SIP User's Manual 484 Document #: LTRT-12801
MP-500 MSBG
Figure 9-31: FXO Device Interfacing between E911 Switch and PSAP
When an IP phone subscriber dials 911, the device receives the SIP INVITE message and
makes a call to the PSAP as follows:
1. The FXO device seizes the line.
2. PSAP sends a Wink signal (250 msec) to the device.
3. Upon receipt of the Wink signal, the device dials MF digits after a user-defined time
(WaitForDialTime) containing the caller's ID (ANI) obtained from the SIP headers From
or P-AssertedID.
4. When the PSAP operator answers the call, the PSAP sends a polarity reversal to the
device, and the device then sends a SIP 200 OK to the IP side.
5. After the PSAP operator disconnects the call, the PSAP reverses the polarity of the
line, causing the device to send a SIP BYE to the IP side.
6. If, during active call state, the device receives a Wink signal (typically of 500 msec)
from the PSAP, the device generates a SIP INFO message that includes a "hookflash"
body, or sends RFC 2833 hookflash Telephony event (according to the
HookFlashOption parameter).
7. Following the "hookflash" Wink signal, the PSAP sends DTMF digits. These digits are
detected by the device and forwarded to the IP, using RFC 2833 telephony events (or
inband, depending on the device's configuration). Typically, this Wink signal followed
by the DTMF digits initiates a call transfer.
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