Roaming LTE Emergency Call MSISDN Retrieval via LRF
Solution Overview
Problem
In cases of Long-Term Evolution (LTE) emergency calls made from non-subscriber user devices roaming on a wireless carrier network, the network may be unable to provide the Public Safety Answering Point (PSAP) with the Mobile Station International Subscriber Directory Number (MSISDN) of the caller, preventing the PSAP from calling back the user in case of a disconnect.
Innovation Solution
The Visited Public Land Mobile Network (VPLMN) modifies its handling of LTE emergency calls by sending a retrieval request with the International Mobile Equipment Identity (IMEI) to the Location Retrieval Function (LRF) in the Gateway Mobile Location Centre, which returns the matching MSISDN, allowing the PSAP to route the call with the user's MSISDN and enable call-back functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the VPLMN routes LTE emergency calls from roaming non-subscriber devices, then emergency call connectivity is enabled, but the PSAP cannot obtain the caller's MSISDN for call-back purposes
Solution Approach 1:
The system performs preliminary actions by retrieving the MSISDN from the HPLMN's HSS database before the emergency call is disconnected. The VPLMN's E-CSCF initiates a location retrieval procedure that queries the GMLC, which then contacts the HPLMN's HSS to obtain the MSISDN associated with the roaming device's IMSI, ensuring the information is available before potential disconnection occurs
Solution Approach 2:
The patent introduces intermediary components (E-CSCF, GMLC, and HSS) that mediate between the VPLMN and HPLMN to retrieve the MSISDN. The E-CSCF acts as an intermediary to initiate the retrieval, the GMLC serves as an intermediary location management function, and the HSS acts as an intermediary database that stores the mapping between IMSI and MSISDN, enabling information exchange between networks without direct connection
2Ease of operation
If the VPLMN handles roaming emergency calls without subscriber authentication, then call accessibility is improved, but the system cannot retrieve caller identification information
Solution Approach 1:
The system extracts the necessary caller identification information (MSISDN) from the HPLMN's HSS database without requiring the roaming device to be authenticated by the VPLMN. The extraction process uses the device's IMSI (which is available during the emergency call setup) to query the HSS and retrieve the associated MSISDN, separating the identification retrieval from the authentication process
Solution Approach 2:
The HSS database serves multiple functions: it stores subscriber authentication data for the HPLMN and also stores the mapping between IMSI and MSISDN that can be queried by external networks (VPLMN) for emergency call identification purposes. This multi-functionality allows the same database to support both authentication and information retrieval without requiring separate systems
Data Source
AI summary
A roaming wireless user device on a Visited Public Land Mobile Network (VPLMN) may initiate a Long-Term Evolution (LTE) emergency call to a Public Safety Answering Point (PSAP). Accordingly, a Proxy Call Session Control Function (P-CSCF) of the VPLMN may receive an anonymous Session Initiation Protocol (SIP) invite message for the LTE emergency call. The anonymous SIP invite message lacks a Mobile Station International Subscriber Directory Number (MSISDN) of the wireless user device but has an International Mobile Equipment Identity (IMEI) of the wireless user device. The anonymous SIP invite message is forwarded to a Location Retrieval Function (LRF) in a Gateway Mobile Location Center (GMLC) of the VPLMN. The LRF obtains a MSISDN of the wireless user device that matches the IMEI in the anonymous SIP invite message from a database of the GMLC, such that the MSISDN is routed with the LTE emergency call to the PSAP.