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Content for  TS 23.167  Word version:  16.3.0

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7  Procedures related to establishment of IMS emergency session

7.1  High Level Procedures for IMS Emergency Services

7.1.1  UE Detectable Emergency Session

The following flow contains a high level description of the emergency service procedures performed when the UE can detect the emergency session is being requested.
Reproduction of 3GPP TS 23.167, Figure 7.1: Terminal Detected Emergency Calls
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The following steps are performed:
Step 1.
The UE detects the request for the establishment of an emergency session. Step 2 to 6 may be skipped based on the conditions specified in clause 6.1.
Step 2.
In the case that the UE has insufficient resources or capabilities to establish an emergency call due to other ongoing sessions then the UE should terminate the ongoing communication and release reserved bearer resources.
Step 3.
In the case that bearer registration is required and has not been performed, the UE shall perform bearer registration to the IP-CAN. If the UE is already bearer registered, then the bearer registration procedures are not required to be performed.
Step 4.
In the case that bearer resources for the transport of the IMS related signalling are required to be reserved in the IP-CAN, the UE shall reserve the resources in the IP-CAN. The IP CAN may support a UE indication that this request is for an emergency service.
If the IP-CAN does not provide an IP address to the UE in step 3, then the IP-CAN shall allocate an IP address to the UE during the bearer resource request procedures.
Step 5.
UE performs a P-CSCF discovery procedure, where the UE discovers a P-CSCF in the local network suitable for use in emergency sessions.
Step 6.
If the UE has sufficient credentials to authenticate with the IMS network, it shall initiate an IMS emergency registration by providing the IP address obtained at step 3 or step 4 to the P-CSCF selected at step \step 5. The IP address used for signalling purposes is allocated in association with step 3 or step 4. The IMS registration request shall include an emergency indication. The implicit registration set of the SIP URI used in the emergency registration request used by the UE when the UE performs a non-emergency registration shall contain an associated TEL URI that is used to call back the UE.
The S-CSCF may set the proposed registration expiration according to the local regulation or operator policy of the serving system. The subsequent registration flows are like any other registration with the considerations defined in clause 6.2.4 and clause 6.2.9.
If the UE does not have sufficient credentials to authenticate with the IMS network, it shall not initiate an IMS emergency registration request, but instead immediately establish an emergency session towards the P-CSCF as described in clause 7.4 and skip step 7.
Step 7.
The UE shall initiate the IMS emergency session establishment using the IMS session establishment procedures containing an emergency service indication, or the eCall type of emergency service indication in the case of eCall and any registered Public User Identifier. If the UE has performed emergency registration, the UE shall use an emergency registered Public User Identifier.
Whether the procedures are activated individually by the UE or some of them are performed automatically depends on the implementation of the terminal and on the UE's configuration. For instance, the multimedia application in the UE could start the application level registration and steps 2-4 would have to be executed in response to support the operation initiated by the application. Interaction with the UE may happen during these steps.
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7.1.2  Non UE detectable Emergency SessionWord‑p. 24
As the UE could not detect the emergency session, the session establishment request will be sent to a P-CSCF in the visited PLMN or a P-CSCF in the home PLMN as per a normal session establishment procedure. The former is only applicable to a roaming situation whereas the latter can apply to both a roaming and non-roaming situation. Prior to sending the session establishment request the UE must be registered in the IMS as per the normal registration procedure.
In the case that the P-CSCF detects that this is a request to establish an emergency session, based upon operator policy (e.g., checking access type):
  • the P-CSCF may reject the session initiation request with an indication that this is for an emergency session. When the UE receives the session rejection then the UE shall:
    • select a domain for the emergency session;
    • if the PS domain is selected, follow the procedure in clause 7.1.1;
    • for systems based on TS 24.008, if the CS domain is selected and a dialled number is available, attempt a normal call (i.e. TS 11, see TS 22.003) using the dialled number if:
      • an emergency service information is included by the P-CSCF with either a country specific emergency subservice type (see TS 24.229) or a emergency subservice type (see TS 24.229) that does not map into an emergency service category for the CS domain; or
      • no emergency service information is included by the P-CSCF;
    • for systems based on TS 24.008, if the CS domain is selected, attempt an emergency call (i.e. TS 12, see TS 22.003) if:
      • a dialled number is not available; or
      • an emergency service information is included by the P-CSCF with no emergency subservice type or a emergency subservice type (see TS 24.229) that maps into an emergency service category for the CS domain;
    • if the CS domain is selected and for CS systems that do not support emergency call handling procedures (e.g. as described by TS12 in TS 22.003 for systems based on TS 24.008 or in systems providing access to IM CN subsystem using a cdma2000 network, for example) a normal call is made;
  • If prior attempting the call in the CS domain the UE receives a list of local emergency numbers, the UE may verify if and recognizes the dialled number is an emergency number and if verified, the UE shall attempt an emergency call set up indicating the appropriate emergency call type.
  • Alternatively, the P-CSCF in the visited PLMN or the P-CSCF in the home PLMN for a non-roaming UE may allow the session initiation request to continue by inserting the explicit emergency indication in the session request. The P-CSCF in the visited PLMN forwards that request to an Emergency CSCF in the same network. The P-CSCF in the home PLMN for a non-roaming UE may forward that request to a Serving CSCF or to an Emergency CSCF in the same network, based on local regulation or operator policy. The E-CSCF shall inform the UE that the session has been marked as an emergency session so the UE can treat the session as an emergency session establishment.
If the AS detects that this is a request to establish an emergency session, the AS shall handle the request as specified in clause 6.2.8 and forward the request marked as an emergency services request to the S-CSCF.
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7.1.3  Emergency Session Establishment using LRF/RDFWord‑p. 25
Figure 7.2 illustrates a high level call flow for the IMS emergency session establishment procedure using LRF/RDF to retrieve location and routing information.
Reproduction of 3GPP TS 23.167, Figure 7.2: Emergency Session Establishment procedure with using LRF/RDF
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Step 1.
UE initiates an emergency session request by sending a SIP INVITE message with including emergency URI.
Step 2.
If required, the IMS network may access the LRF to retrieve the UE's location. For WLAN access, and for non-roaming UEs, if the LRF is configured then it may interact with HSS to provide an NPLI. In some regions, for example in the North American region, ATIS-0700028 [45], if the BSSID of the serving WLAN is available, the LRF may query a database subject to national regulations and operator policies for the dispatchable location associated with the BSSID of the WLAN Access Point.
Step 3.
If required, LRF invokes the RDF to determine the proper PSAP destination. LRF returns the necessary location/routing information (e.g., ESQK for North America or location number for EU) to the IMS network.
Step 4.
The IMS network uses the routing information returned by the LRF to route the emergency session request towards the appropriate PSAP.
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7.2  IMS Registration for Emergency SessionWord‑p. 26
The IMS emergency registration procedure shall follow the procedures as described in clause 5.2.2.3 of TS 23.228 with the following modifications:
  • The UE shall initiate an IMS emergency registration when all of the following conditions are met:
    • either the UE is not already IMS registered or the UE is IMS registered but is roaming outside its home network;
    • the UE has sufficient credentials to authenticate with the IMS network;
    • the UE is able to detect emergency session.
The UE shall also initiate an IMS emergency registration when it receives an "IMS emergency registration required" response as a result of the emergency session request:
  • If the UE initiates an IMS emergency registration, it shall first initiate an emergency access to the IP-CAN if emergency access has been defined for the particular type of IP-CAN. This is to ensure that the session attempt is handled in the VPLMN when the UE is roaming and provides appropriate priority treatment and access to appropriate network elements (e.g. to a particular PDG or UPF and P-CSCF in the VPLMN).
  • If the UE had already performed an emergency access when it receives an "IMS emergency registration required" response as a result of an emergency registration or emergency session request, it shall perform an emergency access followed by an emergency registration using a different VPLMN if available to prevent looping.
  • The UE shall use an indication in the emergency registration request. This indication may be used to inform the home network that roaming restrictions may not be applied.
  • The user's home network should ignore roaming restrictions for emergency registration requests.
P-CSCF handles the registration requests with an emergency indication like any other registration request.
The S-CSCF in the home network may modify the received registration expiration value from the request according to local regulation or operator policy in the serving system. The subsequent registration flows are like any other registration with the considerations defined in clause 6.2.4 and clause 6.2.9.
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7.3  Emergency Session Establishment in the Serving IMS network

If the UE is able to detect that the user is requesting an emergency session then it shall include an emergency service indication in the emergency session establishment request. In the case of NG-eCall, the UE shall include the eCall type of emergency service (automatic or manual) in the emergency session establishment request.
The UE shall follow the requirements in TS 22.101 for domain priority and selection when UE attempts to make an emergency call.
For an attempt in the IM CN Subsystem of the PS domain, the attempt should be in the serving (visited if roaming) IM CN Subsystem of the PS domain.
If the initial attempt is in the CS domain and it fails, the serving (visited if roaming) IM CN Subsystem of the PS domain shall be attempted if the UE is capable and if not disallowed by applicable domain selection rules. If the initial attempt is in the IM CN Subsystem of the PS domain and it fails, the UE shall make the attempt in the CS domain (if the UE is capable and if for an appropriate service e.g., voice).
If the UE is aware that it does not have sufficient credentials to authenticate with the IMS network, it shall not initiate an IMS registration but immediately establish an emergency session towards the P-CSCF, see clause 7.4.
Upon receiving an initial request for an emergency session, the P-CSCF shall follow the rules and procedures described in TS 23.228 with the following additions and clarifications:
  • When a UE using public network traffic initiates an emergency session, the P-CSCF is the IMS network entity, which detects an emergency session.
  • For the case that the initial request carries an indication that the request is for emergency services, and the UE is not registered in the IMS network, see clause 7.4 for details.
  • For the case that UE is IMS registered and the initial request does not carry an indication that the request is for emergency services, and the P-CSCF is able to detect that the request is for emergency services, the P-CSCF shall perform the "Non UE detectable Emergency Session" described in clause 7.1.2 above.
  • For the case that the initial request carries an indication that the request is for emergency services, and the UE is registered in the IMS network, but not performed emergency registration:
    1. the P-CSCF shall reject the request indicating that IMS emergency registration required, if the UE is roaming;
    2. the home P-CSCF may reject the request indicating that IMS emergency registration required, based on operator policy.
  • On receipt of a session establishment request, which is recognized to be for an emergency service, the P-CSCF shall check whether the UE provided a TEL URI as its identity in the request. If a TEL URI is present in the request, the P-CSCF shall check the validity of this TEL URI. If no TEL URI is present in the request and the P-CSCF is aware about the TEL URI associated with the emergency registration, it shall provide the TEL URI to the E-CSCF in the session establishment request.
  • The P-CSCF may query the IP-CAN for the location identifier.
  • P-CSCF shall prioritize emergency sessions over other non-emergency sessions.
  • Emergency IP flows need to be identified by P-CSCF in the Rx interface signalling to allow the PCRF to prioritize emergency service data flows over non-emergency service data flows within IP CAN. The detailed procedures are specified in TS 23.203.
Handling of emergency sessions detected by an AS is specified in clause 6.2.8.
For the case where the emergency session is provided via the interconnect from a private network (as defined in ETSI TS 182 025 [38]), the following procedures apply:
  • For private network traffic where operator policy allows so, do not apply emergency session detection and forward the session according to normal procedures.
  • Otherwise emergency sessions within the IMS are routed to the PSAP via the E-CSCF.
Upon receiving an initial request for an emergency session, the E-CSCF shall perform the following:
  • if location information is not included in the emergency service request or if additional location information is required, the E-CSCF, if required, retrieves the UE's location information as described in clause 7.6 Retrieving Location information for Emergency Session.
  • If location information is included by the UE, the E-CSCF, if required requests the LRF to validate the location information.
  • May determine or may request the LRF to determine the appropriate routing information which could be based on the type of emergency service requested, the UE's location and any indication of an eCall.
  • determine the default PSAP destination if routing based on UE's location is required but the location is unknown.
  • If the PSAP/emergency centre contains a point of presence within the IMS connectivity network, the E-CSCF shall forward the emergency session initiation request directly to the PSAP/emergency centre, including any additional subscriber related identifier(s) received from P-CSCF.
  • If the PSAP/emergency centre has its point of presence in the PSTN/ISDN network or the CS domain, the E-CSCF uses the TEL URI obtained from the LRF and forwards the request to an appropriate BGCF/MGCF for routing in the GSTN. This number shall have the same format as used for CS emergency calls. The MGCF may insert any available location information in the PSTN/CS signalling.
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7.4  IMS Emergency Session Establishment without RegistrationWord‑p. 28
When the UE initiates an emergency session establishment without prior IMS registration, it shall include both the "anonymous user" and "emergency service" indications in the emergency session establishment request to the P-CSCF.
Based on local regulation, the P-CSCF may reject "anonymous user" emergency session establishment with appropriate error code. UE shall not reattempt the "anonymous user" emergency session again via the same network.
When P-CSCF accepts the "anonymous user" emergency session establishment, it forwards this request to an appropriate E-CSCF although no security association between UE and P-CSCF is established. Based on local regulation, P-CSCF may retrieve additional subscriber related identifier(s) from IP-CAN and forward those identifiers to E-CSCF.
The E-CSCF shall follow the same rules and procedure as defined for the Emergency Session Establishment in the Serving IMS network in clause 7.3 to route the anonymous emergency session.
Where required by local regulation, the E-CSCF shall derive a non-dialable callback number to include as the UE's identity in the session establishment request and the location/routeing request (e.g. see Annex C of J-STD-036 [23]).
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