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6.1.2.4 Reference points
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See clause 4.2.6.
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6.1.2.5 Service-based interfaces
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See clause 4.2.5.
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6.1.3 Functional entities
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See clause 4.3.
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6.2 High level functionality and features
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6.2.1 Authorization and Provisioning for A2X communications
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6.2.1.1 General
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Authorization and provisioning of parameters for A2X communication leverages V2X mechanisms as defined in TS 23.287 [11] clause 5.1.1 with the following differences:
- V2X is replaced by A2X.
- The V1 reference point is replaced by the A2X1 reference point.
- The set of parameters defined for A2X are described in clause 6.2.1.2.1 and 6.2.1.3.1.
- The means for PCF including A2X policy/parameters into a Policy section identified by a Policy Section Identifier (PSI) as specified in clause 6.1.2.2.2 of TS 23.503 [9].
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6.2.1.2 Authorization and Provisioning for A2X communications over PC5 reference point
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6.2.1.2.1 Policy/Parameter provisioning
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A2X leverages what is defined for V2X in TS 23.287 [11] clause 5.1.2.1 with the following differences:
- V2X is replaced by A2X.
- The V1 reference point is replaced by the A2X1 reference point.
- Tx Profiles for LTE PC5 are not required.
- Groupcast related policy/parameters are not required.
- In addition to existing parameters for V2X, the radio parameters per PC5 RAT (i.e. LTE PC5, NR PC5) can be configured with Altitude Range per Geographical Area. This additional information may be needed to enable policing the use of PC5 depending on the specific location of the UAV.
- Addition of:
- Deconflicting policy which indicates the communication mode (unicast or broadcast) used for deconflicting is defined for A2X.
NOTE: How frequently a UAV sends deconfliction-related messages is an application layer aspect outside the scope of 3GPP.
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6.2.1.2.2 Principles for applying parameters for A2X communications over PC5 reference point
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A2X leverages what is defined for V2X in TS 23.287 [11] clause 5.1.2.2 with the following differences:
- V2X is replaced by A2X.
- The V1 reference point is replaced by the A2X1 reference point.
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6.2.1.3 Authorization and Provisioning for A2X communications over Uu reference point
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6.2.1.3.1 Policy/Parameter provisioning
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The following set of information may be provisioned to the UAV UE for A2X communication over the Uu reference point using MBS:
1) Mapping of the A2X service type to A2X Application Server address information (consisting of IP address/FQDN and transport layer port#) for unicast.
2) List of FQDNs or IP addresses of the A2X Application Servers, associated with served geographical area information and list of PLMNs that the configuration applies to.
3) Mapping of the A2X service type to MBS session announcement for A2X message reception via MBS.
4) MBS session announcement for receiving A2X Application Server information via MBS (i.e. for A2X Application Sever discovery using MBS).
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6.2.2 A2X communication
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6.2.2.1 A2X communication over PC5 reference point
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6.2.2.1.1 General
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A2X communication over PC5 reference point leverages V2X mechanisms to support Unicast communication mode for NR and Broadcast communication mode for both NR and LTE as defined in clause 5.2.1.1 of TS 23.287 [11] with the following applicable differences:
- V2X is replaced by A2X.
- The distinction between Vehicle UE and Pedestrian UE is not applicable to A2X.
- Groupcast mode for NR based PC5 is not supported.
- For LTE PC5 in EPS, the network scheduled operation mode (defined in referred TS 23.285 [12]) is not supported and A2X uses only the UE autonomous resources selection mode.
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6.2.2.1.2 Broadcast mode communication over PC5 reference point
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A2X leverages what is defined for V2X in clause 5.2.1.2 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- The applicable configuration is described in clause 6.2.1.2.
- PC5 QoS parameters are described in clause 6.2.4.
- Enhanced QoS handling for NR based PC5 reference point is described in clause 6.2.4.
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6.2.2.1.3 Unicast mode communication over PC5 reference point
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A2X leverages what is defined for V2X in clause 5.1.1.3 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Definitions for PC5 QoS flows are described in clause 6.2.4.
- Definitions for A2X identifiers are described in clause 6.2.6.
- Procedures for Unicast mode A2X communication over PC5 reference point are described in clause 6.3.3.2.
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6.2.2.1.4 IP address allocation
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A2X leverages what is defined for V2X in clause 5.2.1.5 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Procedures for Unicast mode A2X communication over PC5 reference point are described in clause 6.3.3.2.
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6.2.2.2 A2X communication over Uu reference point
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6.2.2.2.1 A2X communication via unicast
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A2X leverages what is defined for V2X in clause 5.2.2.1 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
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6.2.2.2.2 A2X reception via MBS
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A2X leverages what is defined for V2X in clause 5.2.2.2 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Multicast MBS session is not supported.
- MBS session announcement is provisioned as described in clauses 6.2.1.1 and 6.2.1.3.1.
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6.2.3 A2X Application Server discovery
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Procedures and mechanisms for V2X Application Server discovery used for V2X communication over Uu operation mode as specified in clause 5.3 of TS 23.287 [11] apply to A2X Application Server discovery used for A2X Broadcast MBS sessions with the following differences:
- The A2X Application Server address information as specified in clause 6.2.1.3.1 may be configured on the UE or provisioned over N1 reference point.
- V2X is replaced by A2X, e.g. V2X services to A2X services, V2X Application Server to A2X Application Server, etc.
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6.2.4 QoS handling for A2X communication
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6.2.4.1 QoS handling for A2X communication over PC5 reference point
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A2X leverages what is defined for V2X in clauses 5.4.1 to 5.4.3 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Groupcast mode communication is not supported.
- Definitions for PC5 QoS handling are described in clause 6.2.4.
- PC5 QoS configuration parameters are described in clause 6.2.1.2.1.
- Definitions for A2X communication over PC5 Reference point are described in clause 6.2.2.1.
- Procedures for A2X unicast mode communication over PC5 Reference point are described in clause 6.3.3.2.
- Procedures for A2X Delivery of PC5 QoS parameters to NG-RAN are described in clause 6.3.5.7.
- PQI values are defined for A2X communication over PC5 reference point. The one-to-one mapping of standardized PQI values that are defined to PC5 QoS characteristics is specified in table 6.2.4.1-1.
Table 6.2.4.1-1: Standardized PQI values to QoS characteristics mapping
PQI
Value
Resource Type
Default Priority Level
Packet Delay Budget
Packet Error
Rate
Default Maximum Data Burst Volume
Default
Averaging Window
Example Services
40
GBR
(NOTE 1)
4
50 ms
10-2
N/A
2000 ms
Unicast mode Detect and Avoid with lower latency requirement
41
4
100 ms
10-2
N/A
2000 ms
Unicast mode Detect and Avoid
42
3
1000 ms
10-3
N/A
2000 ms
C2 Communication - Steer to waypoints;
C2 Communication - video used to aid UAV control (VLOS)
43
3
40 ms
10-3
N/A
2000 ms
C2 Communication - Direct stick steering
44
3
140 ms
10-4
N/A
2000 ms
C2 Communication - video used to aid UAV control (Non-VLOS)
62
Non-GBR
4
100 ms
10-2
N/A
N/A
Broadcast Remote ID with lower latency requirement
63
4
500 ms
10-2
N/A
N/A
Broadcast Remote ID
64
4
50 ms
10-2
N/A
N/A
Broadcast mode Detect and Avoid with lower latency requirement
65
4
100 ms
10-2
N/A
N/A
Broadcast mode Detect and Avoid
NOTE 1: GBR PQIs can only be used for unicast PC5 communications.
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6.2.4.2 QoS handling for A2X communication over Uu reference point
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A2X leverages what is defined for V2X in clause 5.4.5 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Notification on QoS Sustainability Analytics to the V2X Application Server is not supported in this release.
- QoS Change based on extended NG-RAN Notification to support Alternative Service Requirements is not supported in this Release.
- 5QI 75 specified in TS 23.501 [2] is intended to be also used for the delivery of A2X messages over MBS.
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6.2.5 Subscription to A2X service
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A2X leverages what is defined for V2X in clause 5.5 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- The distinction between Vehicle and Pedestrian UE is not supported.
- Definitions for PC5 QoS parameters are described in clause 6.2.4.
- The procedure PCF based A2X Service Authorization and provisioning to UE is described in clause 6.3.2.2.
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6.2.6 Identifiers
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6.2.6.1 Identifiers for A2X communication over PC5 reference point
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A2X leverages what is defined for V2X in clause 5.6.1 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- Groupcast mode communication and corresponding identifiers are not supported.
- IP address allocation for A2X is described in clause 6.2.2.1.4.
- A2X configuration parameters are described in clause 6.2.1.2.1.
- Procedures for Unicast mode A2X communication over PC5 reference point are described in clause 6.3.3.2.
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6.2.7 Interworking between EPS A2X and 5GS A2X
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6.2.7.1 A2X Policy and parameter provisioning
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A2X leverages what is defined for V2X in clause 5.8.1 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
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6.2.7.2 PC5 operation
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A2X leverages what is defined for V2X in clause 5.8.1 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
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6.2.8 MBS Service Description for A2X use
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The announced service information for V2X use as specified in clauses 5.10.2 and 5.10.3 of TS 23.287 [11] is used for the announced service information for A2X use with the following difference:
- V2X is replaced by A2X, e.g. V2X communication to A2X communication, V2X Application Server to A2X Application Server, etc.
- Only broadcast is used as the MBS Service Type in Table 5.10.2-1 of TS 23.287 [11].
- For MBS session announcement for A2X Application Server Discovery, the local Service Information contained in the broadcast is as described in clauses 6.2.3 and 6.3.4.1 and should include similar information defined in clause 6.2.1.3.1.
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6.3 Functional description and information flows
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6.3.1 Control and user plane stacks for NR PC5 reference point supporting A2X services
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6.3.1.1 User plane for PC5 reference point supporting A2X services
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Figure 6.3.1.1-1 depicts a user plane for NR PC5 reference point, i.e. PC5 User Plane Protocol stack.
Legend:
- PC5-U: The SDAP/PDCP/RLC/MAC/PHY functionality is specified in TS 38.300 [15].
- For PDCP SDU type "Non-IP", a "Non-IP Type" header included in the SDU by upper layer to indicate the type of non-IP messages carried will be specified in stage 3 specification.
Figure 6.3.1.1-1: User Plane for NR PC5 reference point
IP and Non-IP PDCP SDU types are supported for the A2X communication over PC5 reference point.
For IP PDCP SDU type, only IPv6 is supported. The IP address allocation and configuration are as defined in clause 5.6.1.1 of TS 23.287 [11].
The Non-IP PDCP SDU contains a Non-IP Type header, which indicates the A2X message family used by the application layer.
NOTE: The Non-IP Type header and allowed values are defined in Stage 3.
The packets from A2X application layer are handled by the A2X layer before transmitting them to the AS layer, e.g. A2X layer maps the IP/Non IP packets to PC5 QoS Flow and marks the corresponding PFI.
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6.3.1.2 Control plane for NR PC5 reference point supporting A2X services
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The protocol stack of clause 6.1.2 of TS 23.287 [11] applies.
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6.3.2 Procedures for A2X service authorization and provisioning to UE
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6.3.2.1 General
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The procedures for service authorization and provisioning to UE may be initiated by the PCF (as described in clause 6.3.2.2), by the UE (as described in clause 6.3.2.3), or by the AF (as described in clause 6.3.2.4).
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6.3.2.2 PCF based A2X Service Authorization and Provisioning to UE
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For PCF based Service Authorization and Provisioning to UE, the Registration procedures as defined in clause 4.2.2.2 of TS 23.502 [3], UE Policy Association Establishment procedure as defined in clause 4.16.11 of TS 23.502 [3] and UE Policy Association Modification procedure as defined in clause 4.16.12 of TS 23.502 [3] apply with the following additions:
- If the UE indicates A2X capability in the Registration Request message and if the UE is authorized to use A2X service based on subscription data, the AMF selects the PCF which supports A2X Policy/Parameter provisioning and establishes a UE policy association with the PCF for A2X Policy/Parameter delivery. PCF discovery and selection mechanism defined in clause 6.3.7.1 of TS 23.501 [2] applies and the AMF may include the A2X capability indication in the Nnrf_NFDiscovery_Request message as the optional input parameter. If provided, the NRF takes the information into account for discovering the PCF instance.
- If the AMF receives the PC5 capability for A2X in the Registration Request message from UE, the AMF further reports the PC5 capability for A2X to the selected PCF. The PCF may determine the A2X Policy/Parameter for specific PC5 RAT based on the received UE's PC5 capability for A2X.
The PCF may update the A2X Policy/parameters to the UE in following conditions:
- UE Mobility, e.g. UE moves from one PLMN to another PLMN. This is achieved by using the procedure of UE Policy Association Modification initiated by the AMF, as defined in clause 4.16.12.1 of TS 23.502 [3].
- When there is a subscription change in the list of PLMNs where the UE is authorized to perform A2X communication over PC5 reference point. This is achieved by using UE Policy Association Modification initiated by the PCF procedure as defined in clause 4.16.12.2 of TS 23.502 [3].
- When there is a change of service specific parameter as described in clause 4.15.6.7 of TS 23.502 [3].
If the serving PLMN is removed from the list of PLMNs in the service authorization parameters, the service authorization is revoked in the UE.
When the UE is roaming, the change of subscription resulting in updates of the service authorization parameters are transferred to the UE by H-PCF via V-PCF.
The UE may perform UE triggered Policy Provisioning procedure to the PCF, after Registration procedure has been completed, as specified in clause 6.3.2.3 when the UE determines the A2X Policy/Parameter is invalid (e.g. Policy/Parameter is outdated, missing or invalid).
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6.3.2.3 Procedure for UE triggered A2X Policy provisioning
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The UE triggered Policy Provisioning procedure is initiated by the UE to request A2X Policy/Parameter from the PCF when UE determines the A2X Policy/Parameter is invalid in the following cases:
- if the validity timer indicated in the A2X Policy/Parameter expires;
- if there are no valid parameters, e.g. for the A2X service type a UE wants to use, for current area, or due to abnormal situation.
Figure 6.3.2.3-1: UE triggered A2X Policy provisioning procedure
1. The UE sends UL NAS TRANSPORT message carrying the UE Policy Container (UE A2X Policy Provisioning Request to request A2X policies) to the AMF.
2. The AMF sends the Namf_Communication_N1MessageNotify request to the PCF including the UE Policy Container received from UE.
3. The PCF receives UE Policy Container which indicates UE Policy Provisioning Request to request A2X policies. If the A2X policies are authorized based on AMF input as specified in clause 6.3.5.2 the PCF performs the UE Policy delivery procedure defined in clause 4.2.4.3 of TS 23.502 [3].
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6.3.2.4 AF-based service parameter provisioning for A2X communications over control plane
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The A2X Application Server can provision the 5GC with A2X service parameters via NEF using the procedure defined in clause 4.15.6.7 of TS 23.502 [3] with the following considerations:
- The AF in TS 23.502 [3] is considered as A2X Application Server in this specification.
- Service Description indicates A2X service domain information.
- Service Parameters include policy/provisioning parameters for A2X communications over PC5 and/or Uu.
NOTE: It is assumed that the A2X service domain information is set based on the Service Level Agreement with the operator.
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6.3.3 Procedures for A2X communication over PC5 reference point
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6.3.3.1 Broadcast mode A2X communication over NR PC5 reference point
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A2X leverages what is defined for V2X in clause 6.3.1 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- The applicable configuration is described in clause 6.2.1.2.
- PC5 QoS parameters are described in clause 6.2.4.
- QoS handling for NR based PC5 reference point is described in clause 6.2.4.1.
- Identifiers for A2X communication over PC5 reference points are described in clause 6.2.6.1.
- Pedestrian UE's and corresponding pedestrian UE power saving as described in clause 5.9 is not supported.
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6.3.3.2 Unicast mode A2X communication over PC5 reference point
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A2X leverages what is defined for V2X in clause 6.3.3 of TS 23.287 [11] with the following differences:
- V2X is replaced by A2X.
- The applicable configuration is described in clause 6.2.1.2.
- PC5 QoS parameters are described in clause 6.2.4.
- A PC5 unicast link is described in clause 6.2.2.1.3.
- IP address allocation for A2X is described in clause 6.2.2.1.4.
- Identifiers for A2X communication over PC5 reference points are described in clause 6.2.6.1.
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6.3.4 Procedures for A2X communication over Uu reference point
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6.3.4.1 A2X Application server discovery using broadcast MBS session
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The procedure for V2X Application Server discovery using broadcast MBS session as specified in clause 6.4.2 of TS 23.287 [11] is used for A2X Application Server discovery using broadcast MBS session with the following difference:
- V2X is replaced by A2X, e.g. V2X message to A2X message, V2X Application Server to A2X Application Server, etc.
The procedure for A2X Application Server discovery using broadcast MBS session may be used by the UE only when it is configured with the information to receive A2X Application Server information via MBS as specified in clause 6.2.1.3.1.
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6.3.4.2 Procedures for A2X communication with MBS
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6.3.4.2.1 MBS service area mapping
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Procedures and mechanisms for MBS service area mapping for V2X communication with MBS as specified in clause 6.4.3.1 of TS 23.287 [11] apply to MBS service area mapping for Broadcast Remote ID using MBS with the following differences:
- V2X is replaced by A2X, e.g. V2X services to A2X services, V2X Application Server to A2X Application Server, etc.
- V1 reference point is replaced by A2X1 reference point.
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6.3.5 Procedures for Service Authorization to NG-RAN for A2X communications over PC5 reference point
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6.3.5.1 General
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In order to enable PC5 radio resource control in NG-RAN, per PC5 RAT A2X service authorization information and PC5 QoS parameters need to be made available in NG-RAN, this clause describes the corresponding procedures and aspects.
NOTE: LTE Uu can control LTE PC5 and/or NR PC5 resource management from the cellular network and NR Uu can control LTE PC5 and/or NR PC5 resource management from the cellular network.
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6.3.5.2 Registration procedure
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The Registration procedure for UE is performed as defined in TS 23.502 [3] with the following additions:
- The UE includes the PC5 Capability for A2X (i.e. LTE PC5 only, NR PC5 only, both LTE and NR PC5) as part of the "5GMM capability" in the Registration Request message. The AMF stores this information for A2X operation. The PC5 Capability for A2X indicates whether the UE is capable of A2X communication over PC5 reference point and which specific PC5 RAT(s) it supports.
- The AMF obtains the A2X Subscription data as part of the user subscription data from UDM during UE Registration procedure using Nudm_SDM service as defined in clause 4.2.2.2.2 of TS 23.502 [3].
- The AMF determines whether the UE is authorized to use A2X communication over PC5 reference point based on UE's PC5 Capability for A2X and the subscription data related to A2X service authorization information (i.e. "A2X services authorized" indication per PC5 RAT and UE-PC5-AMBR per PC5 RAT) received from UDM and stores the A2X service authorization information as part of the UE context.
- The PCF provides the PC5 QoS parameters to AMF and the AMF stores them in the UE context.
- If the UE is PC5 capable for A2X and the UE is authorized to use A2X communication over PC5 reference point based on the subscription data, then the AMF shall include the following information in the NGAP message sent to NG-RAN:
- a "A2X services authorized" indication per PC5 RAT, indicating the UE is authorized to use A2X communication over PC5 reference point.
- UE-PC5-AMBR per PC5 RAT, used by NG-RAN for the resource management of UE's PC5 transmission for A2X services in network scheduled mode.
- the PC5 QoS parameters used by the NG-RAN for the resource management of UE's PC5 transmission for A2X services in network scheduled mode.
- If the UE is authorized to use A2X communication over PC5 reference point, then the AMF should not initiate the release of the signalling connection after the completion of the Registration procedure. The release of the NAS signalling relies on the decision of NG-RAN, as specified in TS 23.502 [3].
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fe86991d3e142214ebb7d1b178b6c24b
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23.256
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6.3.5.3 Service Request procedure
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The Service Request procedure for UE in CM-IDLE state is performed as defined in TS 23.502 [3] with the following additions:
- If the UE is PC5 capable for A2X and the UE is authorized to use A2X communication over PC5 reference point, then the AMF shall include a "A2X services authorized" indication per PC5 RAT in the NGAP message.
- The AMF includes the UE-PC5-AMBR for A2X per PC5 RAT in the NGAP message to the NG-RAN as part of the UE context and NG-RAN may use in resource management of UE's PC5 transmission for A2X services in network scheduled mode.
- The AMF sends the PC5 QoS parameters for A2X to NG-RAN via N2 signalling. The PC5 QoS parameters may be stored in the UE context after the registration procedure. If the UE is authorized to use A2X communication over PC5 reference point but AMF does not have PC5 QoS parameters available, the AMF fetches the PC5 QoS parameters from the PCF.
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fe86991d3e142214ebb7d1b178b6c24b
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23.256
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6.3.5.4 N2 Handover procedure
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The N2 based handover or the Inter-RAT to NG-RAN handover procedures for UE are performed as defined in TS 23.502 [3] with the following additions:
- If the UE is PC5 capable for A2X and the UE is authorized to use A2X communication over PC5 reference point, then the target AMF shall send the "A2X services authorized" indication per PC5 RAT, UE-PC5-AMBR per PC5 RAT and PC5 QoS parameters to the target NG-RAN as follows:
- For the intra AMF handover, the "A2X services authorized" indication per PC5 RAT, UE-PC5-AMBR per PC5 RAT and PC5 QoS parameters are included in the NGAP Handover Request message.
- For the inter AMF handover or Inter-RAT handover to NG-RAN, the "A2X services authorized" indication per PC5 RAT, UE-PC5-AMBR per PC5 RAT and PC5 QoS parameters are included in the NGAP Handover Request message sent to the target NG-RAN.
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23.256
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6.3.5.5 Xn Handover procedure
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The Xn based handover procedures for UE are performed as defined in TS 23.502 [3] with the following additions:
- If the "A2X services authorized" indication is included in the UE context (as described in TS 38.423 [16]), then the source NG-RAN shall include a "A2X services authorized" indication per PC5 RAT, UE-PC5-AMBR per PC5 RAT and PC5 QoS parameters in the XnAP Handover Request message to the target NG-RAN.
- If the UE is PC5 capable for A2X and the UE is authorized to use A2X communication over PC5 reference point, then the AMF shall send the "A2X services authorized" indication per PC5 RAT, the UE-PC5-AMBR per PC5 RAT and PC5 QoS parameters to the target NG-RAN in the Path Switch Request Acknowledge message.
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23.256
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6.3.5.6 Subscriber Data Update Notification to AMF
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The procedure of Subscriber Data Update Notification to AMF is performed as defined in TS 23.502 [3] with the following additions:
- The Nudm_SDM_Notification service operation may contain the "A2X services authorized" indication per PC5 RAT and/or the UE-PC5-AMBR per PC5 RAT.
- The AMF updates the UE Context with the above new A2X subscription data.
- When the AMF updates UE context stored at NG-RAN, the UE context contains the A2X subscription data.
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6.3.5.7 Delivery of PC5 QoS parameters to NG-RAN
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The UE Policy Association Establishment procedure and UE Policy Association Modification procedure, as defined in TS 23.502 [3], is used to provide to the AMF PC5 QoS parameters used by NG-RAN. When receiving Npcf_UEPolicyControl_Create Request from the AMF which includes the PC5 capability for A2X or when receiving the updated subscription data from UDR, the PCF generates the PC5 QoS parameters used by NG-RAN corresponding to a UE. The (V-)PCF provides the information to the AMF as follows: - In the roaming case, the H-PCF includes the PC5 QoS parameters used by NG-RAN in the Npcf_UEPolicyControl_Create Response message or Npcf_UEPolicyControl UpdateNotify Request message sent to V-PCF in an N2 PC5 policy container and V-PCF relays this N2 PC5 policy container as the N2 container in the Namf_Communication_N1N2MessageTransfer message sent to AMF. - In the non-roaming case, the PCF includes the PC5 QoS parameters used by NG-RAN in an N2 container in Namf_Communication_N1N2MessageTransfer message sent to AMF. When the AMF determines that the N2 PC5 policy container comes from (V-)PCF, the AMF stores such information as part of the UE context. In the UE Configuration Update procedure triggered by UE Policy Association Establishment or UE Policy Association Modification: - The AMF forwards the PC5 QoS parameters in the NGAP message to the NG-RAN if a N2 PC5 policy container is received in the Namf_Communication_N1N2MessageTransfer message. - The AMF forwards the PC5 QoS parameters in the NAS message to UE by using the UE Configuration Update procedure for transparent UE Policy delivery procedure defined in clause 4.2.4.3 of TS 23.502 [3] if a UE Policy Container is received in the Namf_Communication_N1N2MessageTransfer message. NOTE 1: If the PC5 QoS parameters are provided to both NG-RAN and UE, both the N2 PC5 Policy Container and the UE Policy Container are included in the Namf_Communication_N1N2MessageTransfer message. NOTE 2: Non-UE specific PC5 QoS parameters, e.g. default PC5 QoS parameters, can also be locally configured in NG-RAN. How such configuration is performed is out of scope of this specification. Annex A (informative): Methods to support C2 communication reliability A.1 Methods to establish redundant C2 connenctions for C2 communication reliability Existing redundant transmission mechanism defined in TS 23.501 [2] can be used for the End-to-End C2 communication reliability with additional redundant information in C2 aviation payload and C2 authorization payload to allow UAV or UTM to synchronize information regarding the usage of redundant C2 connections via 5G system, because existing paring information within the aviation payload only includes one C2 connection. Following principles applies to establish redundant C2 connections for C2 communication reliability: 1. UAV and 5G system support the redundant transmission mechanism defined in clause 5.33.2 of TS 23.501 [2] to be able to establish two redundant C2 connection with a UAV-C or UTM within one network. 2. The C2 redundant information is added into the C2 aviation payload (from UAV to UTM) and C2 authorization payload (from UTM to UAV) to exchange C2 redundant information between UAV and UTM. The redundant information can include the indication of establishment of redundant C2 connections. This C2 redundant information can be part of the C2 paring information associate with the UAV-C. It is up to UAV and UTM to decide which C2 PDU session is primary or secondary. 5G system is only aware of two C2 redundant connections being established and has no knowledge of the primary and secondary C2 connection. 3. The information exchange using C2 aviation payload and C2 authorization payload between UAV and UTM on the status change of C2 redundant connection can occur during C2 PDU connection establishment / modification phase, or UAV controller replacement phase. In the figure A.1-1, the UE initiation PDU session establishment for 2 redundant PDU sessions for C2 communications with a UAV-C is specified. Figure A.1-1: Procedure for PDU Session establishment for C2 redundant 0. The UAV has performed a successful UUAA with the USS. 1. The UE first initiates PDU session establishment request for establishing the first PDU session for C2 connection with the UAV-C. In the PDU Session establishment request CAA-Level UAV ID, a C2 Aviation Payload to be used for C2 authorization which includes the C2 redundant information. 2. The SMF then sends a Nnef_Authentication_AuthenticateAuthorize request including the C2 Aviation Payload, which is used to request authorization redundant C2 communication with 2 redundant C2 connections. 3. The UAS NF/NEF forwards the received authorization request as a Naf_Authentication_AuthenticateAuthorize request to the UTM. 4. The UTM performs C2 authorization based on the received information and sends the Naf_Authentication_AuthenticateAuthorize response to the UAS NF/NEF including the Service Level Device Identity (e.g. the CAA-Level UAV-ID), the C2 Authorization Result and the C2 Authorization Payload (e.g. C2 redundant pairing information). 5. The UAS-NF/NEF forwards the information received from the UTM in the Nnef_Authentication_AuthenticateAuthorize response sent to the SMF. 6. SMF informs the UE about the C2 Authorization Result. SMF conducts PDU session establishment procedure until finalized for the establishment of the first C2 PDU session for the UAV-C. 7. [Conditional] If the C2 authorization is successful, the UTM subscribes via the UAS-NF to a PDU Session Status event for the PDU sessions used for the C2. The UAS NF determines DNN, S-NSSAI corresponding to the PDU session used for the C2 communication and uses this DNN, S-NSSAI to subscribe to SMF for PDU Session Status event. The SMF detects, as described in steps 6-7 of figure 4.15.3.2.3-1 in TS 23.502 [3], when the PDU Session is established and send the PDU Session Status event report to the UTM via UAS NF/NEF including GPSI and the C2 PDU session IP Address. 8. Repeat steps 1-7 to establish the second C2 PDU session. 9. (optional), The UTM stores the received two PDU session IP addresses and invokes the UTM initiated pairing policy configuration procedure (see figure 5.2.5.4.1-1) for the primary C2 connection. 10. (optional), The UTM initiates C2 pairing policy configuration for the secondary C2 connection. A.2 C2 Communication Reliability with Redundant User Plane Paths Annex F in TS 23.501 [2] specifies the mechanism that can be used to achieve redundant user plane paths. The similar mechanism can also be used to enhance the End-to-End C2 communication reliability in the scenarios where: - a UAV has more than one SIM from a single operator and the UAV use them to set up redundant C2 connections to the UTM; - a UAV is present in the area where RAN coverage allows the UAV to connect to multiple gNBs simultaneously. The selection of gNBs can be distinct from each other. NOTE 1: No impact on RAN specifications is expected from the reuse and enhancements of the existing mechanisms. NOTE 2: It is expected that UAV's application layer handles the redundant connections by means outside the scope of the 3GPP. NOTE 3: If this mechanism is being used, the operator needs to ensure that a UAV with multi-SIM single operator is supported and the same UAV ID been used in multiple SIM with single operator. For a UAV with redundant connections to different gNBs to request the establishment of PDU Sessions that use independent RAN and CN network resources, the following per-conditions needs to be met: - separate gNBs nodes are required to achieve user plane redundancy over the 3GPP system. It is, however, up to operator's deployment and configuration to ensure that separate gNBs are available and used; - a UAV is able to establish two (or multiple) independent connections to two (or multiple) independent gNBs; - RAN coverage is so that the UAV is able to connect to multiple gNBs located in the target area. To ensure that the UAV connects to different gNBs, the gNBs need to operate in such a way that allows the UAV to select and connect to different gNBs (e.g. gNB frequency allocation supports such configurations/scenarios); - deployment of the CN and UPF in particular supports configuration/scenarios with redundant user plane paths; - the underlying transport network's topology is aligned with the RAN and UPF deployments and supports configurations/scenarios with redundant user plane paths; - the physical network topology and geographical distribution of network functions also supports configuration/scenarios with redundant user plane paths to the extent deemed necessary by the operator; - operation aspects of the redundant user plane paths (for instance, independent power supplies) are made independent to the extent deemed necessary by the operator. Figure A.2-1 provides the architecture view. The UAV is connected to gNB1 and gNB2, respectively. Connection1 sets up a PDU Session via gNB1 to UPF1, while Connection2 sets up another PDU Session to UPF2 via gNB2. UPF1 and UPF2 interface with SMF1 and SMF2, respectively and UPF1 and UPF2 provide the UAV's connections to the same UTM. Figure A.2-1: Architecture for achieving C2 communication reliability via redundant (disjoint) user plane paths To ensure that the UAV selects different gNBs, UAV's connections and cells of the gNBs are grouped into more than one group (so-called Reliability Group) and the UAV selects the cells from the same reliability group as the connection. Figure A.2-2 shows an example, where Connection 1 and the cells of gNB1 belong to Reliability Group A and Connection 2 and the cells of gNB2 belong to Reliability Group B. Figure A.2-2: Reliability group-based redundancy concept in RAN To determine Reliability Groups for the UAV, the methods shown in Figure F-2 in Annex F of TS 23.501 [2] can be used. A Reliability Group of each of the connections is represented using the existing parameters and sent from the AMF to the RAN during the RAN context establishment; thus, each of the gNBs knows Reliability Groups of the UAV's connections. Reliability groups of gNB cells are pre-configured via the O&M and gNBs can either learn the Reliability Groups of the neighbouring cells as the Xn connectivity is set up or also configured by the O&M. If the UAV leaves the cells of its currently connected Reliability Groups or if the quality of the connection drops below a certain threshold, the connections can be transferred to cells that belong to another Reliability Group. In such case, the gNB initiates the handover to cells in an appropriate Reliability Groups whenever such cells are available. Annex B (informative): Change history Change history Date Meeting TDoc CR Rev Cat Subject/Comment New version 2021-02 SA2#143-e S2-2101029 - - - TS skeleton (approved in S2-2101029) 0.0.0 2021-06 SA#92E SP-210366 - - - MCC editorial update for presentation to TSG SA#92E for information 1.0.0 2021-09 SA#92E SP-210939 - - - MCC editorial update for presentation to TSG SA#93E for approval 2.0.0 2021-09 SA#92E - - - - MCC editorial update for publication after TSG SA#93E approval 17.0.0 2021-12 SA#94E SP-211297 0002 2 F Clarification and EN resolution for SMF Services 17.1.0 2021-12 SA#94E SP-211297 0003 1 F Clarifications for UUAA-SM procedure 17.1.0 2021-12 SA#94E SP-211297 0004 1 F UUAA during default PDN connection at Attach 17.1.0 2021-12 SA#94E SP-211297 0005 - F USS Initiated procedures update 17.1.0 2021-12 SA#94E SP-211297 0006 1 F C2 Authorization for EPS 17.1.0 2021-12 SA#94E SP-211297 0007 - F 23.256 clean-up 17.1.0 2021-12 SA#94E SP-211407 0008 2 F Clean up for List of Aerial UEs in a geographic area 17.1.0 2021-12 SA#94E SP-211297 0009 1 F UUAA-MM Procedure Updates 17.1.0 2021-12 SA#94E SP-211297 0010 3 F Clarification and Correction on AF and NEF authentication service 17.1.0 2021-12 SA#94E SP-211297 0011 2 F Clarify the implicit subscription during the UUAA procedure 17.1.0 2021-12 SA#94E SP-211297 0012 1 F Clarification on UUAA-MM failure 17.1.0 2021-12 SA#94E SP-211297 0013 1 F Clarification on AMF and SMF addressing UAS NF/NEF 17.1.0 2021-12 SA#94E SP-211297 0014 - F Correction on new CAA-level UAV ID allocation 17.1.0 2021-12 SA#94E SP-211297 0015 1 F Correction on UAV tracking mode 17.1.0 2021-12 SA#94E SP-211297 0016 1 F Association of CAA level UAV ID to 3GPP UAV ID in USS 17.1.0 2021-12 SA#94E SP-211297 0020 1 F Procedure for UAV replacement in EPS 17.1.0 2021-12 SA#94E SP-211297 0022 - F Miscellaneous corrections 17.1.0 2021-12 SA#94E SP-211298 0024 1 F Correction on UUAA re-authentication and re-authorization procedure 17.1.0 2021-12 SA#94E SP-211298 0026 1 F Correction and simplification on UAV-C replacement procedure 17.1.0 2021-12 SA#94E SP-211298 0028 1 F UAS architecture figure update for IWK 17.1.0 2021-12 SA#94E SP-211298 0030 1 F TS 23.256: various clarifications and corrections 17.1.0 2021-12 SA#94E SP-211298 0031 3 F TS 23.256: Rapporteur Editorial CR 17.1.0 2021-12 SA#94E SP-211298 0032 1 F Correction of UUAA when aerial subscription is missing 17.1.0 2021-12 SA#94E SP-211298 0035 1 F Clean up for UUAA-MM procedure 17.1.0 2021-12 SA#94E SP-211298 0036 1 F Replace of the term pairing authorization 17.1.0 2021-12 SA#94E SP-211298 0038 1 F Clarifications on UUAA context during revocation procedure 17.1.0 2021-12 SA#94E SP-211298 0039 1 F Clarifications and corrections on UAV Re-authentication 17.1.0 2021-12 SA#94E SP-211298 0040 1 F Correction on UAS NF discovery and UAS NF functionality 17.1.0 2021-12 SA#94E SP-211298 0044 - F Corrections on usage of Nnef_AFsessionWithQoS service for UAS 17.1.0 2022-03 SA#95E SP-220059 0048 - F Clarification on cause of revocation 17.2.0 2022-03 SA#95E SP-220059 0049 1 F UUAA context management 17.2.0 2022-03 SA#95E SP-220059 0051 1 F Clarification on PDU Session Status Event 17.2.0 2022-03 SA#95E SP-220059 0052 1 F Clarification on UAV Re-authorization procedure 17.2.0 2022-03 SA#95E SP-220059 0054 1 F Revocation of C2 authorisation 17.2.0 2022-03 SA#95E SP-220059 0055 1 F Correction on handling the authorized CAA-Level UAV ID provided by a USS 17.2.0 2022-03 SA#95E SP-220059 0056 1 F Clarification on re-authorization 17.2.0 2022-03 SA#95E SP-220059 0057 1 F Corrections to Nnef_Authentication_AuthenticateAuthorize service operation 17.2.0 2022-06 SA#96 SP-220403 0059 1 F Correcting errors for UAV-C address 17.3.0 2022-06 SA#96 SP-220403 0062 F Corrections to Naf(Nnef)_Authentication_AuthenticateAuthorize service operation 17.3.0 2022-06 SA#96 SP-220403 0063 1 F Clarification and Correction on C2 payload 17.3.0 2022-06 SA#96 SP-220403 0064 1 F Corrections to service operation names 17.3.0 2022-09 SA#97E SP-220782 0065 1 F Clarifications on subscription control for UUAA-SM and C2 authorization 17.4.0 2022-09 SA#97E SP-220782 0067 F Correction on reference on UAV re-authentication procedure in 5GS 17.4.0 2022-12 SA#98E SP-221075 0068 2 F Corrections to functionality and procedure at UAS NF for C2 authorization 17.5.0 2022-12 SA#98E SP-221075 0069 1 F Indication of Network Assisted Positioning method for UAV positioning 17.5.0 2022-12 SA#98E SP-221333 0072 1 F Aerial Service Availability Update using UCU 17.5.0 2023-03 SA#99 SP-230041 0073 1 F Addressing Editor Notes 17.6.0 2023-03 SA#99 SP-230076 0075 6 B Support for direct C2 communication 18.0.0 2023-03 SA#99 SP-230076 0076 4 B Architectural enhancements for Rel. 18 UAS features 18.0.0 2023-03 SA#99 SP-230076 0077 1 B Architectural enhancements for support of Broadcast Remote ID 18.0.0 2023-03 SA#99 SP-230076 0078 3 B Architectural enhancements for support of Detect And Avoid 18.0.0 2023-03 SA#99 SP-230076 0079 1 B Ground based DAA for an area (Sol#7) 18.0.0 2023-03 SA#99 SP-230076 0081 2 B Additional architectural enhancements for Rel. 18 UAS features 18.0.0 2023-03 SA#99 SP-230076 0082 1 F Clarification on ProSe capability support for UAV UEs 18.0.0 2023-06 SA#100 SP-230494 0084 1 F Corrections to Direct C2 authorization via UUAA procedure 18.1.0 2023-06 SA#100 SP-230494 0085 5 F Direct C2 authorization exceptions 18.1.0 2023-06 SA#100 SP-230494 0086 1 F Clarification of EN on inter-PLMN A2X 18.1.0 2023-06 SA#100 SP-230494 0087 1 F Corrections to authorization of A2X 18.1.0 2023-06 SA#100 SP-230494 0088 - F Clarification on differences when referring to TS 23.287 18.1.0 2023-06 SA#100 SP-230494 0090 - F Clarification on general concept related to PC5 based functionalities 18.1.0 2023-06 SA#100 SP-230494 0091 - F Clarification on A2X Communication modes 18.1.0 2023-06 SA#100 SP-230494 0092 - F Clarification on A2X Policy 18.1.0 2023-06 SA#100 SP-230494 0094 1 F Removal of cross-rat authorization 18.1.0 2023-06 SA#100 SP-230494 0095 - C Removal of UE requesting UE policies from PCF in REGISTRATION REQUEST 18.1.0 2023-06 SA#100 SP-230494 0097 1 C Ground-based DAA for an area enhancements 18.1.0 2023-06 SA#100 SP-230494 0098 3 B N2 and Xn based HO for UAV 18.1.0 2023-06 SA#100 SP-230494 0100 2 F Clarification of support of A2X capability of UAV UE 18.1.0 2023-06 SA#100 SP-230494 0102 1 F Correction to Abbreviation for A2X and A2X definition 18.1.0 2023-06 SA#100 SP-230494 0089 1 B MBS support for Broadcast Remote ID 18.1.0 2023-06 SA#100 SP-230494 0103 2 F Restructuring to clarify A2X Service 18.1.0 2023-12 SA#102 SP-231274 0108 - F Fixing reference towards UUAA-SM procedure for Direct C2 18.2.0 2023-12 SA#102 SP-231274 0109 1 F New PQI values for A2X communication over PC5 reference point 18.2.0 2023-12 SA#102 SP-231274 0110 - F Clarification on A2X communication over MBS 18.2.0 2023-12 SA#102 SP-231274 0111 2 F Clarifying separation between Common A2X and A2X Service specific descriptions 18.2.0 2023-12 SA#102 SP-231244 0113 - A Cleanup of UUAA-MM and UUAA-SM relation 18.2.0 2023-12 SA#102 SP-231274 0114 1 F Correction for UAS_Ph2 18.2.0 2023-12 SA#102 SP-231274 0115 1 F Correction to authorization for Direct C2 communication 18.2.0 2023-12 SA#102 SP-231274 0116 1 F Clarification on use of A2X Application Layer ID for Direct DAA 18.2.0 2023-12 SA#102 SP-231274 0117 1 F Reference correction in UUAA Revocation 18.2.0 2023-12 SA#102 SP-231274 0118 1 F Minor clarification in USS Discovery 18.2.0 2024-06 SA#104 SP-240585 0120 3 A Correction to UUAA-SM for PDN connection establishment 18.3.0 2024-09 SA#105 SP-241268 0122 2 B Addition of functionalities for UAS_Ph3 to General concept 19.0.0 2024-09 SA#105 SP-241268 0125 2 B Support new functionalities of UAV NF 19.0.0 2024-09 SA#105 SP-241268 0127 3 B KI#1.2: Support for multiple USS serving different geographical areas 19.0.0 2024-09 SA#105 SP-241268 0128 2 B KI#1.2: Adding a new NEF service operations for USS 19.0.0 2024-09 SA#105 SP-241268 0130 2 B USS discovery in multiple USS deployment 19.0.0 2024-09 SA#105 SP-241268 0131 3 B Support of UAV flight planning and monitoring 19.0.0 2024-09 SA#105 SP-241268 0132 2 B Support of C2 communication reliability 19.0.0 2024-09 SA#105 SP-241268 0135 2 B Adding support of network-assisted/ground-based mechanism for Detect And Avoid with 5GS information 19.0.0 2024-12 SA#106 SP-241493 0136 5 B Support of No-Transmit Zones 19.1.0 2024-12 SA#106 SP-241493 0139 2 F Addressing the ENs on geographical description 19.1.0 2024-12 SA#106 SP-241493 0140 4 F Addressing the ENs on message names 19.1.0 2024-12 SA#106 SP-241493 0141 7 F Addressing the EN on Nnef_UAVFlightAssistance_Get service operations 19.1.0 2024-12 SA#106 SP-241493 0142 1 F Provisioning of information about USSs to UAVs and UAS NF 19.1.0 2024-12 SA#106 SP-241493 0143 - F Updates to Network-Based/Assisted DAA Procedures 19.1.0 2024-12 SA#106 SP-241493 0144 2 B Adding C2 Communication Reliability with Redundant User Plane Paths to Annex 19.1.0 2024-12 SA#106 SP-241493 0146 2 F Update to multiple USS serving different geographical areas 19.1.0 2024-12 SA#106 SP-241493 0149 3 F Update of procedure for UAV Triggered Network-assisted DAA 19.1.0 2024-12 SA#106 SP-241493 0150 3 B Instructing a UAV to perform its altitude reporting 19.1.0 2024-12 SA#106 SP-241493 0151 1 F Addressing the EN on security aspects of USS changeover 19.1.0 2024-12 SA#106 SP-241493 0153 1 F GMLC LMF update for Ranging_SL 19.1.0 2025-03 SA#107 SP-250060 0154 1 F Clarifications to NTZ support 19.2.0 2025-03 SA#107 SP-250060 0155 1 F Clarification on reporting velocity in UAV location reporting procedure 19.2.0 2025-03 SA#107 SP-250060 0156 - F Removing EN about border-crossing point 19.2.0 2025-03 SA#107 SP-250030 0159 - A Removing ENs for clean-up 19.2.0 2025-03 SA#107 SP-250050 0161 - A Cleaning-up EN 19.2.0 2025-03 SA#107 SP-250060 0163 2 F Correcting the USS changeover procedure 19.2.0 2025-03 SA#107 SP-250060 0164 - F Indicating optional steps of the pre-flight procedure 19.2.0 2025-03 SA#107 SP-250037 0165 2 F Timestamping UAV's altitude information reports 19.2.0 2025-03 SA#107 SP-250060 0167 1 F Service operation update 19.2.0 2025-03 SA#107 SP-250060 0171 1 F Altitude reporting update 19.2.0 2025-03 SA#107 SP-250060 0175 1 F Correction on USS changeover aspects 19.2.0 2025-03 SA#107 SP-250060 0176 1 F Correction on UAV service operation description 19.2.0 2025-03 SA#107 SP-250030 0184 1 A Unsubscribing from the UUAA-related notifications 19.2.0
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23.272
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1 Scope
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This document defines the Stage 2 architecture and specification for the CS Fallback and for SMS over SGs for EPS or CS Fallback and SMS over S102. The scope of this document includes the architecture enhancements for functionality to enable fallback from E-UTRAN access to UTRAN/GERAN CS domain access and to CDMA 1x RTT CS domain access, and functionality to reuse of voice and other CS-domain services (e.g. CS UDI video / LCS / USSD) by reuse of the CS domain. The functionality specified to support SMS over SGs does not trigger any CS Fallback to UTRAN/GERAN. The functionality specified to support SMS over S102 does not trigger any CS Fallback to CDMA 1xRTT CS domain.
In addition this specification includes an SMS in MME architecture option for SMS services only. This functionality does not trigger any CS Fallback and does not require SGs.
The architecture enhancements to support CS fallback to CDMA 1x RTT CS domain access for UEs with single and dual receiver configurations are specified in Annex B.
In this Release of the specification no mechanisms are specified to support CS Fallback to both UTRAN/GERAN and CDMA 1xRTT in the same PLMN. So, even when a UE has the capability to support both CS Fallback to UTRAN/GERAN CS domain and CS Fallback to CDMA 1xRTT CS domain in a given PLMN, the PLMN implements only one of the two.
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23.272
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 23.401: "GPRS Enhancements for E-UTRAN Access".
[3] 3GPP TS 23.060: "General Packet Radio Service (GPRS); Service description; Stage 2".
[4] 3GPP TS 44.018: "Mobile radio interface layer 3 specification Radio Resource Control (RRC) protocol".
[5] 3GPP TS 23.018: "Basic call handling; Technical realization".
[6] 3GPP TS 48.008: "MSC-BSS interface layer 3 specification; Protocol specification".
[7] 3GPP TS 25.331: "Radio Resource Control (RRC); Protocol specification".
[8] 3GPP TS 23.271: "Functional stage 2 description of Location Services (LCS)".
[9] Open Mobile Alliance, OMA AD SUPL: "Secure User Plane Location Architecture", http://www.openmobilealliance.org.
[10] 3GPP TS 23.090: "Unstructured Supplementary Service Data (USSD); Stage 2".
[11] Void.
[12] 3GPP TS 44.060: "MS-BSS interface; RLC/MAC protocol ".
[13] 3GPP TS 24.010: "Supplementary services specification; General aspects".
[14] 3GPP TS 23.040: "Technical realization of the Short Message Service (SMS)".
[15] 3GPP TS 23.204: "Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) access".
[16] 3GPP2 A.S0008-C: "Interoperability Specification (IOS) for High Rate Packet Data (HRPD) Radio Access Network Interfaces with Session Control in the Access Network".
[17] 3GPP2 A.S0009-C: "Interoperability Specification (IOS) for High Rate Packet Data (HRPD) Radio Access Network Interfaces with Session Control in the Packet Control Function".
[18] 3GPP2 A.S0013-C: "Interoperability Specification (IOS) for cdma2000 Access Network Interfaces – part 3 Features".
[19] 3GPP TR 36.938: "Improved Network Controlled Mobility between E-UTRAN and 3GPP2/Mobile WiMAX Radio Technologies".
[20] 3GPP TS 23.216: "Single Radio Voice Call Continuity (SRVCC); Stage 2".
[21] 3GPP TS 24.008: "Mobile radio interface Layer 3 specification; Core network protocols; Stage 3".
[22] 3GPP2 X.S0042-0: "Voice Call Continuity between IMS and Circuit Switched System".
[23] 3GPP TS 23.236: "Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes".
[24] 3GPP TS 43.055: "Radio Access Network; Dual Transfer Mode (DTM); Stage 2".
[25] 3GPP TS 23.292: "IMS Centralised Services (ICS); Stage 2".
[26] 3GPP TS 23.221: "Architectural Requirements".
[27] 3GPP TS 23.402: "Architecture enhancements for non-3GPP accesses".
[28] 3GPP TS 24.011: "Point-to-Point (PP) Short Message Service (SMS) support on mobile radio interface".
[29] 3GPP TS 25.413: "UTRAN Iu interface Radio Access Network Application Part (RANAP) signalling".
[30] 3GPP TS 48.018: "General Packet Radio Service (GPRS); BSS GPRS Protocol (BSSGP)".
[31] 3GPP TS 23.082: "Call Forwarding (CF) supplementary services; Stage 2".
[32] 3GPP2 C.S0005-A: "Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems - Release A, Addendum 2".
[33] 3GPP TS 36.331: "Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC)".
[34] 3GPP TS 24.301: "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3".
[35] 3GPP TS 36.413: "S1 Application Protocol (S1AP)".
[36] 3GPP TS 22.153: "Multimedia Priority Services".
[37] 3GPP TS 22.067: "enhanced Multi Level Precedence and Pre emption service (eMLPP) - Stage 1".
[38] 3GPP TS 22.011: "Service accessibility".
[39] 3GPP2 A.S0008-C:: "Interoperability Specification (IOS) for High Rate Packet Data (HRPD) Radio Access Network Interfaces with Session Control in the Access Network".
[40] 3GPP2 A.S0009-C: "Interoperability Specification (IOS) for High Rate Packet Data (HRPD) Radio Access Network Interfaces with Session Control in the Packet Control Function".
[41] 3GPP TS 23.146: "Technical realization of facsimile group 3 non-transparent".
[42] 3GPP TS 23.251: "Network sharing; Architecture and functional description".
[43] 3GPP TS 29.338: "Diameter based protocols to support of SMS capable MMEs".
[44] 3GPP TS 29.118: "Mobility Management Entity (MME) -Visitor Location Register (VLR) SGs interface specification".
[45] 3GPP TS 29.274: "Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C)".
[46] Telecommunications Industry Association, TIA-917: "Wireless Priority Service Enhancements for CDMA Systems".
[47] 3GPP TR 22.952: "Priority service guide".
[48] 3GPP TS 23.682: "Architecture enhancements to facilitate communications with packet data networks and applications".
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3 Definitions and abbreviations
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3.1 Definitions
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For the purposes of the present document, the terms and definitions given in TR 21.905 [1] apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
1xCS: The 3GPP2 legacy circuit Switched signalling system as defined in 3GPP2 X.S0042-0 [22].
CSMT flag: A flag in LA update request message used in CS fallback for MT call to avoid missing paging in roaming retry.
CSMO flag: A flag in CM Service Request and LA Update request message used in CS fallback for MO calls.
Service User: See TS 22.153 [36].
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3.2 Abbreviations
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For the purposes of the present document, the abbreviations given in TR 21.905 [1] apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905 [1].
1xCS IWS Circuit Switched fallback Interworking solution Function for 3GPP2 1xCS.
CSMT Circuit Switched fallback Mobile Terminated call.
CSMO Circuit Switched fallback Mobile Originated call.
DRX Discontinuous Reception
eMLPP enhanced Multi Level Precedence and Pre emption
ICS IMS Centralised Services
MNRF Mobile Station Not Reachable Flag
MNRF-MME Mobile Not Reachable Flag in MME for SMS
MPS Multimedia Priority Service
MTRF Mobile Terminating Roaming Forwarding
MWD Message Waiting Data
NAM Network Access Mode
NEAF Non-EPS Alert Flag.
PSM Power Saving Mode
SRVCC Single Radio Voice Call Continuity
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4 Overall Description
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4.1 General Considerations
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The CS fallback in EPS enables the provisioning of voice and other CS-domain services (e.g. CS UDI video/ LCS/ USSD) by reuse of CS infrastructure when the UE is served by E‑UTRAN. A CS fallback enabled terminal, connected to E‑UTRAN may use GERAN or UTRAN to connect to the CS‑domain. This function is only available in case E‑UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.
CS Fallback and IMS‑based services shall be able to co-exist in the same operator's network.
The ICS architecture as defined in TS 23.292 [25] shall be able to co-exist with utilising CS Fallback as the CS domain in the same operator's network.
This specification also specifies the architecture required for SMS over SGs. The MO SMS and MT SMS are signalled over SGs and do not cause any CS Fallback to GERAN/UTRAN RATs, and consequently does not require any overlapped GERAN/UTRAN coverage.
Additionally this specification specifies the SMS in MME architecture option as described in Annex C.
The support of SMS over SGs is mandatory for UE and MME and MSC supporting CS fallback, whereas UE and MME and MSC supporting SMS over SGs are not required to support CS fallback.
NOTE: An MME supporting only SMS over SGs (i.e. not supporting CS fallback) will either reply with "SMS-only" or reject an IMSI attach.
The support of CS fallback (and to a lesser extent SMS over SGs) can impose some operational constraints on Tracking Area boundary planning and the use of the "tracking area list concept" (see TS 23.401 [2]).
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4.2 Reference Architecture
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The CS fallback and SMS over SGs in EPS function is realized by using the SGs interface mechanism between the MSC Server and the MME.
The SGs interface functionality is based on the mechanisms specified for the Gs interface, TS 23.060 [3].
Figure 4.2-1: EPS architecture for CS fallback and SMS over SGs
NOTE 1: The MGW is not shown in the figure 4.2‑1 since neither CS fallback in EPS nor SMS over SGs has any impacts on the U‑plane handling.
NOTE 2: SGSN and S3 have additional functionality related to ISR and CS fallback/SMS over SGs. If ISR is not used, this functionality is not required.
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4.2.1 Reference points
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SGs: It is the reference point between the MME and MSC server. The SGs reference point is used for the mobility management and paging procedures between EPS and CS domain, and is based on the Gs interface procedures. The SGs reference point is also used for the delivery of both mobile originating and mobile terminating SMS. Additional procedures for alignment with the Gs reference point are not precluded.
S3: It is defined in TS 23.401 [2] with the additional functionality to support ISR for CS fallback/SMS over SGs as defined in this specification.
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4.3 Functional entities
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4.3.1 UE
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The CS fallback capable UE supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or UTRAN.
The SMS over SGs capable UE supports access to E-UTRAN/EPC and may support access to the CS domain over GERAN and/or UTRAN.
The support of SMS over SGs is mandatory for a UE that supports CS fallback, whereas a UE that supports SMS over SGs is not required to support CS fallback.
These UEs support the following additional functions:
- Combined procedures specified in this document for EPS/IMSI attach, update and detach.
- CS fallback and/or SMS over SGs procedures specified in this document for using CS domain services.
From the UE's perspective there is no difference whether the MME provides SMS via SGs or by "SMS in MME".
A UE that only supports NB-IoT (see TS 23.401 [2]) may request SMS service and use the EPS Attach, TA Update and EPS Detach procedures.
A UE using CS fallback and/or SMS over SGs supports ISR according to TS 23.401 [2]. In particular a UE deactivates ISR at reception of LAU accept or at reception of combined RAU/LAU accept response with no ISR indication.
The coexistence with IMS services for voice/SMS is defined in clause 4.5.
There are no other CS fallback/SMS over SGs ISR-specifics for the UE compared to ISR description in TS 23.401 [2], i.e. if ISR is active the UE can change between all registered areas and RATs without performing update signalling. The UE listens for paging on the RAT it is currently camped on.
If the UE is a Service User according to TR 22.952 [47] with subscription to CS domain priority service, the UE's USIM belongs to one of Access Class that indicates the priority is needed and the UE sets the RRC establishment cause to "HighPriorityAccess" as specified in TS 36.331 [33].
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4.3.2 MME
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The CS fallback and/or SMS over SGs enabled MME supports the following additional functions:
- Multiple PLMNs selection for the CS domain.
- RAT selection for the CS domain.
- Deriving a VLR number and LAI from the TAI of the current cell and based on the selected PLMN or the selected RAT for CS domain, or using a default VLR number and LAI.
- Deliver the registered PLMN ID for CS domain (included in the LAI) to the eNodeB.
- Includes the PLMN ID for the CS domain (included in the LAI provided to the UE) in the equivalent PLMNs list if it is different compared to the PLMN ID provided as part of the GUTI.
- For CS fallback, generating a TAI list such that the UE has a low chance of "falling back" to a cell in a LA different to the derived LAI (e.g. the TAI list boundary should not cross the LA boundary).
NOTE 1: Alignment of the TAI list boundary with a LA boundary can prevent the MME from making effective use of the "tracking area list" concept. To compensate for this, appropriate cell reselection hysteresis may need to be used within the E-UTRAN.
- Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
- In order for the Serving MSC/VLR to always be up to date with the correct last used LTE PLMN, issuing a LOCATION-UPDATE-REQUEST over SGs when the selected LTE PLMN changes: the serving LTE PLMN is derived from the TAI included in that message.
- Initiating IMSI or EPS detach.
- Initiating paging procedure specified in this document towards eNodeB when MSC pages the UE for CS services.
- Supporting SMS procedures defined in this document.
- Rejecting CS Fallback call request (e.g. due to O&M reasons)
- When configured to support the return to the last used PLMN after CSFB:
- Indicating to the MSC that the MO CS service is due to CSFB as specified in TS 29.118 [44].
- Indicating to the SGSN that the mobility procedure is due to CSFB by adding an indication in Forward Relocation Request and Context Response as specified in TS 29.274 [45].
NOTE 2: This only applies when the S4 reference point is supported between the MME and the SGSN; no equivalent functionality exists on Gn.
- To be sure the Context Request is due to CSFB, a timer is set by the MME at CSFB execution and the indication in Context Response is included only if the timer is not expired.
- If the network supports the CSFB procedures during ongoing IMS voice call with SRVCC capability (i.e. clauses 8.3 and 8.4) and UE has SRVCC and EMM combined capability and CSFB is selected by the network (i.e. not SMS only), then:
- Selecting a MSC/VLR which is both SRVCC and CSFB capable during combined Attach/TAU procedure.
- If CSFB triggers SRVCC during ongoing IMS voice call, a PS to CS HO request is sent to the MSC/VLR that is registered during the combined Attach/TAU procedure, for which the MME selects an SRVCC and CSFB capable MSC/VLR.
An MME that supports "SMS in MME" shall support additional functions specified in Annex C, clause C.4.2.
Whether the MME provides SMS via SGs or by "SMS in MME" is not visible to the UE.
An MME that supports CS Fallback uses the LAI and the TMSI based NRI as provided by the UE or the LAI and a hash value from the IMSI to determine the VLR number as defined in TS 23.236 [23] when multiple MSC/VLRs serve the same LAI. The NRI distribution or IMSI Hash table shall have a consistent configuration in all MMEs in the pool. The same NRI value/hash value/function is used by SGSN to determine the VLR number. An MME that supports SMS over SGs may use the same procedure as for CS Fallback. In some networks, the MME may be configured to select the MSC/VLR for UEs that send low access priority indication with a different load balance to that used for MSC/VLR selection for other UEs. In this case the MME maintains a separate hash/value function for UEs that send low access priority indication.
If the network supports CSFB priority call handling, the MME supports the following additional functions:
- For paging message received on the SGs interface with an indication of the eMLPP priority level, the MME is configured with the priority level(s) for which it provides preferential treatment to the paging of the UE and also the subsequent CS fallback procedure compared to other normal procedures. If the UE needs to be paged, the MME sets the priority level in the S1AP paging request to eNodeB based on the value(s) configured in the MME for CSFB priority call handling. The MME also sets priority indication, i.e. "CSFB High Priority", in the S1AP UE Context Setup/Modification message to the eNodeB, so that the eNodeB can initiate the CSFB procedure with priority, as specified in TS 36.413 [35].
- For an MO CSFB request, the MME determines that the CSFB request needs CSFB priority call handling based on the "HighPriorityAccess" establishment cause received from the eNodeB and/or the MPS CS Priority stored in UE's EPS subscription. The MME provides preferential treatment to this request and also sets priority indication, i.e. "CSFB High Priority", in the S1AP UE Context Setup/Modification message to eNodeB to initiate CSFB procedure with priority, as specified in TS 36.413 [35].
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4.3.3 MSC
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The CS fallback and/or SMS over SGs enabled MSC supports the following additional functions:
- Maintaining SGs association towards MME for EPS/IMSI attached UE.
- Supporting SMS procedures defined in this document.
- When configured to support the return to the last used PLMN after CSFB:
- Indicating to GERAN/UTRAN during the release of an RR connection, that the RR connection was established as a result of CS Fallback, when this is the case.
- During the SGs location update procedure, obtaining the last used LTE PLMN ID via TAI in SGsAP-LOCATION-UPDATE-REQUEST as specified in TS 29.118 [44].
- Indicating the last used LTE PLMN ID to GERAN/UTRAN (immediately following a successful RR connection setup or during the release of the UTRAN RR connection) to help GERAN/UTRAN re-route the UE back to the last used LTE PLMN by selecting the suitable dedicated target frequency list (for idle mode mobility) or target cell (for connected mode mobility in case of UTRAN).
- During a CSFB procedure, including the last used LTE PLMN ID into the Equivalent PLMN list provided to the UE in LAU Accept message.
NOTE 1: The CS Fallback enabled MSC can also be enhanced to support ICS as defined in TS 23.292 [25] and/or SRVCC as defined in TS 23.216 [20].
NOTE 2: In order to speed up the potential LAU procedure during CS fallback the MSC may be configured to lower the frequency of Authentication, TMSI reallocation and Identity check for UEs that are EPS/IMSI attached via the SGs interface.
NOTE 3: The MSC/VLR uses the CSMO flag in LAU and CM service request message and the CSFB indication from the MME for CS Fallback statistics.
NOTE 4: The MSC/VLR can determine that a CS Service is established for CSFB via CSMO or CSMT Flag from the UE or via the CSFB indication from the MME.
If the network supports CSFB priority call handling, the MSC maps, depending on the deployed CS domain priority scheme, the priority level or another priority indication of the IAM message to the eMLPP priority level indication of the paging message that is sent over the SGs interface.
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4.3.4 E-UTRAN
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The CS fallback enabled E-UTRAN supports the following additional functions:
- Forwarding paging request for CS domain to the UE.
- Directing the UE to the target CS capable cell considering the registered PLMN ID and possibly the LAC for CS domain received from the MME.
- The configuration of appropriate cell reselection hysteresis at Location Area boundaries (or across the whole E-UTRAN) to reduce Tracking Area Update traffic.
- To facilitate the configuration of TA boundaries with LA boundaries, the E-UTRAN can gather statistics (from the inbound inter-RAT mobility events of all UEs) of the most common LAs indicated in the RRC signalling.
- Configuration to permit the operator to choose the target 'fallback' RAT and frequency.
For SMS over SGs, no specific E-UTRAN functionality is required.
If the network supports CSFB priority call handling, the E-UTRAN supports the following additional functions:
- For paging message received on S1AP with priority indication, the E-UTRAN should provide preferential treatment to this request compared to other normal paging requests.
- For CS fallback S1AP UE Context Setup/Modification message with priority indication, i.e. "CSFB High Priority", the eNodeB should provide preferential treatment, e.g. in allocating E-UTRAN radio bearer resources compared to other normal resource requests. When CSFB based on PS handover is employed, the eNodeB forwards the priority indication to the target GERAN/UTRAN. When CSFB RRC connection release with redirection is used, the eNodeB sets the "CS Fallback High Priority" indication in the RRC Release message.
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4.3.5 SGSN
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If the SGSN supports ISR, SGSN shall follow the rules and procedures described in TS 23.401 [2] and TS 23.060 [3] with the following additions and clarifications:
- The SGSN shall not send the ISR activated indication at combined RAU/LAU procedure if the UE indicates support of CSFB and/or SMS over SGs.
An SGSN that supports Gs uses LAI and the TMSI based NRI as provided by the UE or LAI and a hash value from the IMSI to determine the VLR number as defined in TS 23.236 [23] when multiple MSC/VLRs serve the same LAI. The same NRI value/hash value/function is used by MME to determine the VLR number.
When configured to support the return to the last used PLMN after CSFB the SGSN shall, during a RAU procedure due to CSFB, include the last used LTE PLMN ID into the Equivalent PLMN list provided to the UE in RAU Accept message.
NOTE: The SGSN finds out the RAU Request message is due to CSFB via the indication from the MME (see clause 4.3.2) when the S4 reference point is supported between the MME and the SGSN; no equivalent functionality exists on Gn..
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4.3.6 BSS
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If the BSS receives indication from the MSC that the CS service (Call, CISS or LCS) was established as a result of CS fallback the BSS may e.g. select to use RR Connection Release with Redirect to send the UE back to E-UTRAN at release of the CS service.
When configured to support the return to the last used PLMN after CSFB and when a last used LTE PLMN ID is indicated after a successful RR connection setup, the BSS shall take the last used LTE PLMN ID into account when selecting the target cell in a PS handover to E-UTRAN or when selecting the dedicated target frequency list for idle mode mobility to E-UTRAN in RR Connection Release procedure. Furthermore, the BSS shall send the last used LTE PLMN ID to the target BSS/RNS during the handover preparation phase.
If the network supports ISR, the CS fallback enabled BSS exhibits the following behaviour:
- Even if the network is operating in NMO II the BSS shall forward Gb interface paging messages onto the radio interface. The BSS in a network operating in NMO II shall not be configured to use PBCCH.
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4.3.7 RNS
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If the RNS receives indication from the MSC that the CS service (Call, CISS or LCS) was established as a result of CS fallback the RNS may e.g. select to use RRC Connection Release with Redirect to send the UE back to E-UTRAN at release of the CS service.
When configured to support the return to the last used PLMN after CSFB and when a last used LTE PLMN ID is indicated after a successful RR connection setup or during the UTRAN RR connection release, the RNS shall take the last used LTE PLMN ID into account when selecting the target cell in a PS handover to E-UTRAN or when selecting the dedicated target frequency list for idle mode mobility to E-UTRAN in RR Connection Release procedure. Furthermore, the RNS shall send the last used LTE PLMN ID to the target RNS/BSS during the handover preparation phase.
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4.4 Control plane
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4.4.1 MME - MSC Server
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Legend:
SGsAP: This protocol is used to connect an MME to an MSC Server based on the BSSAP+.
Stream Control Transmission Protocol (SCTP): This protocol transfers signalling messages.
Figure 4.4.1-1: SGs Interface
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4.5 Co-existence with IMS services
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A CS Fallback and IMS capable UE shall follow the procedures for domain selection for UE originating session/calls according to TS 23.221 [26] 'Domain selection for UE originating sessions / calls'.
An IMS capable UE which supports SMS over IP networks shall follow the procedures for domain selection for UE originating SMS according to TS 23.221 [26] 'Domain selection for UE originating SMS'.
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4.6 Emergency Calls
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4.6.1 General
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When UE is performing CS fallback procedure for Mobile Originating Call for the purpose of emergency call, it shall indicate to the MME that this CS fallback request is for emergency purpose. MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS fallback procedure is for emergency purpose. If PS handover is initiated, E‑UTRAN may indicate priority level of the CS fallback to the target RAT, as specified in TS 25.413 [29], in order to prepare radio resource at target RAT in appropriate way, e.g. priority allocation of the RAB resource.
NOTE 1: E-UTRAN may use the emergency indication for selecting a particular radio access network (2G or 3G) for CS emergency handling.
Based on local operator policy, if the MSC/VLR is required to handle emergeny call setup with IMSI, the network may perform the additional procedure defined in clause 4.6.2.
NOTE 2: When the target RAT for emergency call due to CSFB is selected based on cell re-selection or redirection by the network and the UE moves to a different LA then it is possible that the UE continues the emergency call setup without performing a location updating procedure. In this case, the UE performs the emergency call setup with IMSI.
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4.6.2 Procedures to handle emergency call setup with IMSI
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This procedure is only invoked when permitted by the operator policy.
When MME detects that the CSFB request is due to emergency call and eNodeB indicates that the UE is not available for the PS service at the target RAT, it shall mark the SGs association as not valid (i.e., move to SGs-NULL state) and shall not send IMSI Detach Indication (IMSI) message to the MSC/VLR. After that, MME continues with the CSFB procedure. If ISR is active, MME shall initiate DETACH REQUEST message with the detach type set to "Local Detach" via S3 to the associated SGSN.
The SGSN in NMO I configuration with ISR capability shall support the "EMM combined UE Waiting Flag".
NOTE: If the target RAT is operating in NMO-I, the DETACH REQUEST is to ensure the UE performs combined EPS/IMSI TAU when the UE returns to E-UTRAN as described in clause 6.5.
When MSC/VLR receives CS emergency call setup with IMSI due to CSFB and this IMSI is unknown, the MSC/VLR shall perform a Location Update procedure to the HLR on behalf of the UE after it successfully authenticated the UE. If required by configuration, MSC/VLR shall perform a TMSI reallocation towards the UE using its own non-broadcast LAI and, if the MSC Server is part of an MSC pool for that LA, with its own Network Resource Identifier (NRI).
4.6a Interaction with IMS Emergency session
If the MME receives a paging request which is not for SMS and detects that the UE currently has an ongoing IMS emergency session, the MME shall respond with a Paging Reject towards MSC to stop CS Paging procedure.
NOTE: If the IMS emergency session uses dedicated emergency bearers service for the media, the MME can detect an ongoing IMS emergency session by the presence of at least two EPS bearers with ARP value reserved for emergency services (one bearer for signalling and one for media). If the network allows media for IMS emergency session without a dedicated bearer, the MME can only use the presence of PS bearer with ARP value reserved for emergency services for detection.
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4.7 CSFB Priority Call Handling
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CSFB Priority call handling ensures that, when the network supports eMLPP service (TS 22.067 [37]) or another CS domain priority scheme (e.g. TIA-917 [46] or TR 22.952 [47]), end-to-end priority handling is provided for both mobile originated CSFB calls by a user in E-UTRAN which is entitled for this by a CS domain priority scheme and for mobile terminated CSFB call that is marked for priority and destined to a user in E-UTRAN. An EPS subscription of an UE that is subscribed to a CS domain priority scheme may contain an indication of the users CS domain priority status, i.e. it may contain an MPS CS Priority indication. If the UE is subscribed to a CS domain priority scheme, the UE's USIM may belong to one of Access Class 11 to 15, e.g. when the UE is a Service User according to TR 22.952 [47] the UE is assigned to an Access Class in the range of 11 - 15.
For mobile terminated CS fallback calls belonging to the eMLPP CS domain priority scheme, the MSC provides the call's eMLPP level to the MME along with the SGs paging message. Operator configuration in the MME defines which eMLPP level(s) indicated from MSC to MME require CSFB priority call handling. For CSFB priority call handling the MME is configured with the corresponding paging priority level(s) (i.e., mapped from eMLPP level received) that the MME shall set in the message to the eNodeB when requesting the eNodeB to page the UE if the UE is idle.
For mobile originated CS fallback calls, the MME determines that the CSFB request requires priority handling based on the "HighPriorityAccess" establishment cause received from the eNodeB and/or the UE's MPS CS Priority. For both mobile originated and mobile terminated CSFB priority call handling, the MME shall also provide priority indication to the eNodeB, i.e. "CSFB High Priority", when requesting the eNodeB to execute the CSFB priority procedure as specified in TS 36.413 [35]. The eNodeB should handle the S1AP paging message with the indicated paging priority level. And the eNodeB shall prioritize the CS fallback procedure to GERAN/UTRAN or 1xRTT according to the CSFB priority indicated in the S1AP UE context setup/modification message. If PS handover to GERAN or UTRAN is initiated, E-UTRAN may forward CS fallback priority indicator to the target RAT, as specified in TS 25.413 [29], in order to prepare radio resource at target RAT in appropriate way, e.g. priority allocation of the RAB resource
NOTE 1: For a Mobile Terminating Call from a normal user to a user entitled by a CS domain priority scheme to receive priority, no special handling is required.
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4.8 CS services handling in specific cases
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4.8.1 SGs paging without LAI
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If the SGs Paging Request does not contain the LAI, and if the MME can retrieve the S-TMSI associated to the IMSI, the MME shall temporarily store this MSC/VLR number and force the UE to re-attach to the non-EPS services based on:
- If the UE is in connected mode, the MME shall not send a SGs Service Request to the MSC/VLR as it normally would, but instead shall send a Detach Request (IMSI Detach) to the UE as described in clause 5.3.2;
- If the UE is in idle mode, the MME shall send Detach Indication (Local Detach) message to the SGSN via S3 interface if ISR is active. If the mobile reachable timer is still running the MME shall page the UE with IMSI and the CS indicator (for CSFB) or with S‑TMSI and the PS indicator (for SMS). At the reception of Extended Service Request (for CSFB) or Service Request (for SMS) from the UE, the MME shall not send a SGs Service Request to the MSC/VLR as it normally would, but instead shall send a Detach Request (IMSI Detach) to the UE as described in clause 5.3.2.
The MME shall not send IMSI Detach Indication message to the MSC/VLR in the above scenarios.
MME shall then wait for the UE to re-attach to non-EPS services via a Tracking Area Update Request; the MME shall IMSI attach the UE to the MSC/VLR using the stored MSC/VLR number. After successful IMSI attach, if the paging timer has not expired, the MSC/VLR shall page the UE again using SGs Paging Request with LAI.
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4.8.2 Handling of SGs paging when MME cannot retrieve the S-TMSI
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Upon receiving SGs Paging Request, if the MME cannot retrieve the S-TMSI associated with IMSI, the MME shall page the UE with IMSI and PS indicator to force the UE to re-attach to EPS and non-EPS services.
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5 Mobility Management
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5.1 General
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The CS fallback and SMS over SGs in EPS is realized by using the SGs interface mechanism between the MSC Server and the MME. The SMS over SGs NAS procedures are also used for the SMS in MME option.
The use of the "pool-area" concept as specified in TS 23.236 [23] allows to minimize the occurrence of MSC change at CS fallback.
5.1A TAI list and LAI allocation
For CS fallback, the fallback procedure is likely to be faster if the network can allocate a Location Area to the UE that is the LA of the overlapping target RAT's coverage. For this situation, the MME should avoid allocating TAI lists that span multiple Location Areas of the target RAT (which may be contrary to the normal usage of the "tracking area list" concept described in TS 23.401 [2]).
This can be achieved by:
- configuring the E-UTRAN cell's TAI to take into account the LA boundary of the target RAT;
- the MME being configured to know which TAIs are within which LA; and
- the MME using the TAI of the current E-UTRAN cell to derive the LAI.
The operator should be able to configure the MME as to whether it either:
- provides normal usage of the "tracking area list" concept, or,
- the TAI list allocation is adjusted, for CS fallback mobiles, to provide "TAI lists that do not span multiple LAs".
The MME may use alternative approaches for LAI and TAI list allocation. In particular, this is appropriate for:
- the case of SMS over SGs without overlapping GERAN/UTRAN coverage; and
- the case when not all MSCs in the VPLMN support the SGs interface.
In these situations, one approach is to configure the MME to allocate a default (e.g. non-broadcast) LAI which is associated with a VLR that supports the SGs interface.
If "SMS in MME" is used for a UE, then the MME shall allocate a non-broadcasted LAI (not associated with any VLR) and a reserved TMSI, if needed, for that UE.
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5.2 Attach procedure
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The attach procedure for the CS fallback, SMS over SGs or "SMS in MME" in EPS is realized based on the combined GPRS/IMSI Attach procedure specified in TS 23.060 [3].
Figure 5.2-1: Attach Procedure
1) The UE initiates the attach procedure by the transmission of an Attach Request (parameters as specified in TS 23.401 [2] including the Attach Type, old LAI and Mobile Station Classmark 2) message to the MME. The Attach Type indicates that the UE requests a combined EPS/IMSI attach and informs the network that the UE is capable and configured to use CS fallback and/or SMS over SGs. If the UE needs SMS service but not CSFB, the UE shall include an "SMS-only" indication in the combined EPS/IMSI Attach Request. See clause 5.6.
A UE that only supports NB-IoT (see TS 23.401 [2]) may request SMS service but set the Attach Type to EPS attach instead of combined EPS/IMSI attach.
2) Step 3 to step 16 of the EPS Attach procedure are performed as specified in TS 23.401 [2] with the differences as described below when "SMS in MME" applies:
- If the MME is enabled to use "SMS in MME" and the MME is not registered with an HSS for that UE (i.e. the MME stores no subscriber data for the UE), the MME performs a registration with HSS as described in Annex C, clause C.8.
- If the MME is enabled to use "SMS in MME" and the MME is registered with an HSS for that UE but has no valid SMS subscriber data (i.e. the MME stores subscriber data for the UE but SMS subscriber data have not been requested before or have been removed by the HSS), the MME may perform a re-registration with HSS as described in Annex C, clause C.8 to obtain SMS subscriber data and to provide the HSS with the MME identity to be used for MT-SMS delivery.
- If the HSS supports "SMS in MME", the HSS follows the registration procedures described in Annex C, clause C.8.
If the UE subscribes to the eMLPP (TS 22.067 [37]) service in the CS domain or to another CS domain priority scheme (e.g., based on TIA-917 [46] or TR 22.952 [47]) and has a priority level that qualifies for CSFB priority call handling, the UE EPS subscription received from HSS contains the MPS CS Priority indication.
3) If the Attach Request message includes an Attach Type indicating that the UE requests a combined EPS/IMSI attach, or an NB-IoT-only UE requests SMS and the Attach Type indicates EPS attach, the MME allocates a new LAI for the UE as described in clause 5.1A.
The MME does not perform any registrations with a VLR, (i.e. it skips steps 4 to 7 and no SGs association is created), if the Network Access Mode information in the subscriber data indicate that the subscription has no CS subscriber data; or if the HSS registered the MME for "SMS in MME" for the UE as described in Annex C, clause C.8; or if local configuration, e.g. on MSISDN/IMSI segment, indicates that MME should not perform registration with a VLR for the UE (e.g. NB-IoT only UE).
If the registration with a VLR is required and multiple PLMNs are available for the CS domain, the MME performs selection of the PLMN for CS domain and CS domain operator if the selected CS network is shared network configuration, based on the PLMN ID contained in the current TAI, old LAI and operator selection policies on preferred RAT for CS domain. If the target network is a shared GERAN, the MME shall also take into account the UE capability of support or non-support of GERAN network sharing when selecting the PLMN for the CS domain as specified in TS 23.251 [42]. The PLMN selected for CS should be the same that is used for this UE as a target PLMN for PS handovers or for any other mobility procedures related to CSFB. The MME may take any access restrictions provided by the HSS into account, if the network is using separate location areas for GERAN and UTRAN cells. The selected PLMN ID is included in the newly allocated LAI which is sent to MSC/VLR in step 4 and in Attach Accept to the UE.
The MME derives a VLR number based on the newly allocated LAI and the TMSI based NRI as provided by the UE or on the newly allocated LAI and an IMSI hash function defined in TS 23.236 [23]. The MME starts the location update procedure towards the new MSC/VLR upon receipt of the subscriber data from the HSS in step 2). This operation marks the MS as EPS-attached in the VLR.
4) The MME sends a Location Update Request (new LAI, IMSI, MME name, Location Update Type, selected CS domain operator) message to the VLR. MME name is a FQDN string. The cases in which the MME includes the selected CS domain operator towards the VLR are specified in TS 23.251 [42].
5) The VLR creates an association with the MME by storing MME name.
6) The VLR performs the normal subscription checks for CS and if all checks are successful performs Location Updating procedure in CS domain.
For GWCN configuration, the MSC selects the core network operator as specified in TS 23.251 [42].
7) The VLR responds with Location Update Accept (VLR TMSI) to the MME.
8) The EPS Attach procedure is completed by performing step 17 to step 26 as specified in TS 23.401 [2].
If the UE requested a combined EPS/IMSI Attach, the Attach Accept message includes the parameters as specified in TS 23.401 [2]: VLR TMSI and LAI as allocated in step 3 above. The existence of LAI and VLR TMSI indicates successful attach to CS domain.
If the UE requests combined EPS/IMSI Attach Request without the "SMS-only" indication, and if the network supports SGs procedures only for SMS or the network decided to provide "SMS in MME" for the UE, the MME shall indicate in the Attach Accept message that the IMSI attach is for "SMS-only". When the network accepts a combined EPS/IMSI attach without limiting to "SMS-only", the network may provide a "CSFB Not Preferred" indication to the UE.
If the UE requests combined EPS/IMSI Attach Request with the "SMS-only" indication, and if the network supports SGs procedures only for SMS or if it supports CSFB and SMS over SGs or the network decided to provide "SMS in MME" for the UE, the MME shall indicate in the Attach Accept message that the IMSI attach is for "SMS-only".
If the MME provides "SMS in MME" for a UE that requested combined EPS/IMSI Attach, then the TMSI and LAI are provided as specified in clause C.4.2.
The network provides the "SMS-only" or "CSFB Not Preferred" indications based on locally configured operator policies based on e.g. roaming agreement.
The UE behaviour upon receiving such indications is described in TS 23.221 [26].
If the PLMN ID for the CS domain (included in the LAI provided to the UE) differs from the PLMN ID provided as part of the GUTI, the equivalent PLMNs list includes the PLMN ID for the CS domain.
9) If the VLR has updated the SGs association and if a paging timer is still running for a MT service for this UE, the VLR shall repeat SGs Paging Request towards the updated SGs association.
NOTE 2: The case of unsuccessful attach to CS domain is documented in stage 3 specifications, taking into account reachability for CS services of UEs that have the user preference to prioritize voice over data services and are not configured/supporting to use IMS voice services.
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5.3 Detach procedure
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5.3.1 UE-initiated Detach procedure
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The UE-initiated Detach procedure for the CS fallback and SMS over SGs in EPS is realized based on the MS-Initiated Detach Procedure specified in TS 23.060 [3].
Figure 5.3.1-1: UE-initiated Detach Procedure
1) The UE initiates the detach procedure by the transmission of a Detach Request (parameters as specified in TS 23.401 [2], Detach Type) message to the MME. Detach Type indicates which type of detach is to be performed, i.e., IMSI Detach only, EPS Detach only or combined EPS and IMSI Detach.
2) The UE-initiated Detach procedure for E-UTRAN is continued as specified in TS 23.401 [2].
3a) If the detach type indicates "IMSI Detach only" or "combined EPS and IMSI Detach", the MME sends an IMSI Detach Indication (IMSI) message to the MSC/VLR.
3b) If the detach type indicates "EPS Detach only", the MME sends an EPS Detach Indication (IMSI) message to the MSC/VLR.
4) The MSC/VLR removes the association with the MME.
5) The MME sends a Detach Accept message to the UE as specified in TS 23.401 [2]. When the UE receives the Detach Accept message and the Detach Type indicated "EPS Detach only" in step 1, the UE disables E-UTRAN, selects an appropriate GERAN or UTRAN cell.
6) The UE-initiated Detach procedure for E-UTRAN is completed with step 12 as specified in TS 23.401 [2].
If no SGs association exists for the UE steps 3a-4 are not executed.
5.3.1A UE-initiated Detach procedure for GERAN/UTRAN with ISR activated
When ISR is activated, UE initiates detach procedure as specified in TS 23.401 [2], clause 5.3.8.2.2. The procedure is performed with the exception as follows:
- In step 4, the SGSN sends Detach Notification (Cause, Detach type) message to the associated MME. Cause indicates "IMSI Detach only" when UE performs IMSI Detach only procedure. Otherwise, Cause indicates "complete detach", and Detach type indicates "PS detach" in case of UE-initiated GRPS Detach only procedure, or indicates "combined PS/CS detach" in case of UE-initiated combined GPRS/IMSI detach procedure. If Gs association exists, the SGSN also sends an IMSI Detach Indication (IMSI) message or a GPRS Detach Indication (IMSI) message to the VLR depending on the detach type sent by the UE as specified in TS 23.060 [3], clause 6.6.1.
- When the MME receives the Detach Notification message, it sends an IMSI Detach Indication (IMSI) message to the MSC/VLR if the cause indicates "IMSI Detach only" or the detach type indicates "combined PS/CS detach", or sends an EPS Detach Indication (IMSI) message to the MSC/VLR if the detach type indicates "PS detach".
- If Cause indicates "IMSI Detach only", the MME shall not deactivate ISR and steps 5 to 9 shall be skipped.
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5.3.2 MME-initiated Detach procedure
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The MME-initiated detach procedure for the CS fallback and SMS over SGs in EPS is realized based on the SGSN-Initiated Detach Procedure specified in TS 23.060 [3].
Figure 5.3.2-1: MME-initiated Detach Procedure
1) The MME-initiated Detach procedure is performed as specified in TS 23.401 [2].
2a) If EPS service is not allowed for the UE the MME sends an EPS Detach Indication (IMSI detach from EPS service) message to the MSC/VLR.
2b) If the UE is required to be IMSI detached, the MME sends an IMSI Detach Indication (IMSI) message to the MSC/VLR.
3) The MSC/VLR removes the association with the MME.
4) The MME-initiated Detach procedure is completed with step 11 to step 12 as specified in TS 23.401 [2].
5) If ISR is activated in SGSN and the UE is EMM combined procedures capable, when SGSN receives Detach Indication message sent from MME with "Detach Type" IE that indicates "Local Detach" or Delete Bearer Request message from SGW for last PDP Context, then the SGSN shall send Detach Request (IMSI Detach) to the UE. The SGSN turns on "EMM Combined UE Waiting Flag". When SGSN may have the capability to use this flag and how the SGSN uses this flag is described in clause 5.5.
If no SGs association exists for the UE steps 2a-3 are not executed.
5.3.2A SGSN-initiated Detach procedure with ISR activated
When ISR is activated, SGSN initiates detach procedure as specified in TS 23.401 [2], clause 5.3.8.3A. The procedure is performed with the exception as follows:
- In step 4, the SGSN sends Detach Notification (Cause, Detach type) message to the associated MME. If this detach is local to the SGSN (e.g. implicit detach), Cause indicates local detach. Otherwise, Cause indicates complete detach, and Detach type indicates "PS detach". If Gs association exists, the SGSN also sends a GPRS Detach Indication (IMSI) message to the VLR as specified in TS 23.060 [3], clause 6.6.2.1..
- When the MME receives the Detach Notification message, it sends an EPS Detach Indication (IMSI) message to the MSC/VLR if the detach type indicates "PS detach". If the cause indicates local detach, the MME shall not remove SGs association.
- If Cause indicates local detach, the MME deactivates ISR and steps 5 to 9 shall be skipped.
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5.3.3 HSS-initiated Detach procedure
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The HSS-initiated detach procedure for the CS fallback and SMS over SGs in EPS is realized based on the HLR-Initiated Detach Procedure specified in TS 23.060 [3].
Figure 5.3.3-1: HSS-initiated Detach Procedure
1) The HSS-initiated Detach procedure is performed as specified in TS 23.401 [2].
2) The MME sends an EPS Detach Indication (IMSI) message to the MSC/VLR.
3) The MSC/VLR removes the association with the MME.
4) The HSS-initiated Detach procedure is completed with step 8a to step 10a as specified in TS 23.401 [2].
If no SGs association exists for the UE steps 2-3 are not executed.
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5.3.4 Administration of the MME - MSC/VLR Association
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The MME - MSC/VLR association is created at the following occasions:
- Combined EPS/ IMSI attach in clause 5.2.
- Combined TA/LA Update in clause 5.4.
The association is updated on the following occasions:
- When an UE changes MME.
The MME - MSC/VLR association is removed at the following occasions:
- UE-initiated Detach in clause 5.3.1.
- MME initiated Detach in clause 5.3.2.
- HSS initiated Detach in clause 5.3.3.
- Gs association establishment in 2/3G, see TS 23.060 [3].
- MSC/VLR receives a LA update via the A or Iu interface.
- Based on configuration (e.g., if the network is not using NMO-I), MSC/VLR may remove the SGs association when MSC/VLR receives Relocation Complete/Handover Complete during PS to CS SRVCC from E-UTRAN, see TS 23.216 [20].
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5.4 TA/LA Update procedure
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5.4.0 General
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When a CS fallback and/or SMS over SGs capable UE is EPS/IMSI attached, it initiates the combined TA/LA procedure based on the triggers specified in TS 23.401 [2].
When a CS fallback and/or SMS over SGs capable UE is not EPS/IMSI attached, it may initiate a combined TA/LA procedure in order to use CS Fallback or SMS over SGs services.
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5.4.1 Combined TA/LA Update Procedure
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NOTE 1: The combined TA/LA Update procedure for the CS fallback, SMS over SGs or "SMS in MME" in EPS is realized based on the combined RA/LA Update procedure specified in TS 23.060 [3].
Figure 5.4.1-1: Combined TA / LA Update Procedure
1) The UE detects a change to a new TA by discovering that its current TAI is not in the list of TAIs that the UE registered with the network or the UE's TIN indicates the need for a TAU when re-selecting to E-UTRAN. The combined TA/LA Update Procedure is also performed in order to re-establish the SGs association.
A UE that only supports NB-IoT (see TS 23.401 [2]) may request SMS service but send a TA Update instead of a combined TA/LA Update.
2) The UE initiates the TAU procedure by sending a TAU Request (parameters as specified in TS 23.401 [2] including the Update Type, old LAI and Mobile Station Classmark 2) message to the MME. The Update Type indicates that this is a combined Tracking Area/Location Area Update Request or a combined Tracking Area/Location Area Update with IMSI attach Request. If the UE needs SMS service but not CSFB, the UE shall include an "SMS-only" indication in the combined TA/LA Update procedure, see clause 5.6.
3) Step 4 to step 19 of the EPS TAU procedure are performed as specified in TS 23.401 [2] with the differences as described below when "SMS in MME" applies:
- If the MME is enabled to use "SMS in MME" and the MME is not registered with an HSS for that UE (i.e. the MME stores no subscriber data for the UE), the MME performs a registration with HSS as described in Annex C, clause C.8.
- If the MME is enabled to use "SMS in MME" and the MME is registered with an HSS for that UE but has no valid SMS subscriber data (i.e. the MME stores subscriber data for the UE but SMS subscriber data have not been requested before or have been removed by the HSS), the MME may perform a re-registration with HSS as described in Annex C, clause C.8 to obtain SMS subscriber data and to provide the HSS with the MME identity to be used for MT-SMS delivery.
- If the HSS supports "SMS in MME" the HSS follows the registration procedures described in Annex C, clause C.8.
The MME does not perform any registrations with a VLR, (i.e. it skips steps 4 to 6 and no SGs association is created), if the Network Access Mode information in the subscriber data indicate that the subscription has no CS subscriber data or if the HSS registered the MME for "SMS in MME" for the UE as described in Annex C, clause C.8.
4) If multiple PLMNs are available for CS domain, the MME performs selection of the PLMN for CS domain and CS domain operator if the selected CS network is shared network configuration, based on current TAI, old LAI and operator selection policies on preferred RAT for CS domain. If the target network is a shared GERAN, the MME shall also take into account the UE capability of support or non-support of GERAN network sharing when selecting the PLMN for the CS domain as specified in TS 23.251 [42]. The PLMN selected for CS should be the same that is used for this UE as a target PLMN for PS handovers or for any other mobility procedures related to CSFB. The MME may take any access restrictions provided by the HSS into account, if the network is using separate location areas for GERAN and UTRAN cells. The selected PLMN ID is included in the newly allocated LAI. If the association has to be established or if the LA changed, the new MME sends a Location Update Request (new LAI, IMSI, MME name, Location Update Type, selected CS domain operator) message to the VLR. The cases in which the MME includes the selected CS domain operator towards the VLR are specified in TS 23.251 [42]. The MME retrieves the corresponding VLR number from the determined LAI. If multiple MSC/VLRs serve this LAI, the TMSI based NRI as provided by the UE or an IMSI hash function is used to retrieve the VLR number for the LAI as defined in TS 23.236 [23]. The Location Update Type shall indicate normal location update. The MME name is a FQDN string.
5) The VLR performs the normal subscription checks for CS and if all checks are successful performs Location Update procedure in CS domain.
For GWCN configuration, the MSC selects the core network operator as specified in TS 23.251 [42].
6) The VLR responds with Location Update Accept (VLR TMSI) to the MME.
7) The MME sends a TAU Accept (parameters as specified in TS 23.401 [2], LAI, VLR TMSI) message to the UE. The VLR TMSI is optional if the VLR has not changed. LAI is determined in step 4 above. The presence of the LAI indicates to the UE that it is IMSI attached. If the UE requests combined TA/LA Update Request without the "SMS-only" indication, and if the network supports SGs for SMS only, the network shall perform the IMSI attach and the MME shall indicate in the TAU Accept message that the IMSI attach is for "SMS-only".
If the UE requests combined TA/LA Update (or combined TA/LA Update with IMSI attach) without the "SMS-only" indication, and if the network supports SGs procedures only for SMS or the network decided to provide "SMS in MME" for the UE, the MME shall indicate "SMS-only" in the TAU Accept message. However, if the network supports CSFB and SMS over SGs and accepts a combined TA/LA Update procedure but does not indicate "SMS-only", the MME may provide a "CSFB Not Preferred" indication to the UE.
If the UE requests combined TA/LA Update (or combined TA/LA Update with IMSI attach) with the "SMS-only" indication, and if the network only supports SGs procedures for SMS or if it supports CSFB and SMS over SGs or if the network decided to provide "SMS in MME" for the UE, the MME shall indicate in the TAU Accept message that the combined TA/LA Update procedure is for "SMS-only".
If the MME provides "SMS in MME" for a UE that requested combined TA/LA Update with IMSI attach, then the TMSI and LAI are provided as specified in clause C.4.2.
The network provides the "SMS-only" or "CSFB Not Preferred" indications based on locally configured operator policies based on e.g. roaming agreement.
The UE behaviour upon receiving such indications is described in TS 23.221 [26].
If the PLMN ID for the CS domain (included in the LAI provided to the UE) differs from the PLMN ID provided as part of the GUTI, the equivalent PLMNs list includes the PLMN ID for the CS domain.
8) The UE may send a TAU complete message as specified in TS 23.401 [2] for the TAU procedure.
9) If the VLR has updated the SGs association and if a paging timer is still running for a MT service for this UE, the VLR shall repeat SGs Paging Request towards the updated SGs association.
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5.4.2 Periodic TA and LA Update Procedure
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When the UE is camped on E-UTRAN, periodic LA updates shall not be performed, but periodic TA updates shall be performed. In this case, an SGs association is established and the MSC/VLR shall disable implicit detach for EPS-attached UEs and instead rely on the MME to receive periodic TA updates.
When a periodic TA update is not received in the MME, the MME clears the PPF. The lack of periodic TA update may be caused by reselection or handover to GERAN/UTRAN when ISR is active. To ensure CS paging can reach the EPS/IMSI attached UE, the UE shall perform combined RA/LA update in NMO I or LAU in NMO II when the periodic TAU timer expires and the UE is in GERAN/UTRAN (or next returns to coverage in GERAN/UTRAN) and ISR is active.
In addition, when a periodic TA update is not received in the MME, the MME may implicitly detach the UE as specified in TS 23.401 [2]. This MME implicit detach does not affect any SGSN attach status. At an implicit detach, the MME also releases the SGs association with the MSC/VLR. The MSC continues to maintain the registered LA for the UE. The MSC changes to supervise LA updates and pages in the still registered LA when mobile terminated services arrive.
When the UE camps on GERAN/UTRAN it may perform combined RA/LA updates. The combined RA/LA update procedures and the conditions for their usage are described in TS 23.060 [3].
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5.4.3 Non-EPS Alert procedure
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The MSC/VLR may request an MME to report activity from a specific UE. In this case, the MSC/VLR shall send a SGsAP Alert Request (IMSI) message to the MME where the UE is currently EPS-attached.
Upon reception of the SGsAP Alert Request (IMSI) message, the MME shall set NEAF (Non-EPS Alert Flag). If NEAF is set for an UE, the MME shall inform the MSC/VLR when the next activity from that UE (and the UE is both IMSI- and EPS attached) is detected, and shall clear NEAF.
If the activity detected by the MME leads to a procedure towards the MSC/VLR, the MME shall just follow this procedure. If the activity detected by the MME does not lead to any procedure towards the MSC/VLR, the MME shall send an UE Activity Indication (IMSI) message towards the MSC/VLR.
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aac84734dd828878c9914c9a3545338d
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23.272
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5.4.4 Void
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aac84734dd828878c9914c9a3545338d
|
23.272
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5.5 Idle Mode Signalling Reduction
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In relation with CSFB and/or SMS over SGs, when ISR is activated, the UE follows regular ISR behaviour. It may reselect between E-UTRAN and GERAN/UTRAN without a need to update the CN. When a mobile terminated service arrives, the MSC/VLR sends a paging message via SGs to the MME. The MME pages in the TA(s) registered for the UE, and, the MME uses the S3 interface to request the SGSN (i.e. the SGSN that has an ISR relation with the MME for that UE) to page the UE in the registered RA. When the UE is already connected with the MME, the MME forwards the paging request only to the UE via the established signalling connection.
When the UE is IMS registered for voice service, even if the UE is configured for CSFB or SMS over SGs, it may need to ignore ISR activation based on the conditions for ISR activation/de-activation for UEs registered for IMS voice service as defined in TS 23.401 [2].
A UE that supports CSFB and/or SMS over SGs includes the "EMM combined procedures capability" indication as part of the "MS Network Capability" in the Attach, RAU or combined RAU/LAU Request message, if the UE supports CSFB service or SMS over SGs. SGSN stores the "EMM combined procedures capability" indication for ISR operation as part of the stored "MS Network Capability". If the UE does not support CSFB or SMS over SGs, it shall not include the "EMM combined procedures capability" indication in the Attach, RAU or combined RAU/LAU Request message to SGSN.
ISR remains activated until the UE performs a combined RAU/LAU procedure (e.g. a UE in NMO I moves to a new RA or LA or the periodic TAU timer expires while the UE is in NMO I of GERAN/UTRAN) or separate LAU procedure (e.g. a UE moves to a different LA in NMO II or the periodic TAU timer expires while the UE is in NMO II of GERAN/UTRAN). Normal re-selection between registered RA/TA(s) does not cause ISR deactivated condition. When the UE needs to perform a combined RAU/LAU, the SGSN checks the "EMM combined procedures capability" bit in MS Network Capability and if it indicates that CSFB and/or SMS over SGs is supported then the SGSN deactivates ISR by not indicating ISR activated in the combined RAU Accept message, which is a regular ISR functionality as specified in TS 23.401 [2]. So an SGSN in a CSFB/SMS over SGs configuration never indicates ISR activated in combined RAU procedures for a UE that supports CSFB/SMS over SGs. An SGSN with ISR capability in a CSFB/SMS over SGs configuration shall always maintain ISR by indicating ISR Activated in Periodic RAU Accept for a UE that supports CSFB/SMS over SGs if the SGSN has the status that ISR is activated for the UE. After a combined RA/LA update procedure, the MSC pages via Gs for mobile terminated services. When Gs is not used, the MSC/VLR pages in the LA via Iu/A for mobile terminated services.
If ISR is deactivated and the UE re-selects to E-UTRAN with the TIN indicating "P-TMSI", it initiates a TAU procedure, which is a regular ISR functionality as specified in TS 23.401 [2], and ISR can be activated again. The UE that supports CS fallback/SMS over SGs shall perform this TAU procedure as a combined TA/LA Update Procedure.
In case of the detach procedure for E-UTRAN when ISR is activated, the MME notifies the associated SGSN with indicating detach cause (i.e. local detach or complete detach) as specified in clause 5.3.1, 5.3.2, 5.3.3 and TS 23.401 [2] except UE-initiated IMSI detach only procedure.
In case of the detach procedure for GERAN/UTRAN when ISR is activated, the SGSN sends an IMSI or GRPS Detach indication message to the VLR depending on the detach type sent by the UE as specified in TS 23.060 [3], clause 6.6.1, and notifies the associated MME with indicating detach cause (i.e. local detach, complete detach or IMSI detach only) and detach type (i.e. PS detach or combined PS/CS detach) in case of complete detach, and the MME sends IMSI Detach Indication or EPS Detach Indication message to the MSC/VLR accordingly, which is specified in clause 5.3.1A and 5.3.2A.
When the MME receives a SGs paging for CS service, if ISR is activated and the mobile reachable timer has expired but the Implicit Detach timer has not expired, the MME shall not notify the MSC/VLR that the UE is unreachable via SGs and instead shall page the UE via S3 interface.
When implicit detach timer expires in the MME, the MME shall send EPS Detach Indication message. .
The SGSN in NMO I configuration with ISR capability shall support the "EMM combined UE Waiting Flag". The "EMM Combined UE Waiting Flag" shall be turned on if SGSN sends Detach Request (IMSI Detach) to the UE as specified in clause 5.3.2.
If "EMM Combined UE Waiting Flag" is turned on in the SGSN, then the SGSN shall behave as follows:
- If the SGSN receives Periodic RAU from the UE, the SGSN shall send Detach Request (IMSI Detach) to the UE.
- If the SGSN receives Combined RAU from the UE, the SGSN shall continue the procedure as specified in TS 23.060 [3].
- If the SGSN receives Service Request from the UE, the SGSN handles the Service Request normally and shall send Detach Request (IMSI Detach) to the UE.
- The SGSN shall turn off the "EMM Combined UE Waiting Flag" if the SGSN receives Combined RAU from the UE.
The MME may activate ISR if "SMS in MME" for the UE is used.
For PS-only Service Provision per TS 23.060 [3] clause 5.3.18, if SGSN provides SMS services via PS domain for a UE, the SGSN may activate ISR for this UE by indicating ISR Activated in Combined RAU Accept message.
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aac84734dd828878c9914c9a3545338d
|
23.272
|
5.6 Mobility Management for SMS over SGs only UEs
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UEs that need SMS service but not CSFB indicate this specific condition with the "SMS-only" indication in the EPS/IMSI Attach Request and combined TA/LA update procedures (or, for a UE that only supports NB-IoT, optionally using the EPS Attach Request and TA update procedures, see TS 23.401 [2]). This allows an operator to deploy the SGs for SMS delivery over LTE only without the need for CSFB support. In addition, this allows the MME to use a dedicated algorithm for the selection of the MSC that supports those UEs. An MME that supports "SMS in MME" may directly handle SMS services for the UE under the conditions described in clause C.4.2.
NOTE: SMS delivery via SGs does not cause the terminal to fallback to the CS-capable network. It is possible that only certain MSCs in the network (one in minimum) are configured to support SGs when the network only supports SMS and no other (CS) services via SGs. However such a minimal configuration can cause inter-MSC location updates to be performed at every movement into/out of E‑UTRAN coverage.
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aac84734dd828878c9914c9a3545338d
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23.272
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5.7 Access to CS services after selecting a non-3GPP access
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When a UE, which obtained voice and/or SMS services using an EPS/IMSI registration via E-UTRAN, selects a non-3GPP access network, the UE tries to retain these services by actions described in TS 23.221 [26], clause 7.2a.
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aac84734dd828878c9914c9a3545338d
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23.272
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6 Mobile Originating Call
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