FXS Device Emulating PSAP using DID Loop-Start Lines
The device's FXS interface can be configured to emulate PSAP (using DID loop start lines), according to the Telcordia GR-350-CORE specification.
The call flow of an E911 call to the PSAP is as follows:
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The E911 tandem switch seizes the line. |
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The FXS device detects the line seize, and then generates a wink signal (nominal 250 msec). The wink can be delayed by configuring the parameter [DelayBeforeDIDWink] to 200 (for 200 msec or a higher value). |
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The switch detects the wink and then sends the MF Spill digits with ANI and (optional) Pseudo-ANI (P ANI). |
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The FXS device collects the MF digits, and then sends a SIP INVITE message to the PSAP with all collected MF digits in the SIP From header as one string. |
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The FXS device generates a mid-call wink signal (two subsequent polarity reversals) toward the E911 tandem switch upon either detection of an RFC 2833 "hookflash" telephony event, or if a SIP INFO message with a "hooflash" body is received from the PSAP (see the example below). The duration of this "flashhook" wink signal is configured using the parameter [FlashHookPeriod] (usually 500 msec). Usually the wink signal is followed by DTMF digits sent by PSAP to perform call transfer. Another way to perform the call transfer is to use SIP REFER messages, as described below. |
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The FXS device supports call transfer initiated by the PSAP. If it receives a SIP REFER message with the Refer-To URI host part containing an IP address that is equal to the device's IP address, the FXS device generates a 500-msec wink signal (double polarity reversals), and then (after a user-defined interval configured by the parameter [WaitForDialTime]), plays DTMF digits according to the transfer number received in the SIP Refer-To header URI userpart. |
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When the call is answered by the PSAP operator, the PSAP sends a SIP 200 OK to the FXS device, and the FXS device then generates a polarity reversal signal to the E911 switch. |
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After the call is disconnected by the PSAP, the PSAP sends a SIP BYE to the FXS device, and the FXS device reverses the polarity of the line toward the tandem switch. |
The following parameters need to be configured:
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[EnableReversalPolarity = 1] |
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[PolarityReversalType = 1] |
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[FlashHookPeriod = 500] - for 500 msec "hookflash" mid-call Wink |
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[WinkTime = 250] - for 250 msec signalling Wink generated by the FXS device after it detects the line seizure |
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[EnableTransfer = 1] - for call transfer |
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[LineTransferMode = 1] - for call transfer |
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[WaitforDialTime = 1000] - for call transfer |
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[SwapTEl2IPCalled&CallingNumbers = 1] |
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[DTMFDetectorEnable = 0] |
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[MFR1DetectorEnable = 1] |
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[DelayBeforeDIDWink = 200] - for 200 msec and can be configured in the range from 0 (default) to 1000 |
Modification of the [WinkTime] parameter requires a device restart.
The outgoing SIP INVITE message contains the following headers:
INVITE sip:Line@DomainName
From: <sip:*81977820#@sipgw>;tag=1c143
To: <sip:Line@DomainName>
Where:
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Line = as configured in the Endpoint Phone Number Table |
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SipGtw = configured by the [SIPGatewayName] parameter |
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From header/user part = calling party number as received from the MF spill |
The ANI and the pseudo-ANI numbers are sent to the PSAP either in the From and/or P-AssertedID SIP header.
Typically, the MF spills are sent from the E911 tandem switch to the PSAP, as shown in the table below:
Dialed MF Digits Sent to PSAP
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8 digits "nnnnnnnn" (ANI)
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"KPnnnnnnnnST"
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12 digits "nnnnnnnnnnnn" (ANI)
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"KPnnnnnnnnnnnnSTP"
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12 digits ANI and 10 digits PANI
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"KPnnnnnnnnnnnnSTKPmmmmmmmmmmST"
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two digits "nn"
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"KPnnSTP"
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The MF KP, ST, and STP digits are mapped as follows:
For example, if ANI and PANI are received, the SIP INVITE contains the following From header:
From: <sip:*nnnnnnnnnnnn#*mmmmmmmmmm#@10.2.3.4>;tag=1c14
It is possible to remove the * and # characters, using the device's number manipulation rules.
If the device receives the SIP INFO message below, it then generates a "hookflash" mid-call Wink signal:
INFO sip:4505656002@192.168.13.40:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.13.2:5060
From: port1vega1 <sip:06@192.168.13.2:5060>
To: <sip:4505656002@192.168.13.40:5060>;tag=132878796-1040067870294
Call-ID: 0010-0016-D69A7DA8-1@192.168.13.2
CSeq:2 INFO
Content-Type: application/broadsoft
Content-Length: 17
event flashhook