From a 3GPP presentation by Hannu Hietalahti:
Emergency calls in LTE
Regulatory requirement of emergency calls is supported in Rel-9 for LTE:
1. Detection of emergency numbers in UE
2. Indication and prioritisation of emergency calls
3. Location services, both for routing and user location data for PSAP (Public Safety Answering Point)
4. Callback is possible, but processed as normal call without exceptions
UE matches digits dialledby the user with list of known emergency numbers
1. Emergency number list in the UE is common for CS and PS domain use
2. Default 112 and 911, USIM pre-configuration, downloaded in MM procedure
3. In case of match, the UE shall initiate the call as an emergency call
In IMS emergency calls the UE translates dialled number into emergency service URN
1. Service URN with a top-level service type of "sos" as specified in RFC5031
2. Additionally, sub-service type can be added to indicate emergency category if information on the type of emergency service is known (fire, ambulance, police,…)
P-CSCF (Proxy - Call Session Control Function) must also be prepared to detect emergency call if the UE is not aware of local emergency call
1. This is backup for those cases when the (roaming) UE does not have full information of all local emergency call numbers and initiates a normal call
2. From EPC perspective, it will be a normal PDN connection
Benefit of location information
1. P-CSCF discovers the regionally correct PSAP to take the emergency call
2. PSAP gets information on the precise user location
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