Subject: [Ecrit] Query: How to derive Emergency Call from SIP INVITE Message

From: "NC Reddy" <[email-masked]>
Date: Fri, 29 Feb 2008 16:15:06 -0500

Hi,
    I have the following queries regarding emergency call interpretation at
SIP Application Server (SIP-AS).

scenario:
========
AP-  Access Point
AS - SIP Application server
MGCF - Media Gateway Control Function (Softswitch )
PSAP -  *Public Safety Answering Point

*AP<----SIP--->AS<---SIP----->MGCF<----Non-SIP---->PSAP

Use case1 : When subscriber(AP) makes emergency call to PSAP (AP
------->PSAP)
Use case2 : When PSAP callbacks to subscriber.  (PSAP ------->AP)

Now Questions related the mechanisms for analyzing the emergency calls at
SIP Application Server. By considering the cases of emergency call numbers
are different in different countries and context.

Option 1:  SIP-AS can do the digit analysis from request uri of the userpart
(ex: 911) and figureout the call as emeregncy and handle the call.
Option 2 :  If the AP/MGCF done the digit analysis and add the "priority :
emergency" header, SIP-AS can rely on that header and route the call.

So which option is suitable in context of SIP-AS implementation for
emeregency calls.

Thanks in advance.

Regards
Channa
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---

From: "Brian Rosen" <[email-masked]>
Date: Fri, 29 Feb 2008 16:44:27 -0500

Generally, the IETF has not spent a lot of time on the specifics of how a
PSAP without an IP connection handles a call that originated in an IP
network.  So, the IETF documents would just treat the “MGCF” in your diagram
as the PSAP, and whatever it does to get to the PSTN connected PSAP is up to
it.   

The ecrit LoST protocol describes how the AP learns the dialing sequence for
its location.  We have another document that describes how emergency calls
are marked (‘urn:service:sos where there is a single emergency number).
LoST also describes how calls should route to the proper PSAP.  

So if the AP does digit analysis, it would obtain the dialing string(s) to
look for from LoST.  Once it found one, it would signal with the service URN
(in the R-URI) and the PSAP URI obtained from LoST in a Route header.

It is difficult for the AS (or MGCF) to do the digit analysis, because it
would need to know the location of the AP to determine what to look for.  If
the network forces the AS to be in a “visited” network, that may not be so
hard, but few networks can guarantee that will always be the case.  That is
why the IETF recommends the AP do the digit analysis and provides the LoST
dial “Service number” mechanism.

Brian

________________________________________
From: [email-masked] [mailto:[email-masked]] On Behalf Of NC
Reddy
Sent: Friday, February 29, 2008 4:15 PM
To: [email-masked]
Cc: [email-masked]
Subject: [Ecrit] Query: How to derive Emergency Call from SIP INVITE Message

Hi,

scenario:
========
AP-  Access Point
AS - SIP Application server
MGCF - Media Gateway Control Function (Softswitch )
PSAP -  Public Safety Answering Point

AP<----SIP--->AS<---SIP----->MGCF<----Non-SIP---->PSAP

Thanks in advance.

Regards
Channa