Version 5.8 389 September 2009
SIP User's Manual 9. IP Telephony Capabilities
9.4.1.1 FXO Device Interworking SIP E911 Calls from Service Provider's IP
Network to PSAP DID Lines
The FXO device can interwork SIP emergency E911 calls from the Service Provider's IP
network to the analog PSAP DID lines. The standards that define this interface include TR-
TSY-000350 or Bellcore’s GR-350-Jun2003. This protocol defines signaling between the
E911 tandem switch (E911 Selective Router) and the PSAP, using analog loop-start lines.
The FXO device can be implemented instead of an E911 switch, by connecting directly to
the PSAP DID loop-start lines.
Figure 9-6: 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).
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