SIP User's Manual 488 Document #: LTRT-12801
MP-500 MSBG
4. 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.
5. 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.
6. 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.
7. 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.
8. 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:
îš„ EnableDIDWink = 1
îš„ EnableReversalPolarity = 1
îš„ PolarityReversalType = 1
îš„ FlashHookPeriod = 500 (for 500 msec "hookflash" mid-call Wink)
îš„ WinkTime = 250 (for 250 msec signalling Wink generated by the FXS device after it
detects the line seizure)
îš„ EnableTransfer = 1 (for call transfer)
îš„ LineTransferMode = 1 (for call transfer)
îš„ WaitforDialTime = 1000 (for call transfer)
îš„ SwapTEl2IPCalled&CallingNumbers = 1
îš„ DTMFDetectorEnable = 0
îš„ MFR1DetectorEnable = 1
îš„ DelayBeforeDIDWink = 200 (for 200 msec) - can be configured in the range from 0
(default) to 1000.
Note: Modification of the WinkTime and FlashHookPeriod parameters require a
device reset.
The outgoing SIP INVITE message contains the following headers:
INVITE sip:Line@DomainName
From: <sip:*81977820#@sipgw>;tag=1c143
To: <sip:Line@DomainName>
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