Wednesday, April 4, 2012

IMS Emergency Calls over LTE – Short description

As operators are moving towards LTE, the need for emergency calls support over LTE is high, as they have to comply with local authorities. 3GPP has defined a nice procedure for emergency calls in Release 9(?). The support for emergency calls is required across the network and UE. Below is how the procedure works. 
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. 

Another thing to note is 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. 
There are several nuts and bolts described in 3GPP TS 23.401, 24.301 and 29.274 if you want to look at the procedure in detail.


Anonymous said...

Crisp explanation !

Anonymous said...

Hi Santosh,

Are you aware of any devices or applications supporting these calls? Or the users just have to type the standard emergency number, e.g. 112 or 911, and then it's the network's job to handle it?

Santosh said...

As of now I am not aware of any UE that are available off the hook and support IMS. I am also not aware if any of the service providers use IMS for their voice network.

However I have recent reports from qualcomm that they are testing VoLTE with IMS capable UEs.

Anonymous said...

E-PDN attach can be of 2 types
1. IMEI based
2. UIMSI based(un-authenticated IMSI.Here UIMSI flag is set by the MME in the CSReq Msg).

Also Multi-PDN calls are not possible once the E-PDN call is up. The MME has to make sure of this in CN.

Anonymous said...

Hello Santosh,

Nice explaination indeed..Planning to work on LTE in ear future and it helped

I have an offmark question by the way...I am looking for a IMS UE with features such as Presence,File sharing,IM etc...Can you suggest some. Free/Paid whatever...Also the OS doesnt matter much...

PLs help if u can..

Santosh Kumar Dornal said...

Sorry , I am not aware such UE.