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Content for  TS 23.402  Word version:  16.0.0

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4.5.7  IMS Emergency Session Support |R9|

4.5.7.1  Overview |R13|

Support for IMS Emergency Session for E-UTRAN access connected to the EPC with GTP-based S5/S8 is covered in TS 23.401. Corresponding changes that apply for PMIP-based S5/S8 interface are covered in clause 5 of this specification.
For this Release of the specification, IMS Emergency Session Support for non-3GPP accesses connected to EPC is limited to:
  • Support of handover of emergency sessions from E UTRAN access to HRPD access and is covered in clause 9 of this specification with an overview provided in clause 9.2.2.
  • Support of IMS Emergency Session Support over WLAN access to EPC as described in clause 4. 5.7.2
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4.5.7.2  IMS Emergency Session Support over WLAN access to EPC |R13|

4.5.7.2.1  Introduction
This clause provides an overview about the EPC functionality for emergency PDN connections used to support IMS Emergency Session over WLAN untrusted or trusted access to EPC defined in TS 23.167. The specific functionality is described in the affected procedures and functions of this specification. For discrepancies between this overview clause and the detailed procedure and function descriptions the latter take precedence.
UEs request a PDN Connection for emergency services (also called an emergency PDN connection) when they are aware they need to establish an IMS emergency session.
The UE shall not issue an emergency session over WLAN access to EPC if the emergency session can be established via 3GPP access.
In this Release of the specification, the same four behaviours of IMS emergency session support as identified in TS 23.401, clause 4.3.12 are applicable.
To get EPC access for emergency services in case of untrusted WLAN, the UE shall select an ePDG that supports emergency services as specified in clause 4.5.4a. Then, if a new ePDG is selected, the UE shall execute the procedure of Initial attach for S2b emergency services described in clause 7.2.5. Otherwise, if an existing ePDG connection is reused, the UE shall perform the UE-initiated Connectivity to Additional PDN to Emergency Service PDN connectin described in clause 7.6.3.
An ePDG/TWAG that supports emergency services is configured with Emergency Configuration Data that are applied to all PDN Connections for emergency services. The Emergency Configuration Data contain the Emergency APN which is used to derive a PDN GW, the statically configured PDN GW for the Emergency APN and optionally a fallback statically configured PDN GW, and may also contain information on the default QoS to apply to a PDN Connection for emergency services (as defined in clause 4.5.7.2.4).
The following procedures apply for emergency PDN connections for untrusted WLAN case:
  • procedures defined in clause 7.4.3 ("UE/ePDG-initiated Detach Procedure and UE-Requested PDN Disconnection");
  • procedures defined in clause 7.6 ("UE-initiated Connectivity to Additional PDN");
  • procedures defined in clause 7.9.2 ("PDN GW initiated Resource Allocation Deactivation");
  • procedures defined in clause 7.10 ("Dedicated S2b bearer activation");
  • procedures defined in clause 7.11.1 ("PDN GW initiated bearer modification");
  • procedures defined in clause 8 ("Handovers without Optimizations Between 3GPP Accesses and Non-3GPP IP Accesses").
As part of these procedures, the UE local IP address and optionally UDP or TCP source port number (if NAT is detected) are reported from ePDG to the PDN GW. When Access Network Information reporting has been set by the PCRF, the UE local IP address and optionally UDP or TCP source port number (if NAT is detected) is reported to the PCRF.
To get EPC access for emergency services in case of trusted WLAN, the UE shall select a Trusted WLAN that supports emergency services. This is defined in clause 4.8.2b. Then the UE executes the procedure of Initial attach for S2a emergency services described in clause 16.2.1a
The following procedures apply for emergency PDN connections for trusted WLAN case:
  • procedures defined in clause 16.3 ("Detach and PDN disconnection in WLAN on S2a");
  • procedures defined in clause 16.4 ("PDN GW initiated Resource Allocation Deactivation in WLAN on S2a");
  • procedures defined in clause 16.5 ("Dedicated bearer activation in WLAN on GTP S2a");
  • procedures defined in clause 16.6.1 ("PDN GW Initiated Bearer Modification");
  • procedures defined in clause 16.7.1.1 ("UE/TWAN Initiated Detach Procedure in WLAN on GTP S2a");
  • procedures defined in clause 16.7.2.1 ("UE/TWAN Initiated Detach Procedure in WLAN on PMIP S2a");
  • procedures defined in 16.8 ("UE Initiated PDN connectivity request procedure in WLAN on S2a for Multi-connection Mode");
  • procedures defined in clause 16.9 ("UE/TWAN Initiated PDN disconnection for Multi-connection Mode");
  • procedures defined in clause 16.10 ("Handover procedure from 3GPP access to WLAN on S2a");
The emergency PDN connection is not a subscribed service. Thus procedures related with HSS Initiated Subscribed QoS Modification in clause 7.11.2, procedures related with HSS Initiated Bearer Modification in clauses 16.6.2 and 16.7.2.2 or procedures related with "HSS/AAA-initiated Detach Procedure" in clauses 7.4.4, 16.3.1.2 and 16.3.2.2 do not apply to emergency PDN connections.
Procedures related with S2c do not apply to emergency PDN connections.
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4.5.7.2.2  Architecture Reference Model for Emergency ServicesWord‑p. 43
In this Release of the specification, both the non-roaming architecture defined in Figure 4.2.2-1 and the roaming architecture defined in Figure 4.2.3-1 apply for emergency services.
4.5.7.2.3  PDN GW selection function for Emergency Services
The PDN GW selection does not depend on subscriber information in the HSS since emergency service support is not a subscribed service but a local service. Upon reception from the UE indication that a PDN connection for emergency services needs to be established, the ePDG/TWAG looks up its configured Emergency Configuration Data. The Emergency Configuration Data contains the Emergency APN to be used to derive a PDN GW, or may also contain the statically configured PDN GW for the Emergency APN.
When a PDN GW is selected based on the Emergency APN, the PDN GW selection function described in clause 4.3.8.1 of TS 23.401 for normal bearer services is applied to the Emergency APN. The PDN GW selection function shall always derive a PDN GW in the local PLMN.
This functionality is used by the Initial Attach procedure for emergency services as described in clause 7.2.5 for untrusted WLAN and clause 16.2.1a for trusted WLAN.
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4.5.7.2.4  QoS for Emergency Services
The Default QoS values used over S2b/S2a for establishing emergency PDN connections are configured in the Emergency Configuration Data.
4.5.7.2.5  PCC for Emergency Services
The same mechanisms than defined for 3GPP access in clause 4.3.12.6 of TS 23.401 apply.
4.5.7.2.6  IP Address Allocation
The same mechanisms than defined for 3GPP access in clause 4.3.12.8 of TS 23.401 apply.
4.5.7.2.7  Handling of PDN Connections for Emergency Bearer Services
The same mechanisms as those defined for 3GPP access in clause 4.3.12.9 of TS 23.401 apply with the only difference being that it is the ePDG/TWAG (and not the MME) that shall reject any additional emergency PDN Connection requests.
4.5.7.2.8  Network provided WLAN Location Information
In the case of trusted WLAN Access, WLAN Location Information is provided as specified in clause 16.1.7.
In the case of untrusted WLAN access, WLAN Location Information is provided as follows:
When as part of procedures for Authentication and Authorization on an Access Point based on USIM credentials, the WLAN Access Network provides WLAN Access Network location information to the 3GPP AAA server that it considers as network provided location, the 3GPP AAA server stores this information and provides it to the ePDG at the SWm Authentication and or Autorization procedure or upon request of the ePDG.
This location information is called WLAN Location Information and contains the same information as is contained in the TWAN Identifier defined in clause 16.1.7. The Age of the WLAN Location information is provided in conjunction with the WLAN Location information.
The 3GPP AAA server shall update its storage of WLAN Location Information associated with an UE when it receives WLAN Access Network location information from a WLAN AN that it considers as trustworthy for network provided location. The 3GPP AAA server shall remove its storage of WLAN Location Information associated with an UE when it becomes aware that the WLAN session of the UE is terminated or when it receives WLAN Access Network location information from a WLAN AN that it considers as not trustworthy for network provided location.
The ePDG shall store WLAN Location Information associated with an UE when it receives WLAN Access Network location information from the 3GPP AAA server. The ePDG shall remove its storage of WLAN Location Information associated with an UE when it receives from the 3GPP AAA server an indication that no WLAN Access Network location information is available for this UE.
The WLAN Location Information information and its Age, when available, are propagated by the ePDG to the PDN GW and then via PCC as defined in TS 23.203. This takes place at the UE-initiated connectivity to an initial PDN connection (Attach Procedure), at the UE-initiated connectivity to an additional PDN connection or, as described below, when the ePDG needs to send User Location Information about an already established PDN connection.
When the AAA server has sent WLAN Location Information at the UE-initiated connectivity to an initial (Attach Procedure) or additional PDN connection, and when later the ePDG needs to send User location Information towards the PDN GW over S2b, the ePDG may initiate a WLAN Location Information Request to fetch the most up to date WLAN Location Information in conjunction with the age of this Information.
[not reproduced yet]
Figure 4.5.7.2.8-1: EPDG retrieval of WLAN Location Information
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Step 0.
When the 3GPP AAA server detects that the UE has moved between WLAN AN, it locally updates or removes the WLAN Location Information information and its Age it stores for the UE.
1) A procedure is triggered that requires the ePDG to provide User Location Information over S2b for an already established PDN connection. The corresponding procedures are:
  • 7.4.3 UE/ePDG-initiated Detach Procedure and UE-Requested PDN Disconnection with GTP on S2b.
  • 7.9.2 PDN GW initiated Resource Allocation Deactivation with GTP on S2b.
  • 7.10 Dedicated S2b bearer activation with GTP on S2b.
  • 7.11 S2b bearer modification with GTP on S2b.
Step 2.
When the AAA server has sent WLAN Location Information at the set-up of a SWm session and the ePDG has detected a change of the outer IP address of the UE, the ePDG initiates a WLAN Location Information Request (IMSI) towards the 3GPP AAA server.
Step 3.
The 3GPP AAA server provides a WLAN Location Information Answer that may contain WLAN location information and WLAN location information Age or an indication that no WLAN location information is available. The ePDG replaces any WLAN location information andWLAN location information Age it may have stored beforehand by the information received from the 3GPP AAA server. When the WLAN Location Information Answer contains an indication that no WLAN location information is available, the ePDG removes any WLAN location information and WLAN location information Age it may have stored beforehand about the UE.
Step 4.
The ePDG issues S2b signalling with User Location Information. The User Location Information shall include UE local IP address and optionally UDP or TCP source port number (if NAT is detected). The User Location Information includes WLAN Location Information (and its Age) only when the ePDG has such information currently available about the UE. When the PDN GW receives no WLAN Location Information from the ePDG it shall delete any such information it may have stored for the PDN connection.
Step 5.
If requested by the PCRF the PDN GW forwards to the PCRF following information extracted from User Location Information it may have received from the ePDG:
  • The UE local IP address and optionally UDP or TCP source port number (if NAT is detected).
  • WLAN location information in conjunction with the Age of this information,
When the PCRF receives no WLAN location information from the PDN GW within User Location Information the WLAN location information is considered as not any longer valid.
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4.5.7.2.9  Determination of location |R14|Word‑p. 46
If the UE needs to provide location information when requesting an emergency session, the UE may determine its location by using its own implementation-specific means (e.g. by using its GPS receiver). If, however, the UE cannot determine its location by its own means, the UE may retrieve its location from a WLAN AP, prior or after association with the AP, by requesting the Civic Location ANQP element, the Geospatial Location ANQP element or both as specified in IEEE Std 802.11-2012 [64]), using ANQP procedures described in HS2.0 Rel 2 specification [75].
_NOTE: Location determination based on WLAN ANQP procedures should be considered as less reliable than e.g. location determination based on GPS receiver.
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4.5.7.2.10  Support of PS handover with 3GPP EPC |R14|
Seamless PS handover between WLAN and 3GPP EPC is supported if the following conditions are satisfied:
  • For UEs without UICC or with an unauthenticated IMSI and for authenticated roaming UEs, the ePDG/TWAG is configured with a static PDN GW identity and optionally a fallback PDN GW identity as part of the Emergency Configuration Data.
    • When the UE initiates an Emergency Attach with handover indication to the MME, the MME uses the static PDN GW identity locally configured instead of querying the HSS as described in TS 23.401.
  • For authenticated non-roaming UEs, based on operator policy, the ePDG/TWAG may select a PDN GW based on DNS look up for the emergency APN, which is configured in the Emergency Configuration Data. In such case:
    • During the initial establishment of the PDN connection for emergency services, the "PDN GW currently in use for emergency services", which comprises the PDN GW address and an indication that the PDN connection is for emergency services is provided to the HSS by the PDN GW via the 3GPP AAA server over S6b and SWx as described in clause 7.2.5 for untrusted WLAN and clauses 16.2.1a and 16.8.1 for trusted WLAN and by the MME over S6a as described in TS 23.401. The HSS stores it as a specific data "PDN GW currently in use for emergency services", not associated with any APN. If the HSS detects that the UE is attached to the other access (3GPP or WLAN), the HSS updates the corresponding serving node (MME, ePDG or TWAN) with the "PDN GW currently in use for emergency services".
    • During the handover procedure from WLAN to 3GPP EPC, the "PDN GW currently in use for emergency services" information is provided by the HSS as part of the subscription information sent to the MME over S6a as described in clause 8.6.1.1 for untrusted WLAN and clause 16.11 for trusted WLAN.
    • During the handover procedure from 3GPP EPC to WLAN, the "PDN GW currently in use for emergency services" information is provided by the HSS as part of the subscription information sent to the ePDG / TWAG via the 3GPP AAA server over SWx and SWm / STa as described in clause 8.6.2.1 for untrusted WLAN and clauses 16.10.1.1 and 16.10.2.1 for trusted WLAN.
  • Alternatively for non-roaming authenticated UEs, based on operator policy (e.g. the network supports handovers to/from HRPD), the ePDG/TWAG may be configured with a static PDN GW identity as part of the Emergency Configuration Data. In such case, when the UE initiates an Emergency Attach with handover indication to the MME, the MME uses the static PDN GW identity locally configured instead of querying the HSS as described in TS 23.401.
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4.5.8  APN congestion Control Function for eHRPD |R12|Word‑p. 47
The PDN GW may provide mechanisms for avoiding and handling overload situations for eHRPD over S2a. These include the rejection of PDN connection requests from UEs.
When performing overload control the PDN GW shall operate as specified in clause 4.3.7.5 of TS 23.401.
When receiving the rejection from the PDN GW, the HSGW shall operate as specified in clause 4.13 of the 3GPP2 X.S0057 [51].
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4.5.9  GTP-C signalling based Load and Overload Control for trusted and untrusted WLAN |R12|

4.5.9.1  GTP-C load control

GTP-C Load Control feature is an optional feature which allows a GTP control plane node to send its Load Control Information to a peer GTP control plane node which the receiving GTP control plane peer node uses to augment existing PDN GW selection procedure.
GTP-C Load Control feature allows the PDN GW to send its Load Control Information to the TWAN/ePDG (for enhanced load balancing across PDN GWs during Attach or new PDN connectivity request scenarios).
This feature is supported over S2a and S2b interfaces via GTPv2 control plane protocol.
The same concepts as described in TS 23.401, clause 4.3.7.1a.1 for PGW Load Control apply with the TWAN/ePDG playing a similar role as the MME/SGSN.
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4.5.9.2  GTP-C overload control

GTP-C Overload Control feature is an optional feature. Nodes using GTP control plane signalling may support communication of Overload Control Information in order to mitigate overload situation for the overloaded node through actions taken by the peer node(s).
This feature is supported over S2a and S2b interfaces via GTPv2 control plane protocol.
The Overload Control Information may convey information regarding the node itself and/or regarding specific APN(s) status.
GTP-C Overload Control feature allows the PDN GW to send its Overload Control Information to the TWAN/ePDG.
GTP-C Overload Control feature allows the TWAN/ePDG to send its Overload Control Information to the PDN GW.
An ePDG may apply certain restrictions towards PDN GW that have indicated overload, e.g.:
  • reject PDN connection requests from the UE (e.g. Initial Attach, UE-initiated Connectivity to Additional PDN, Attach and PDN Connectivity Request at handover to Untrusted WLAN) and locally set a back-off timer. As long as the back-off timer is running, the ePDG shall reject the subsequent PDN connection requests from the UE;
  • reduce/throttle messages towards the PDN GWs indicating overload status;
  • apply other implementation specific mechanisms, which are outside the scope of 3GPP specifications.
A TWAN may during access authentication in Transparent Single-Connection Mode, Single-Connection Mode and during WLCP procedures in Multi-Connection Mode apply certain restrictions towards PDN GW that have indicated overload, e.g.:
  • reject PDN connection requests from the UE (e.g. Initial Attach with PDN Connectivity, UE Initiated PDN connectivity request, Attach and PDN Connectivity Request at handover to Trusted WLAN) as follows:
    • for Transparent Single-Connection Mode, locally set a back-off timer and prevent the UE from accessing the SSID. For any further request for the same UE and the same SSID, as long as the back-off timer is running, the TWAN prevents the UE from accessing the SSID.
      • The TWAN reselects another PDN GW to retry PDN connection establishment, if more than one PDN GW supports the target APN,
      • If possible, the TWAN rejects UEs in Single-Connection Mode and Multi-Connection Mode before rejecting UEs in Transparent Single-Connection Mode when the PDN GW(s) have indicated overload.
    • for Single-Connection Mode, reject EPC access requests from the UE with a Session Management back-off timer that instructs the UE to not request new PDN connectivity to the same APN for the indicated time.
    • for Multi-Connection Mode, reject WLCP PDN connection requests for the same APN from the UE with a Session Management back-off timer that instructs the UE to not request new PDN connectivity to the same APN for the indicated time.
  • reduce/throttle messages towards the PDN GWs indicating overload status;
  • apply other implementation specific mechanisms, which are outside the scope of 3GPP specifications.
The same concepts as described in TS 23.401, clause 4.3.7.1a.2 for PGW Overload Control apply with the TWAN/ePDG playing a similar role as the MME/SGSN.
If the UE has received a Session Management back-off timer over non-3GPP access from the TWAG, the UE shall not send any Session Management requests related to that APN to the network via WLAN as long as the Session Management back-off timer is running.
A Session Management back-off timer received over non-3GPP access has no impact on the UE behaviour in 3GPP access. A Session Management back-off time received over 3GPP access has no impact on the UE behaviour in non-3GPP access.
A PDN GW may apply certain restrictions towards TWAN/ePDG that have indicated overload, e.g. apply similar policies as those described in TS 23.401, clause 4.3.7.1a.2 in the case of an MME or an SGW has indicated overload.
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