Detecting and Handling E9-1-1 Calls

The ELIN device identifies E9-1-1 calls and translates their incoming E9-1-1 calling numbers into ELIN numbers, which are sent to the PSAP. The device handles the received E9-1-1 calls as follows:

1. The device identifies E9-1-1 calls if the incoming SIP INVITE message contains a PIDF-LO XML message body. This is indicated in the SIP Content-Type header, as shown below: 
Content-Type: application/pidf+xml
2. The device extracts the ELIN number(s) from the ELIN field in the XML message. The ELIN field corresponds to the <CompanyName> column in the Location Information Server (LIS). The device supports up to five ELIN numbers per XML message. The ELINs are separated by a semicolon. The digits of the ELIN number can be separated by hyphens (xxx-xxx-xxx) or they can be adjacent (xxxxxxxxx), as shown below: 
<ca:ELIN>1111-222-333; 1234567890 </ca:ELIN>

For backward compatibility, if the ELIN field doesn't appear in the PIDF-LO, the device extracts the ELINs from the NAM field.

3. The device saves the From header value of the SIP INVITE message in its ELIN database table ('Call From' field). The ELIN table is used for PSAP callback, as discussed later in PSAP Callback for Dropped E9-1-1 Calls. The ELIN table also stores the following information:
● ELIN: ELIN number
● Time: Time at which the original E9-1-1 call was terminated with the PSAP
● Count: Number of E9-1-1 calls currently using the ELIN

An example of the ELIN database table is shown below:

ELIN

Time

Count

Index

Call From

4257275678

22:11:52

0

2

4258359333

4257275999

22:11:57

0

3

4258359444

4257275615

22:12:03

0

0

4258359555

4257275616

22:11:45

0

1

4258359777

The ELIN table stores this information for a user-defined period (see Configuring the E9-1-1 Callback Timeout), starting from when the E9-1-1 call, established with the PSAP, terminates. After this time expires, the table entry with its ELIN is disregarded and no longer used (for PSAP callback). Therefore, table entries of only the most recently terminated E9-1-1 callers are considered in the ELIN table. The maximum entries in the ELIN table is 300.

4. The device uses the ELIN number as the E9-1-1 calling number and sends it in the SIP INVITE (as an ANI / Calling Party Number) to the SIP Trunk.

An example of a SIP INVITE message received from an E9-1-1 caller is shown below. The SIP Content-Type header indicating the PIDF-LO and the ELIN field listing the ELINs are shown in bold typeface.

INVITE sip:911;phone-context=Redmond@192.168.1.12;user=phone SIP/2.0
From: "voip_911_user1"<sip:voip_911_user1@contoso.com>;epid=1D19090AED;tag=d04d65d924
To: <sip:911;phone-context=Redmond@192.168.1.12;user=phone>
CSeq: 8 INVITE
Call-ID: e6828be1-1cdd-4fb0-bdda-cda7faf46df4
VIA: SIP/2.0/TLS 192.168.0.244:57918;branch=z9hG4bK528b7ad7
CONTACT: <sip:voip_911_user1@contoso.com;opaque=user:epid:R4bCDaUj51a06PUbkraS0QAA;gruu>;text;audio;video;image
PRIORITY: emergency
CONTENT-TYPE: multipart/mixed; boundary= ------=_NextPart_000_4A6D_01CAB3D6.7519F890
geolocation: <cid:voip_911_user1@contoso.com>;inserted-by="sip:voip_911_user1@contoso .com"
Message-Body:
------=_NextPart_000_4A6D_01CAB3D6.7519F890
Content-Type: application/sdp ; charset=utf-8
v=0
o=- 0 0 IN IP4 Client
s=session
c=IN IP4 Client
t=0 0
m=audio 30684 RTP/AVP 114 111 112 115 116 4 3 8 0 106 97
c=IN IP4 172.29.105.23
a=rtcp:60423
a=label:Audio
a=rtpmap:3 GSM/8000/1
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=ptime:20------=_NextPart_000_4A6D_01CAB3D6.7519F890
Content-Type: application/pidf+xml
Content-ID: <voip_911_user1@contoso.com>
<?xml version="1.0" encoding="utf-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf" xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10" xmlns:bp="urn:ietf:params:xml:ns:pidf:geopriv10:basicPolicy" xmlns:ca="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr" xmlns:ms="urn:schema:Rtc.LIS.msftE911PidfExtn.2008" entity="sip:voip_911_user1@contoso.com"><tuple id="0"><status><gp:geopriv><gp:location-info><ca:civicAddress><ca:country>US</ca:country><ca:A1>WA</ca:A1><ca:A3>Redmond</ca:A3><ca:RD>163rd</ca:RD><ca:STS>Ave</ca:STS><ca:POD>NE</ca:POD><ca:HNO>3910</ca:HNO><ca:LOC>40/4451</ca:LOC>
<ca:ELIN>1111-222-333; 1234567890 </ca:ELIN> <ca:PC>98052</ca:PC></ca:civicAddress></gp:location-info><gp:usage-rules><bp:retransmission-allowed>true</bp:retransmission-allowed></gp:usage-rules></gp:geopriv><ms:msftE911PidfExtn><ms:ConferenceUri>sip:+14255550199@contoso.com;user=phone</ms:ConferenceUri><ms:ConferenceMode>twoway</ms:ConferenceMode><LocationPolicyTagID xmlns="urn:schema:Rtc.Lis.LocationPolicyTagID.2008">user-tagid</LocationPolicyTagID ></ms:msftE911PidfExtn></status><timestamp>1991-09-22T13:37:31.03</timestamp></tuple></presence>
------=_NextPart_000_4A6D_01CAB3D6.7519F890--