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Content for  TR 23.771  Word version:  13.0.0

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1  ScopeWord‑p. 7

The present document studies the mechanisms needed to support IMS Emergency sessions over WLAN (both in the Trusted -S2a - and in the untrusted -S2b- cases)
The study is split up into 2 phases corresponding to 2 SID (FS_SEW1 and FS_SEW2).
Topics to be studied in phase 1 (FS_SEW1) are:
  • Providing access to EPC over WLAN in order to support an emergency session only in case of "Valid" UEs as in the case 1 in clause 4.3.12.1 of TS 23.401: the support of emergency session is only studied for UEs which (a) have valid credentials to access EPC over WLAN and (b) are authorized to connect to EPC over WLAN in the location where they initiate an emergency session.
  • Providing Location information (in particular for untrusted WLAN). Only existing location determination mechanisms are considered i.e. location determination mechanisms that should not require standard effort. No location accuracy requirements are considered in phase 1.
Phase 1(FS_SEW1) will not consider solutions that imply specification activities in other SDO(s).
Topics to be studied in phase 2 (FS_SEW2) correspond to all the features not covered by phase 1, e.g.:
  • Support of other cases for the access to EPC over WLAN (e.g. the UE is authenticated but is in a location where it is restricted from regular service, or is unauthenticated or has no USIM).
  • Roaming cases (user not in his / her home country).
  • Support of session continuity of emergency sessions at inter-access mobility.
  • TWAN access.
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2  References

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]
TR 21.905: "Vocabulary for 3GPP Specifications".
[2]
TS 22.101: "Service aspects; Service principles".
[3]
TS 23.401: "General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access".
[4]
TS 23.167: " IP Multimedia Subsystem (IMS) emergency sessions".
[5]
IEEE Std 802.11-2012: "IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications".
[6]
WiFi Alliance Technical Committee, Hotspot 2.0 Technical Task Group: "Hotspot 2.0 (Release 2) Technical Specification", ver.5.00.
[7]
TS 23.402: "Architecture enhancements for non-3GPP accesses".
[8]
TS 23.237: "IP Multimedia Subsystem (IMS) Service Continuity; Stage 2".
[9]
TS 23.271: "Functional stage 2 description of Location Services (LCS)".
[10]
RFC 7315:  "Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the 3GPP".
[11]
TS 23.003: "Numbering, addressing and identification".
[12]
TS 24.229: "IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3".
[13]
TS 29.172: "Location Services (LCS); Evolved Packet Core (EPC) LCS Protocol (ELP) between the Gateway Mobile Location Centre (GMLC) and the Mobile Management Entity (MME); SLg interface".
[14]
TS 33.402: "3GPP System Architecture Evolution (SAE); Security aspects of non-3GPP accesses e".
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3  Definitions and abbreviationsWord‑p. 8

3.1  Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905.
Emergency Session Transfer Number for DRVCC:
A number used by the UE to request the EATF AS to perform Session Transfer from PS to CS access. The E-STN-DR is dynamically assigned by EATF during IMS emergency session procedure with WLAN.
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3.2  Abbreviations

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply.
An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
E-STN-DR
Emergency Session Transfer Number for DRVCC

4  Architectural assumptions

4.1  General architectural assumptions

General assumptions:
  1. SEW solutions do not support pre Rel-13 devices and networks.
  2. no new requirements are defined for the UE for undetected emergency calls.
  3. In the case of S2b connectivity, the authentication mechanisms used by the UE to authenticate with WLAN for local IP access are out of scope of the SEW work. The SEW work does not consider the development of new authentication mechanisms for this case.
  4. During the establishment of connectivity for emergency services over WLAN, the UE provides an indication that the connectivity request is for emergency services. The mechanisms defined for LTE (i.e. specific request type and mechanisms in the MME) are adopted, with an indication in the connectivity request being added by the UE to the TWAG and the ePDG.
  5. A UE establishing connectivity for emergency services over WLAN does not provide an emergency APN.
  6. It is assumed that the same mechanisms adopted in the MME for PDN GW selection for emergency services can be extended to connectivity establishment for IMS emergency services over WLAN.
  7. In SEW work no new mechanisms are defined for P-CSCF Selection for IMS emergency services over WLAN.
  8. In S2a case, mechanisms have been defined (as part Netloc_TWAN) to provide location information from TWAG to IMS nodes upon bearer creation / deletion / modification. No more work is assumed for the S2a procedures and flows in order to support providing network asserted location information from TWAG to IMS nodes. This does not preclude addition of relevant new information in TWAN ID.
  9. An UE shall select an ePDG of the local country when connecting to an ePDG in order to issue an emergency session.
  10. A PLMN should be able to determine whether a UE is at a location where an emergency call can be supported by the PLMN (e.g. routed to the correct PSAP).
  11. For UE detected emergency calls, a UE already registered to IMS for regular IMS services (i.e. not emergency services) over WLAN shall:
    1. For S2b connectivity, establish a new PDN connection for emergency services according the procedures defined for IMS emergency services over S2b, and (re)register for emergency services to IMS.
    2. For S2a SCM connectivity, disconnect and reconnect to establish a PDN connection for emergency services according the procedures defined for IMS emergency services over S2a SCM, and (re)register for emergency services to IMS.
    3. For S2a MCM connectivity, establish a new PDN connection for emergency services according the procedures defined for IMS emergency services over S2a, and (re)register for emergency services to IMS.
  12. If the network detects a UE undetected emergency call, the IMS core shall reject the call subject to local regulations for a UE already registered to IMS for regular IMS services (i.e. not emergency services) over WLAN.
  13. (for phase 2) An ePDG that supports Emergency sessions supports all kinds of emergency sessions required by local regulations.
  14. (for phase 2) A TWAN that supports Emergency sessions supports all kinds of emergency sessions required by local regulations.
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4.2  Architectural assumptions for phase 1Word‑p. 9

  1. a UE will attempt emergency calls over WLAN only in case of unsuccessful emergency call attempts over the cellular technologies supported by the UE.
  2. WLAN selection for the establishment of IMS emergency calls over WLAN is based on mechanisms defined in Rel-12, including ANDSF WLAN_NS, operator configuration, WLAN configuration provided by RAN, and manual selection.
  3. For the S2a case, the EAP-AKA' authentication procedure specified in Rel-12 is used by the UE to authenticate with the TWAN to obtain access to the EPC for IMS emergency services; no new authentication mechanisms are considered in the SEW work for this case. This does not preclude addition of relevant new parameters in EAP-AKA'.
  4. For the S2b case, the EAP-AKA authentication procedure specified in Rel-12 is used by the UE to authenticate with the ePDG to obtain access to the EPC for IMS emergency services; no new authentication mechanisms are considered in the SEW work for this case. This does not preclude addition of relevant new parameters in EAP-AKA.
  5. UE is assumed to be in its Home country.
  6. Any local policy using location information at initial connection for voice services will only be implemented by one of the network operator's IMS nodes.
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