User's Manual 554 Document #: LTRT-27045
Mediant 1000B Gateway & E-SBC
26.11.1 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.
Figure 26-10: FXS Device Emulating PSAP using DID Loop-Start Lines
The call flow of an E911 call to the PSAP is as follows:
1. The E911 tandem switch seizes the line.
2. 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).
3. The switch detects the wink and then sends the MF Spill digits with ANI and (optional)
Pseudo-ANI (P ANI).
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.
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