5.10 IMS Emergency Session Support

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

5.10.1 Introduction

Emergency bearer services are provided to support IMS emergency sessions. Emergency bearer services are functionalities provided by the serving network when the network is configured to support emergency bearer services. Emergency bearer services are provided to normal attached UEs and to UEs that are in limited service state. Receiving emergency services in limited service state does not require a subscription. To provide emergency bearer services as a local service, node configuration parameters may be used to set service values that would otherwise be obtained from subscription data.

When a PLMN supports IMS and emergency bearer services in UTRAN, all SGSNs in that PLMN shall have the same capability to support emergency bearer services.

NOTE: IMS emergency session may be provided over GPRS without emergency procedures as specified from this release of specifications. In such a scenario, GPRS is unaware of the emergency session and thus provides no specific support explicitly required for such access.

For emergency services other than eCall, a UE that is not in in limited service state, as specified in TS 23.122 [7b] determines from a NAS indicator whether that additional emergency numbers/types received via WLAN from trusted sources may be used for detecting emergency calls.

5.10.2 PS Domain Functions for IMS Emergency Session Support General

IMS emergency sessions over emergency access via packet core are supported for UTRAN access as well as inter-RAT handover between UTRAN and E-UTRAN. Support for Packet Core (GPRS and EPS) emergency bearer services over GERAN networks is not included in this Release and thus PS handover to GERAN access should not be performed when IMS emergency sessions over EPS or GPRS using emergency bearer services are active.

The MS shall signal a cause specific for emergency as defined in TS 25.331 [52] when it requests an RRC connection in relation to an emergency session. Specific situations that require setting the RRC establishment cause to emergency are described in TS 24.008 [13]. Reachability Management for Emergency Attached MS in PMM-IDLE state

An emergency attached MS when its periodic RA update timer expires shall not initiate a periodic RAU procedure but enter PMM- DETACHED state. The SGSN assigns the periodic RAU timer value to emergency attached MS. This timer keeps the MS emergency attached after change to PMM-IDLE state to allow for a subsequent emergency service without a need to emergency attach again. For emergency attached MS the SGSN runs a mobile reachable timer with a similar value to the MS’s periodic RAU timer. Any time after expiry of this timer the SGSN may change the PMM state of an emergency attached MS to PMM- DETACHED. Mobility and Access Restrictions for Emergency Services

When Emergency Services are supported and local regulation requires Emergency Sessions to be provided regardless of mobility or access restrictions, regional subscription restrictions or access restrictions (see TS 23.221 [80] and TS 23.008 [79]) e.g. CSG restrictions, should not be applied to MSs receiving emergency services. When the RABs for emergency bearers are established, the ARP value for emergency bearer services indicates the usage for emergency services to the UTRAN.

During handover, the source UTRAN and source SGSN ignore any MS related restrictions during handover evaluation when there are active emergency bearers. UTRAN shall not initiate handover to GERAN PS domain. During handover to a CSG cell, if the UE is not a CSG member of the target CSG cell and has emergency bearer services, the target RNC only accepts the emergency bearers and the target SGSN releases the non-emergency bearers that were not accepted by the target RNC according to clause Such UEs behave as emergency attached.

During Routing Area Update procedures, including a RAU as part of a handover, the target SGSN ignores any mobility or access restrictions for MS with emergency bearer services where required by local regulation. Any non emergency bearer services are deactivated, according to clause, by the target SGSN when not allowed by the subscription for the target location. Such MSs behave as emergency attached. To allow the emergency attached MS to get access to normal services after the emergency session has ended and when it has moved to a new area that is not stored by the MS as a forbidden area, the MS may explicitly detach and reattach to normal services without waiting for the emergency PDN connection deactivation by the PDN GW or GGSN.

This functionality applies to all mobility procedures.

5.10.3 Attach handling

An MS in limited service state, as specified in TS 23.122 [7b], initiates the GPRS Attach procedure by indicating that the attach procedure is for emergency services. The Attach Request message for emergency attach purpose is indicated as Attach Type "GPRS Emergency Attach". Also MSs that had attached for normal service and do not have emergency bearers established and are camped on a cell, where the MS is in limited service state (e.g. because of change to a restricted Tracking Area or not allowed CSG), shall initiate this Attach procedure, indicating that the attach is to receive emergency services. The network which support emergency services for an MS in limited service state provides emergency attach service to this MS, according to regulatory requirements. The UEs in limited service state determine that the cell supports emergency bearer services for IMS based emergency services over UTRAN from a broadcast indicator in AS.

TS 23.401 [89] describes in clause "IMS Emergency Session Support" how the network provides emergency services according to different local regulations. Roaming or mobility restrictions are not applied when the network supports emergency services. Emergency Attach is supported only in UTRAN access.

An MS that camps normally on a cell, i.e. without any conditions that result in limited service state, initiates the normal GPRS attach procedure if not already attached. A normal attached IMS enabled MS is assumed to have a non-emergency PDN connection. A normal attached MS initiates the GPRS PDP Context Activation procedure by indicating emergency service to receive emergency bearer services. The UEs that camp normally on a cell are informed that the PLMN supports emergency bearer services for IMS based emergency services over UTRAN from the Emergency Service Support indicator in the Attach and RAU procedures. UEs that camp normally on a cell may also use the emergency attach procedure under conditions specified in TS 24.008 [13], e.g. when the MM back-off timer is running.

NOTE 1: Failure of the normal initial attach may occur e.g. when the network rejects the request with a back-off time.

NOTE 2: The establishment of the emergency bearer services may fail e.g. when the MS needs to perform a RAU procedure prior to the GPRS PDP Context Activation procedure, i.e. the MS moved into a non-registered Routeing Area with the MM back-off timer running in the MS. Another example is when the MS needs to perform a Service Request procedure prior to the GPRS PDP Context Activation procedure, i.e. the MS is in PMM-IDLE state when the emergency service is triggered.

The details for Emergency Attach are included in the Combined GPRS/IMSI Attach procedure, see clause 6.5.3.

5.10.4 PDP Context Activation for emergency bearer services

The procedure is executed after the MS has successfully performed a GPRS attach and is valid for both normal and limited service state. The MS shall always activate a new PDP Context when emergency bearer services are invoked. When the MS detects a request for emergency service and it is required to establish a PDP context within the same network as the SGSN, the MS performs a PDP context activation with an indication of emergency usage so that emergency bearer services handling can be provided by the local network per configuration without subscription limitations. The request for emergency bearer services is indicated as Request Type "emergency" in the PDP Context Activation message.

MS request for emergency PDP context activation is added into the PDP Context Activation Procedure in clause

The enhancements to support MS request for emergency PDP context activation are:

a) The PDP Context Activation Request includes an indication of emergency usage.

b) Configuration parameters in the network are used to override subscription limitations such as bearer QoS.

The PDN GW/GGSN selection function in the SGSN shall derive a PDN GW/GGSN identity in the visited PLMN by using the Emergency APN. The PDN GW/GGSN address is derived from the PDN GW/GGSN identity by using the Domain Name Service function. If the Domain Name Service function provides a list of PDN GW/GGSN addresses, one PDN GW/GGSN address is selected from this list. If the selected PDN GW/GGSN cannot be used, e.g. due to an error, then another PDN GW/GGSN is selected from the list. The specific interaction between the SGSN and the Domain Name Service function may include functionality to allow for the retrieval or provision of additional information regarding the PDN GW capabilities (e.g. whether the PDN GW supports PMIP based or GTP-based S5/S8, or both).

If there is already an emergency bearer activated, the SGSN shall reject any PDP context activation request for normal services if the mobility and access restrictions do not allow the MS to access normal services.

If the PDN GW/GGSN identity is statically configured in the SGSN, then it may be a FQDN or an IP Address[es].

5.10.5 Handling of PDN Connections for Emergency Bearer Services

The PDP contexts (described in clause 5.10.4) of a PDN Connection associated with the emergency APN shall be dedicated for IMS emergency sessions and shall not allow any other type of traffic. The Emergency PDP contexts shall not be changed to normal PDP contexts and vice versa.

The PDN GW/GGSN shall block any traffic that is not from or to addresses of network entities (e.g. P-CSCF) providing IMS emergency service. Dynamic PCC shall be deployed in order to support IMS emergency sessions, the procedures are as described in TS 23.203 [88]. If there is already an emergency PDN connection, the MS shall not request another emergency PDN Connection. The SGSN shall reject any additional emergency PDN Connection requests. The MS shall not initiate any Secondary PDP Context Activation Procedure or any PDP Context Modification Procedure for the emergency PDN connection. The PDN GW/GGSN shall reject any MS initiated Secondary PDP Context Activation Procedure or any PDP Context Modification Procedure that is for the emergency PDN Connection.

The ARP reserved for emergency bearer service shall only be assigned to bearers associated with an emergency PDN Connection.

For emergency attached MS, SGSN initiates an implicit detach based on an inactivity timeout specific to emergency.

NOTE: In case of IMS emergency services prior to this release of specification, dynamic PCC support was not required in the specifications. In such case if PCC is not used the PDN GW/GGSN provides static policy.