As everyone is moving towards LTE so the support for emergency calls over LTE is needed, as they have to comply with authorities. 3GPP has defined a nice procedure for emergency calls. The support for emergency calls is required across the network and UE.
Emergency service is provided to the Lte enabled UE in the following situations:
- User calls an emergency number.
- UE is in the state where network is not available to serve the UE.
- User can make IMS emergency call even if does not have a SIM card.
- UE is in limited service state i.e limited service state where UE is not allowed to call or roam.
- To make an IMS Emergency call, UE sends Attach type as Emergency attach and request type as Emergency in Attach Request.
- MME(Lte Network) should be configured with the Emergency support enabled otherwise MME rejects the emergency call.
- Following are the various configurations in MME according to which MME proceeds the Emergency call.
First thing to consider is any UE can make an emergency call, provided network supports it. This means there is no subscription for that UE in HSS. This also means there is need for an emergency APN to be defined! I will avoid eNBs here as RRC always goes above my thinking capabilities.
When UE wants to perform emergency attach, it will send an attach request with attach type as Emergency Attach. Upon receiving this attach MME may or may not contact HSS for Authentication. If MME supports, then call may proceed even if Authentication fails if at all HSS was contacted. If authentication fails then MME may not contact HSS for Update location request. In a nut shell, when Emergency Attach is received Auth/Security procedure becomes optional.
This brings the next point. APN. This is where 3GPP defines the magic term “MME Emergency Configuration Data” that is needed on MME, where Emergency APN is stored. Optionally QoS parameters and static PDN gateway address can be stored. So upon receiving an emergency attach MME knows where to send the Create Session Request.
a UE may not send IMSI in the attach request, if it doesn’t have one. Well, I should be able to make an emergency call even if I don’t have a SIM card. In this case IMEI is required. Basically mobile network is doomed without IMSI or IMEI. This needs SGW/PGW to be prepared for receiving Create Session requests without IMSIs or IMEIs. Another software modification! The QoS values, like ARP can be statically configured on MME or if Dynamic PCC is deployed then PCRF may provide these details. The PDN created for emergency service cannot be per-empted, so proper ARP values should be used. It is also expected that Emergency APN supports IPv4v6 PDN type just to be on safer side.
Once everything is set, MME may send Create Session Request to SGW/PGW and based the Response the attach procedure may be completed. A UE may make an emergency call now. Once the call is complete UE need not detach. As soon as UE goes to idle mode, MME may go ahead and release the call.
Emergency Bearer in Limited Service:
3GPP specs TS 23.401, 24.301 and 29.274 can be referred to check the procedure in detail :).