Emergency Call Setup and Location Tracking

This section describes the IMS emergency call setup and location tracking procedures.

We first elaborate on the call setup procedure and the Location Polling scheme proposed in 3GPP [13, 14]. Then we propose the Active Location Reporting scheme that improves the performance of location tracking.

Establish Emergency Call Report Initial Location

UE

CS-6. MAP_PROVIDE_SUBSCRIBER_LOCATION

CS-8. Positioning CS-10. MAP_PROVIDE_SUBSCRIBER_LOCATION_ack CS-9. Location Report

CS-7. Location Reporting Control CS-3. Emergency Location Request

CS-11. Emergency Location Response

CS-15. Location Information

PSAP GMLC E-CSCF SGSN RAN

CS-2. SIP INVITE GGSN

CS-12. SIP INVITE CS-13. 200 OK

CS-13 200 OK CS-14. SIP ACK CS-14. SIP ACK

HSS

CS-1. PDP Context Activation

CS-4. MAP_SEND_ROUTING_INFO_FOR_LCS CS-5. MAP_SEND_ROUTING_INFO_FOR_LCS_ack

Figure 2.2: IMS Emergency Call Setup

2.2.1 Emergency Call Setup

Before the IMS emergency call is set up, the UE has attached to the network through the RAN. Figure 2.2 illustrates IMS emergency call setup message flow defined in 3GPP [13]

with the following steps:

Step CS-1. The UE performs PDP context activation that establishes the IP connectiv-ity to the IMS through the GPRS network [4].

Step CS-2. The UE sends the SIP INVITE message to the E-CSCF. This message in-cludes the supported positioning methods of the UE (i.e., Cell-ID-based in our example).

Step CS-3. The E-CSCF uses the received information to determine a GMLC and sends the Emergency Location Request message to the GMLC, which includes all UE-related information received at Step CS-2.

Steps CS-4 and 5. The GMLC exchanges the MAP SEND ROUTING INFO FOR LCS and MAP SEND ROUTING INFO FOR LCS ack message pair with the HSS to obtain the SGSN address for the UE.

Step CS-6. The GMLC sends the MAP PROVIDE SUBSCRIBER LOCATION message to the SGSN to request the UE’s location. In this message, the locationEstimateType parameter is set to “initialLocation”.

Step CS-7. Upon receipt of the MAP PROVIDE SUBSCRIBER LOCATION message, the SGSN sends a Location Reporting Control message to the RAN to trigger the posi-tioning procedure. In this message, the Request Type is set to “report directly”.

Step CS-8. The RAN and the UE exercise the Cell-ID-based positioning procedure to obtain the location estimate information of the UE (i.e., the SA identity of the UE).

Step CS-9. The RAN returns the Location Report message with the SA identity to the SGSN.

Step CS-10. The SGSN returns the SA identity to the GMLC through the MAP PROVIDE-SUBSCRIBER LOCATION ack message.

Step CS-11. The GMLC selects a suitable PSAP according to the SA of the UE and replies the Emergency Location Response message with the selected PSAP address to the E-CSCF.

Steps CS-12-14. The E-CSCF forwards the SIP INVITE to the PSAP. The PSAP and the UE exchange the 200 OK and the SIP ACK messages through the E-CSCF. After the PSAP has received the SIP ACK message, the emergency call is established.

Step CS-15. The GMLC sends the location information obtained at Step CS-10 to the PSAP after the call has been established.

2.2.2 Location Polling

A UE may move during an emergency call, and the PSAP may need to monitor the UE’s location in real time. In 3GPP TS 23.271 [14], the UE’s location is monitored through a polling procedure where the PSAP periodically queries the UE’s location. In each polling query, the following steps are executed (see Figure 2.3).

Step LP-1. The PSAP sends the Location Information Request message to the GMLC.

Steps LP-2-8. These steps are similar to Steps CS-4-10 in Figure 2.2 except that the parameter locationEstimateType in the MAP PROVIDE SUBSCRIBER LOCATION message is set to “currentLocation”.

Step LP-9. The GMLC returns the SA identity of the UE to the PSAP.

Steps LP-10-12. When the emergency call is terminated, the E-CSCF exchanges the Emergency Location Release and Response message pair with the GMLC to terminate location tracking.

Report Current Location

UE

LP-1. Location Information Request

LP-9. Location Information

LP-11. Emergency Location Release LP-12. Emergency Location Response

PSAP GMLC E-CSCF GGSN SGSN RAN

LP-4. MAP_PROVIDE_SUBSCRIBER_LOCATION

LP-6. Positioning LP-7. Location Report

LP-5. Location Reporting Control

LP-8. MAP_PROVIDE_SUBSCRIBER_LOCATION_ack HSS

LP-10. Emergency Call Release Call Setup

LP-2. MAP_SEND_ROUTING_INFO_FOR_LCS LP-3. MAP_SEND_ROUTING_INFO_FOR_LCS_ack

Figure 2.3: Location Polling

2.2.3 Active Location Reporting

In the Location Polling scheme, if the UE does not change its location between two queries, the second query is wasted. On the other hand, if the UE has moved to several new loca-tions between two location queries, then the PSAP may lose track of the UE. To resolve this issue, we propose the Active Location Reporting scheme that reports the UE’s location upon change of its SA. Our scheme introduces two new locationEstimateTypes “initiate-ActiveReport” (to trigger Active Location Reporting) and “terminate“initiate-ActiveReport” (to terminate Active Location Reporting) in the MAP PROVIDE SUBSCRIBER LOCATION message, and one new LCS event type “ActiveReporting” (to indicate the Active Lo-cation Reporting event). At emergency call setup, the loLo-cationEstimateType is set to

“initiateActiveReport” at Step CS-6, and the Request Type is set to “change of service area” at Step CS-7. Since the IP connectivity exists during the IMS emergency call, the UE is in the Cell-Connected state and is tracked by the RAN at the cell level [4]. There-fore, the RAN can detect the movement of the UE at the cell level and report the new SA identity to the SGSN. In this approach, the GMLC maintains a UE-PSAP mapping table, and the (UE, PSAP) pair is stored in the GMLC at Step CS-11. The GMLC does not need to query the HSS to obtain the SGSN address of the UE (therefore, Steps LP-2 and LP-3 are eliminated). The Active Location Reporting scheme is illustrated in Figure 2.4 with the following steps:

Step ALR-1. When the UE moves to a new SA, the RAN detects this movement at the cell tracking mode [4] and then triggers the positioning procedure.

Step ALR-2. After the positioning procedure is executed, the UE’s SA identity is ob-tained.

Call Setup

Report Current Location

UE

ALR-5. Location Information

PSAP GMLC E-CSCF GGSN SGSN RAN

ALR-2. Positioning

ALR-4. MAP_SUBSCRIBER_LOCATION_REPORT

ALR-3. Location Report

ALR-1. Change of Service Area

ALR-7. Emergency Location Release ALR-8. Emergency Location Response

ALR-6. Emergency Call Release

Figure 2.4: Active Location Reporting

Step ALR-3. The RAN sends the Location Report message with the SA identity of the UE to the SGSN.

Step ALR-4. The SGSN sends the MAP SUBSCRIBER LOCATION REPORT message with the SA identity to the GMLC.

Step ALR-5. From the UE-PSAP mapping table, the GMLC retrieves the PSAP ad-dress of the UE stored at Step CS-11 and then sends the updated location informa-tion to the PSAP.

When the emergency call is terminated, the following steps are executed.

Step ALR-6. When the IMS call is released, the UE moves from the Cell-Connected

Step ALR-7. The E-CSCF sends the Emergency Location Release message to the GMLC to terminate location tracking.

Step ALR-8. The GMLC returns the Emergency Location Response message to the E-CSCF and then deletes the (UE, PSAP) mapping from the UE-PSAP table.

The major difference between Active Location Reporting and Location Polling is at Steps ALR-1 and ALR-2. Active Location Reporting is triggered when the RAN detects the movement of the UE (through the standard tracking procedure at the Cell-Connected mode). Note that the HSS query (see Steps LP-2 and LP-3 in Figure 2.3) is not required for the Active Location Reporting scheme because of the active reporting of the RAN.

The GMLC needs to maintain the (UE, PSAP) mapping so that when a UE changes the SA, the GMLC can report the location update to the corresponding PSAP.