Sequence of 911 Trunk Call to MAARS PSAP System
by jadedragon
(glossary of terms at bottom)

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1) 911 call initiated by caller and routed to PSAP based ESN (Emergency Service Number).
2) Tandem switch looks for idle 911 trunk to PSAP, identified as idle by -48v on ring and ground
   on tip.                                                              POWER AND POLARITY
3) Tandem signals the PSAP with a loop closure on the 911 trunk that generates current flow.
4) MAARS TIU looks for this loop closure once and then again 200 milliseconds late for loop
   current from tandem.                                                 DEBOUNCE OR BUFFER
5) If loop is still there, MAARS TIU reverses tip and ring, (WINK-250msec REVERSAL)
6) The Tandem seeing the wink will transmit (10 MULTIFREQUENCY TONE to the PSAP)  KP, NPD, 
   ABC-XXXX,ST
7) TIU decodes the multifreq tones and stores this information (ANI-Automatie Number
   Identification)
8) TIU provides ringing to the 400 KTU (Key Telephone Unit)(or other) line card and audible ring
   back to the initiator of the call.
9) TIU transmits a message which goes over the ELAN to all stations to turn on the buzzers in the
   APU's and display a visual symbol on the ANI screen  
10) TIU transmits a message over the ELAN to the DBU containing the ANI requesting a query over
    the data links for ALI   (Automatic Location Identification)
11) DBU transmist the ALI request over the data links
12) DMS(Data Management System) responds with an "ACK" (Acknowledgement) message.
13) DMS then transmits the ALI information over one data links back to the PSAP. This information
    is stored in the TIU.
14) Position attendant (telecommunicator) answer call and comes OFF HOOK at station LOOP CURRENT
15) The MAARS PSAP seizes the 911 trunk by reversing tip and ring back to the Tandem.  
    SUPERVISON
16) After the APU goes off hook the TIU transmits a Data Over Voice message over the common tip
    and ring to the position in an attempt to discover which position is off hook. (This message
    is broadcast to any APU that comes off hook).   TIU# AND ANI WITH NO HYPHEN
17) The APU receives the DOV message and then sends a message on the ELAN to the TIU requesting  
    the ALI and informing the TIU of the APU's address (Position Number)
18) This address is stored in the TIU
19) The TIU transmits the ALI information via the ELAN to the appropriate APU address. The ANI 
    will include the HYPHEN.
20) Telecommunicator sees ANI and ALI information displayed and deals with the call
21) When finished, the telecommunicator and caller go on hook.
22) 911 trunk is released by MAARS and goes idle.
23) TIU transmits the call record to the CRU via the ELAN.
24) CRU transmits message over parallel or serial port to the printer printing call records.


**Terms**
-MAARS: Modular ANI/ALI Retrieval System
-PSAP: Public Safety Answering Points
-TIU: Trunk Interface Unit
-ELAN: Emergency Local Area Network
-APU: Answering Point Unit
-DBU: Data Base Unit
-DOV: Data Over Voice (message)
-CRU: Call Record Unit