applies.

AE.3.2  At call setup  p. 442

The P-CSCF receives from the PCF the satellite identifier of the satellite serving the UE and receives from the P-CSCF serving the remote UE the satellite identifier of the satellite serving the remote UE according to clause AE.5.1 and clause AE.5.3. Subject to operator policy, the P-CSCF uses this information to determine whether or not activate optimized media routing. With the lack of the satellite identifier of the satellite either serving the local UE or the remote UE, the P-CSCF determines not to activate optimized media routing.
If P-CSCF determines the activation of optimized media routing, the P-CSCF instructs the PCF to authorize the necessary resources according to clause AE.5.1 and clause AE.5.3, so that the PCF proceeds to establish a 5GS user plane path for optimized media routing.
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AE.3.3  At change of satellite  p. 443

The P-CSCF is notified of the identifier of the target satellite from the PCF when the serving satellite of the UE changes according to clause AE.5.2.1. The P-CSCF determines whether optimized media routing continues to be possible between the target satellite and the satellite serving the remote UE based on the identifiers of those satellites. If there is no target satellite identifier in the notification, the P-CSCF determines that optimized media routing is no longer possible.
If the P-CSCF determines that optimized media routing can continue at change of satellite, the P-CSCF invokes Npcf_PolicyAuthorization_Update service operation to the PCF as defined in clause 4.3.6.3 of TS 23.502 and according to clause AE.5.2.1 to establish a path through the target satellite for optimized media routing.
If the P-CSCF determines that optimized media routing is no longer possible e.g. due to a change of satellite, the P-CSCF executes the ground fallback procedure as per clause AE.5.2.2 to route the media via the ground.
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AE.4  Interaction with IMS AGW  p. 443

AE.4.1  General  p. 443

If UE-Satellite-UE communication in IMS is supported and if enabled subject to operator policy, the following behaviour described in clause AE.4 applies.

AE.4.2  At call setup  p. 443

The P-CSCF serving the MO UE selects an IMS AGW on ground at call setup according to TS 23.334, clause AE.5.1 and clause AE.5.3. The P-CSCFs serving the MO UE and MT UE proceed with the determination of whether UE-Satellite-UE communication is possible as described in clause AE.3.2. If the P-CSCF determines the activation of optimized media routing is possible, after deciding to use the optimized media routing (e.g. with consideration of whether early media is in progress or not), the P-CSCF releases the IMS AGW on ground if allocated and select instead an IMS AGW on satellite according to TS 23.334], clause AE.5.1 and clause AE.5.3. If early media is in progress during call setup, the P-CSCF serving the MO UE does not release the IMS-AGW on ground until the IMS session is answered, then selects the IMS-AGW on satellite according to clause AE.5.3 for using the optimized media routing.
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AE.4.3  At change of satellite  p. 443

When P-CSCF is informed that the satellite serving UE changes as defined in clause 4.3.6.3 of TS 23.502 and according to clause AE.5.2, the P-CSCF selects an IMS AGW on the target satellite according to clause AE.5.2.1 or on ground according to clause AE.5.2.2, while keeping the IMS AGW on the source satellite, depending on whether optimized media routing continues to be possible or not.
After configuring and reserving resources in the IMS AGW on the target satellite or on ground, the P-CSCF requests the IMS AS to initiate SIP re-INVITE to inform the remote network and then the local UE to use the new IMS AGW(s) according to clauses AE.5.2.1 and AE.5.2.2.
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AE.5  Procedures for optimizing media routing  p. 444

AE.5.1  At call setup  p. 444

When the originating P-CSCF receives the initial INVITE request from the originating UE using regenerative NR satellite access, it selects the IMS-AGW on ground and triggers media negotiation by sending SDP offer to the terminating side indicating that satellite access is used on the originating side. If the terminating P-CSCF activates UE-satellite-UE communication, the originating P-CSCF re-selects an IMS-AGW on satellite. See Figure AE.5.1-1 that describes the procedure. In the procedure, UE A and P-CSCF A represent originating side function or entity, UE B and P-CSCF B represent terminating side function or entity.
Reproduction of 3GPP TS 23.228, Fig. AE.5.1-1: Session establishment procedure with activation of optimizing media routing
Figure AE.5.1-1: Session establishment procedure with activation of optimizing media routing
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Step 1.
UE A sends a SIP INVITE request, containing an initial SDP offer, to P-CSCF A. The access information in the SIP INVITE request indicates that the UE is accessing from NR satellites access as defined in TS 24.229.
Step 2.
After receiving the SIP INVITE request indicating NR satellite access, if UE A is not roaming and the P-CSCF A supports UE-Satellite-UE communication, the P-CSCF requests Access Network Information, which includes an indication for requesting satellite identifier information, for UE A from the PCF as described in TS 23.503 via Npcf_PolicyAuthorisation_Create service operation. The PCF requests Access Network Information, which includes the identifier of UE A's serving satellite, from the SMF via Npcf_SMPolicyControl_UpdateNotify service operation. Then the PCF receives the Access Network Information via Npcf_SMPolicyControl_Update service operation as described in TS 23.502 and TS 23.503. This satellite identifier is subsequently conveyed to the P-CSCF in the Access Network Information Notification via Npcf_PolicyAuthorization_Notify service operation.
Step 3.
The P-CSCF A allocates an IMS-AGWon the ground as default. P-CSCF A interacts with IMS-AGW on the ground to allocate transport resources based on the mechanism defined in TS 23.334.
Steps 4-20 are performed if the UE-Satellite-UE communication is possible based on step 2 (i.e. UE A is using NR regenerative payload satellite access and is not roaming). Otherwise, procedures for IMS user plane traffic routing with IMS-AGW on ground are performed (details have been omitted from the diagram).
Step 4.
P-CSCF A updates the SIP INVITE request inserting the identifier of the satellite serving UE A and updates SDP offer with the IMS-AGW transport addresses allocated in step 3. P-CSCF A forwards the updated SIP INVITE request to the terminating side via IMS core (details have been omitted from the diagram).
Step 5.
After receiving the SIP INVITE request which contains a satellite identifier indicating that UE A is using regenerative satellite access, if UE B is not roaming and the P-CSCF B supports UE-Satellite-UE communication, the P-CSCF B requests Access Network Information, which includes an indication for requesting satellite identifier information, for UE B from PCF as described in TS 23.503 via Npcf_PolicyAuthorization_Create service operation.. The PCF obtains Access Network Information, which may include an identifier of the satellite serving UE B, from the SMF via Npcf_SMPolicyControl_UpdateNotify and Npcf_SMPolicyControl_Update service operations as described in TS 23.502 and TS 23.503. This satellite identifier, if received, is subsequently conveyed to the P-CSCF in the Access Network Information Notification via Npcf_PolicyAuthorization_Notify service operation.
Step 6.
Based on the Access Network Information received at step 5, if UE B is using regenerative payload satellite access and if the two satellites identified in step 4 and 5 are the same or connected with ISL(s), P-CSCF B determines to activate the UE-Satellite-UE communication in IMS.
and If UE-Satellite-UE communications in IMS is not activated, P-CSCF B selects an IMS-AGW on ground for UE B; otherwise, P-CSCF B selects an IMS-AGW on satellite for UE B.
Step 6a-6b.
Based on the decision in step 6, P-CSCF B interacts with IMS-AGW onboard the satellite or IMS-AGW on ground to allocate transport resources based on the mechanism defined in TS 23.334.
Steps 7-20 are performed if the UE-Satellite-UE communication is activated. Otherwise, SIP message routing procedures with IMS-AGW on the ground should be performed after P-CSCF B selecting a ground IMS-AGW for usage (details have been omitted from the diagram).
Step 7-9.
P-CSCF B forwards the updated SIP INVITE request to UE B. UE B returns an 18X response with SDP Answer as normal SIP message routing procedures.
Step 9a-9b.
After receiving the SDP Answer, P-CSCF B interacts with the IMS-AGW onboard the satellite to allocate transport resources based on the mechanism defined in TS 23.334.
Step 10.
P-CSCF B instructs the PCF to authorize the resources necessary to establish a QoS flow for media via Npcf_PolicyAuthorization_Update service operation. To indicate the SMF to insert a UL CL/BP and L-PSA UPF for IMS PDU session to enable UE-Satellite-UE communication, the P-CSCF B should send following parameters via the request to PCF based on Application Function influence on traffic routing mechanism described in clause 5.6.7 of TS 23.501 and clause 4.3.6 of TS 23.502:
  • A flow description information which contains the IP address of the IMS-AGW onboard the satellite;
  • A DNAI associated with the IMS-AGW onboard the satellite and optionally corresponding N6 traffic routing information.
Step 10a.
To be notified of satellite change and perform IMS-AGW relocation at change of satellite as described in clause AE.5.2, P-CSCF B subscribes to notification of User Plane management events (i.e. UP path change) via Npcf_PolicyAuthorization_Update service operation to the PCF, which in turn subscribes to the events from SMF via Npcf_SMPolicyControl_UpdateNotify service operation and receives the event notifications from the SMF via Nsmf_EventExposure_Notify service operation as specified in clause 4.3.6 of TS 23.502. The P-CSCF B inserts the following parameters in its subscription request (i.e. Npcf_PolicyAuthorization_Update service operation) as specified in clause 5.2.5.3.3 of TS 23.502:
  • Early and late notifications about UP path management events.
Step 11.
P-CSCF B updates the 18X response to carry the identifier of the satellite serving UE B to indicate that UE-Satellite-UE communication is activated. Then, P-CSCF B returns the updated 18X response to originating side via IMS core (details have been omitted from the diagram).
Step 12.
Based on the identifier of the satellite serving UE B in step 11, P-CSCF A determines to perform UE-satellite-UE communication and select an IMS-AGW onboard the satellite.
Step 12a-12c.
P-CSCF A interacts with IMS-AGW onboard the satellite to allocate transport resources for both IMS access and IMS Core network sides. P-CSCF A also releases the IMS-AGW on ground.
Step 13.
P-CSCF A instructs the PCF to authorize the resources necessary to establish a QoS flow for media via Npcf_PolicyAuthorization_Update service operation. To indicate to the SMF to insert a UL CL/BP and L-PSA UPF for the IMS PDU session to enable UE-Satellite-UE communication in IMS, P-CSCF B should send following parameters via the request to PCF based on Application Function influence on traffic routing mechanism described in clause 5.6.7 of TS 23.501 and clause 4.3.6 of TS 23.502:
  • A flow description information which contains the IP address of the IMS-AGW onboard satellite;
  • A DNAI associated with the IMS-AGW onboard satellite and optionally corresponding N6 traffic routing information.
Step 13a.
To be notified of satellite change and perform IMS-AGW relocation at change of satellite as described in clause AE.5.2, P-CSCF A subscribes to notification of user plane management events (i.e. UP path change) via Npcf_PolicyAuthorization_Update service operation to the PCF, which in turn subscribes to the events from SMF via Npcf_SMPolicyControl_UpdateNotify service operation and receives the event notifications from the SMF via Nsmf_EventExposure_Notify service operation as specified in clause 4.3.6 of TS 23.502. The P-CSCF A inserts the following parameters in its subscription request (i.e. Npcf_PolicyAuthorization_Update service operation) as specified in clause 5.2.5.3.3 of TS 23.502:
  • Early and late notifications about UP path management events.
Step 14.
P-CSCF A updates the SDP answer in the 18X response with the IMS-AGW transport addresses allocated in step 12 and forwards the updated 18X response to UE A.
Step 15.
UE A acknowledges the 18X Response and sends the Response Confirmation to P-CSCF A.
Step 16-17.
P-CSCF A forwards the Response Confirmation to P-CSCF B. The Response Confirmation includes SDP offer modified/generated by P-CSCF A which contains the IMS-AGW transport address obtained in step 12. Upon receiving the SDP offer, P-CSCF B updates the allocated IMS-AGW on the satellite for UE B with the transport address in the SDP offer.
Step 18.
P-CSCF B sends the Response Confirmation to UE B.
Step 19.
Procedure continues to setup the call as defined in clauses 5.6 and 5.7.
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AE.5.2  IMS AGW relocation and media routing path change due to change of satellites  p. 447

AE.5.2.1  Continued optimized media routing procedure  p. 447

Figure AE.5.2.1-1 depicts a signalling flow diagram for continuation of optimized media routing after change of satellites that serve a UE. The procedure is written in such a way that change of satellite occurs in the originating network for the purpose of the explanation, while change of satellite can occur in the terminating network as well.
Use of the N5 interface between IMS and 5GC is assumed.
Based on the procedure described in clause AE.5.1, P-CSCF is expected to subscribe from 5GC for the early and the late notification of the satellite user plane management events associated with UE-Satellite-UE communication media traffic as specified in clause 5.6.7 of TS 23.501 and clause 4.3.6.3 of TS 23.502.
Copy of original 3GPP image for 3GPP TS 23.228, Fig. AE.5.2.1-1: Continued optimized media routing procedure
Figure AE.5.2.1-1: Continued optimized media routing procedure
(⇒ copy of original 3GPP image)
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The steps in the call flow are as follows:
Step 1.
A media path in both directions is established between UEs. IMS AGWs on satellite forward voice/video media via ULCL and L-PSA on satellite between the UEs.
Step 2.
P-CSCF receives the early notification of the satellite user plane management events associated with UE-Satellite-UE communication media traffic from PCF as defined in clause 4.3.6.3 of TS 23.502. This early notification contains satellite ID of a target satellite that has gNB to which the UE gets connected and an indication being set "EARLY", indicating that 5GC is prepared to change the user plane path for optimized media routing to the one through this target satellite.
Step 3.
P-CSCF determines that optimized media routing continues to be possible based on the satellite ID received in step 2 for the originating network and the satellite ID stored for the terminating network. P-CSCF determines to continue activating optimized media routing.
If P-CSCF determines that optimized media routing cannot continue, P-CSCF follows the ground fallback procedure as defined in clause AE.5.2.2 for subsequent steps.
Step 4.
P-CSCF requests IMS AGW on the target satellite to configure the IP address allocated in UE, which P-CSCF has stored, to be used by the IMS AGW on the target satellite as the destination of media traffic towards the UE and to reserve an IP address in the IMS AGW on the target satellite to be used by the UE as the destination of media traffic. In addition, the P-CSCF requests the IMS AGW on the target satellite to configure context information other than IP addresses of the connection point towards the UE based on the corresponding context in the IMS AGW on the source satellite. This step 4 is performed according to clause 8.2 of TS 23.334.
Step 5.
P-CSCF requests IMS AGW on the target satellite to configure the IP address allocated in IMS AGW on the remote satellite in the terminating network, which P-CSCF has stored, to be used by the IMS AGW on the target satellite as the destination of media traffic towards the terminating network and to reserve an IP address in the IMS AGW on the target satellite to be used by the IMS AGW on the remote satellite in the terminating network as the destination of media traffic towards the originating network. In addition, the P-CSCF requests the IMS AGW on the target satellite to configure context information other than IP addresses of the connection point towards the terminating network based on the corresponding context in the IMS AGW on the source satellite. This step 5 is performed according to clause 8.2 of TS 23.334.
Step 6.
P-CSCF replies to PCF by invoking Npcf_PolicyAuthorization_Update service operation as defined in clause 4.3.6.3 of TS 23.502 to the early notification received in step 2. The Npcf_PolicyAuthorization_Update request is a positive response indicating that the change of the user plane paths for optimized media routing to the one through the target satellite should be performed. This request also includes the IP address allocated in IMS AGW on the target satellite to be used by UE as the destination of media traffic and optional N6 traffic routing information associated with target DNAI. SMF in 5GC establishes UL CL/BP and L-PSA on the target satellite, with the UL CL/BP configured with traffic filters containing this IP address to route the IMS media towards the L-PSA, according to clause 4.3.5.7 of TS 23.502. If N6 traffic routing information associated with target DNAI is received, SMF also configures the N6 traffic routing information on the L-PSA.
Step 7.
P-CSCF updates via PCF the packet filter list of the QoS rule in UE for media traffic to additionally contain the IP address allocated in IMS AGW on the target satellite to be used by UE as the destination of media traffic according to clause 4.3.3.2 of TS 23.502.
Step 8.
P-CSCF receives the late notification of the satellite user plane management events associated with UE-Satellite-UE communication media traffic from PCF as defined in clause 4.3.6.3 of TS 23.502. This late notification contains an indication being set "LATE" that indicates that 5GC has established the user plane path for optimized media routing through the target satellite.
Step 9.
P-CSCF sends a SIP MESSAGE to IMS AS to request it to send SIP re-INVITE to the terminating network (i.e. Step 10) and then towards the UE in the originating network after receiving the SDP answer from the terminating network (i.e. Step 16). This SIP message contains the IP address allocated in IMS AGW on the target satellite to be used by the terminating network as the destination of media traffic. This SIP message also contains the satellite ID of the target satellite.
Step 10.
IMS AS sends SIP re-INVITE to the terminating network. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on the target satellite to be used by the terminating network as the destination of media traffic. This SIP re-INVITE also contains a SIP header for conveying the satellite ID of the target satellite.
The following steps 11-15 are performed in the terminating network.
Step 11.
P-CSCF requests IMS AGW on satellite to configure the IP address received in step 10 to be used by the IMS AGW on satellite as the destination of media traffic towards the originating network. This step 11 is performed so far according to clause 8.4 of TS 23.334. In addition, P-CSCF stores the satellite ID of the target satellite in the originating network for future use.
Step 12.
The media path from the originating network to the terminating network remains the same. The media path from the terminating network to the originating network is via UL CL/BP, L-PSA and IMS AGW on the remote satellite in the terminating network and further via IMS AGW, L-PSA and UL CL/BP on the target satellite in the originating network.
Step 13.
P-CSCF sends SIP re-INVITE containing an SDP offer to UE.
Step 14.
UE sends SIP 200 OK containing an SDP answer to P-CSCF.
Step 15.
P-CSCF sends SIP 200 OK to the originating network. This SIP 200 OK also contains a SIP header for conveying the satellite ID of the satellite in the terminating network and an SDP answer. The satellite ID is the same as the one sent before the satellite change in the originating network.
The following steps 16-22 are performed in the originating network.
Step 16.
IMS AS sends SIP re-INVITE to P-CSCF. This SIP re-INVITE also contains a SIP header for conveying the satellite ID of the satellite in the terminating network and an SDP offer.
Step 17.
P-CSCF sends SIP re-INVITE to UE. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on the target satellite to be used by UE as the destination of media traffic.
Step 18.
The media path in both directions is via ULCL, L-PSA and IMS AGW on the target satellite in the originating network and UL CL/BP, L-PSA and IMS AGW on the remote satellite in the terminating network.
Step 19.
UE sends SIP 200 OK containing an SDP answer to P-CSCF.
Step 20.
P-CSCF sends SIP 200 OK containing an SDP answer to IMS AS.
Step 21.
SMF in 5GC releases UL CL/BP and L-PSA on the source satellite according to steps 11 and 12 in clause 4.3.5.7 of TS 23.502.
Step 22.
P-CSCF releases IMS AGW on the source satellite sometime after receiving SIP 200 OK in step 19. This step 22 is performed according to clause 8.5 of TS 23.334.
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AE.5.2.2  Ground fallback procedure  p. 450

Figure AE.5.2.2-1 depicts a signalling flow diagram for the case that optimized media routing is abandoned and the routing with the media transiting through the ground segment is selected after change of satellites that serve a UE. The procedure is written in such a way that change of satellite occurs in the originating network for the purpose of the explanation, while change of satellite can occur in the terminating network as well.
Use of the N5 interface between IMS and 5GC is assumed.
Based on the procedure described in clause AE.5.1, P-CSCF is expected to subscribe from 5GC for the early and the late notification of the satellite user plane management events associated with UE-Satellite-UE communication media traffic as specified in clause 5.6.7 of TS 23.501 and clause 4.3.6.3 of TS 23.502.
Copy of original 3GPP image for 3GPP TS 23.228, Fig. AE.5.2.2-1: Ground fallback procedure
Figure AE.5.2.2-1: Ground fallback procedure
(⇒ copy of original 3GPP image)
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The steps in the call flow are as follows:
Step 1.
Step 1 of clause AE.5.2.1 applies.
Step 2.
P-CSCF receives the early notification of the satellite user plane management events associated with UE-Satellite-UE communication media traffic from PCF as defined in clause 4.3.6.3 of TS 23.502. This early notification contains an indication being set "EARLY". This early notification may contain satellite ID of a target satellite that has gNB to which the UE gets connected, which indicates that 5GC is prepared to change the user plane path for optimized media routing to the one through this target satellite.
Step 3.
P-CSCF determines that optimized media routing cannot continue if there is no target satellite ID included in the early notification as received in step 2, or the two satellites identified by the satellite ID received in step 2 for the originating network and the satellite ID stored for the terminating network have no ISLs. The P-CSCF determines to activate ground fallback routing.
Step 4.
P-CSCF requests IMS AGW on ground to configure the IP address allocated in UE, which the P-CSCF has stored, to be used by the IMS AGW on ground as the destination of media traffic towards the UE and to reserve an IP address in the IMS AGW on ground to be used by the UE as the destination of media traffic. In addition, the P-CSCF requests the IMS AGW on ground to configure context information other than IP addresses of the connection point towards the UE based on the corresponding context in the IMS AGW on satellite. This step 4 is performed according to clause 8.2 of TS 23.334.
Step 5.
P-CSCF requests IMS AGW on ground to reserve an IP address in the IMS AGW on ground to be used by the terminating network as the destination of media traffic towards the originating network. In addition, the P-CSCF requests the IMS AGW on ground to configure context information other than IP addresses of the connection point towards the terminating network based on the corresponding context in the IMS AGW on satellite. This step 5 is performed according to clause 8.2 of TS 23.334.
Step 6.
P-CSCF replies to PCF by invoking Npcf_PolicyAuthorization_Update service operation as defined in clause 4.3.6.3 of TS 23.502 to the early notification received in step 2. The Npcf_PolicyAuthorization_Update request indicates that the change of the user plane paths for the continuation of optimized media routing should not be performed. P-CSCF also sends to PCF the IP address allocated in IMS AGW on ground to be used by UE as the destination of media traffic, so that 5GC updates the packet filter list of the QoS rule in the UE for media traffic to additionally contain this IP address according to clause 4.3.3.2 of TS 23.502.
Step 7.
P-CSCF sends a SIP MESSAGE to IMS AS to request it to send SIP re-INVITE to the terminating network (i.e. Step 8) and then towards the UE in the originating network after receiving the SDP answer from the terminating network (i.e. Step 18). This SIP message contains the IP address allocated in IMS AGW on ground to be used by the terminating network as the destination of media traffic. This SIP message does not contain any satellite ID.
Step 8.
IMS AS sends SIP re-INVITE to the terminating network. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on ground to be used by the terminating network as the destination of media traffic. This SIP re-INVITE does not contain any satellite ID.
The following steps 9-16 are performed in the terminating network.
Step 9.
P-CSCF receives the SIP re-INVITE.
Step 10.
P-CSCF requests IMS AGW on ground to configure the IP address allocated in UE, which the P-CSCF has stored, to be used by the IMS AGW on ground as the destination of media traffic towards the UE and to reserve an IP address in the IMS AGW on ground to be used by the UE as the destination of media traffic. In addition, the P-CSCF requests the IMS AGW on ground to configure context information other than IP addresses of the connection point towards the UE based on the corresponding context in the IMS AGW on satellite. This step 10 is performed according to clause 8.2 of TS 23.334.
Step 11.
P-CSCF requests IMS AGW on ground to configure the IP address received in step 9 to be used by the IMS AGW on ground as the destination of media traffic towards the originating network and to reserve an IP address in the IMS AGW on ground to be used by the originating network as the destination of media traffic towards the terminating network. In addition, the P-CSCF requests the IMS AGW on ground to configure context information other than IP addresses of the connection point towards the originating network based on the corresponding context in the IMS AGW on satellite. This step 11 is performed according to clause 8.2 of TS 23.334.
Step 12.
P-CSCF sends to PCF the IP address allocated in IMS AGW on ground to be used by UE as the destination of media traffic, so that 5GC updates the packet filter list of the QoS rule in UE for media traffic to additionally contain this IP address according to clause 4.3.3.2 of TS 23.502.
Step 13.
P-CSCF sends SIP re-INVITE to UE. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on ground to be used by UE as the destination of media traffic.
Step 14.
The media path from the originating network to the terminating network remains the same. The media path from the terminating network to the originating network is via UL CL on satellite, PSA on ground, IMS AGW on ground in the terminating network and further via IMS AGW on ground, PSA on ground and UL CL on satellite in the originating network.
Step 15.
UE sends SIP 200 OK containing an SDP answer to P-CSCF. P-CSCF sets an implementation-specific timer for releasing IMS AGW on satellite that expires sometime after UE in the originating network receives SIP re-INVITE in step 20.
Step 16.
P-CSCF sends SIP 200 OK to the originating network. This SIP 200 OK contains an SDP answer that has the IP address allocated in IMS AGW on ground to be used by the originating network as the destination of media traffic towards the terminating network. This SIP 200 OK does not contain any satellite ID.
The following steps 17-23 are performed in the originating network.
Step 17.
IMS AS receives the SIP 200 OK.
Step 18.
IMS AS sends SIP re-INVITE to P-CSCF. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on ground in the terminating network to be used by the originating network as the destination of media traffic towards the terminating network. This SIP re-INVITE does not contain any satellite ID.
Step 19.
P-CSCF requests IMS AGW on ground to configure the IP address received in step 18 to be used by the IMS AGW on ground as the destination of media traffic towards the terminating network. This step 19 is performed according to clause 8.4 of TS 23.334.
Step 20.
P-CSCF sends SIP re-INVITE to UE. This SIP re-INVITE contains an SDP offer that has the IP address allocated in IMS AGW on ground in the originating network to be used by UE as the destination of media traffic.
Step 21.
The media path in both directions is via UL CLs on satellite, PSAs on ground and IMS AGWs on ground.
Step 22.
UE sends SIP 200 OK containing an SDP answer to P-CSCF.
Step 23.
P-CSCF sends SIP 200 OK containing an SDP answer to IMS AS. This SIP 200 OK does not contain any satellite ID.
Step 24.
Both in the originating network and in the terminating network, SMF in 5GC releases UL CL and L-PSA on satellite according to steps 11 and 12 in clause 4.3.5.7 of TS 23.502.
Step 25.
P-CSCF in the originating network releases IMS AGW on satellite in the originating network sometime after receiving SIP 200 OK in step 22. P-CSCF in the terminating network releases IMS AGW on satellite in the terminating network after the timer being set in step 15 expires. This step 25 is performed according to clause 8.5 of TS 23.334.
Step 26.
The media path in both directions is via PSAs on ground and IMS AGWs on ground.
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AE.5.3  At call setup with early media  p. 453

During the call setup with both originating and terminating UEs using regenerative NR satellite access, the originating P-CSCF uses the IMS-AGW on ground to transfer early media then, after the IMS session is answered, releases the IMS-AGW on ground and selects the IMS-AGW on satellite to transfer regular media. See Figure AE.5.3-1 that describes the procedure.
In this procedure, the IMS AS for early media uses existing method for providing early media and updating the IMS session when answered.
Copy of original 3GPP image for 3GPP TS 23.228, Fig. AE.5.3-1: IMS session establishment procedure with early media
Figure AE.5.3-1: IMS session establishment procedure with early media
(⇒ copy of original 3GPP image)
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Step 1-11.
Steps 1-11 of clause AE.5.1 apply with the difference that the IMS AS serving early media allocates early media resources with MRF, as well as includes the P-Early-Media header field and a modified SDP answer for early media in the 18X response.
The IMS AS may instruct the MRF to play early media any time before step 12.
Step 12.
Based on the identification of the satellite serving UE B and the P-Early-Media header field in step 11, P-CSCF A determines to perform UE-satellite-UE communication with early media. P-CSCF A selects an IMS-AGW A on satellite but does not release the IMS-AGW A on ground in order to use it for early media.
Step 12a-12b.
P-CSCF A interacts with IMS-AGW A on ground for early media to allocate and configure transport resources for IMS access side as well as configure transport resources for IMS core network side.
Step 13.
Step 13 of clause AE.5.1 applies.
The early media is able to be sent to the UE A via the IMS AGW A on ground, the UPF A on ground and the UPF A on satellite.
Step 14.
Step 14 of clause AE.5.1 applies with the difference that P-CSCF A includes in the SIP 18X response an SDP answer containing the transport address of IMS-AGW A on ground obtained in step 12.
Step 15-16.
Steps 15-16 of clause AE.5.1 apply with difference that P-CSCF A does not generate/modify SDP offer in step 16. Step 17 of clause AE.5.1 is not performed because no SDP offer for regular media is received.
Step 17-18.
Steps 18-19 of clause AE.5.1 apply and UE B is ringing.
Step 19.
The UE B answers the IMS session and sends SIP 200 response for the INVITE.
Step 20.
The IMS AS serving early media instructs the MRF to stop playing early media. UE A may play local ring tone when early media is stopped.
Step 21.
The IMS AS serving early media triggers the SDP re-negotiation procedure between UE A and UE B using existing mechanism, e.g. sending SIP ACK and SIP reINVITE without SDP information towards UE B.
Step 22.
During the SDP re-negotiation procedure, P-CSCF A releases the IMS AGW A on ground and interacts with the IMS-AGW A on satellite selected in step 12 for regular media to allocate and configure transport resources for both IMS access side and IMS core network side.
Step 23.
The SDP re-negotiation procedure continues.
After receiving the SIP 200 response for the INVITE, the UE A is able to send regular media and is ready to receive regular media. The regular media uses UE-satellite-UE communication path.
Step 24.
During the SDP re-negotiation procedure, P-CSCF B updates the allocated IMS-AGW on the satellite for UE B with the transport address in the received SDP information for regular media to configure transport resources for IMS core network side.
Step 25.
P-CSCF B forwards the SIP ACK towards UE B.
After receiving the SIP ACK, the UE B is able to send regular media.
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