With enhanced 911 the
citizens telephone call is instantly routed to our
When a call goes out from your phone, your voice isn't
the only thing being transmitted in the network. The phone
company switch that serves your phone is also sending out an
Automatic Number Identification (ANI) signal to the network.
Originally, ANI signaling was designed to assist the
phone company in accessing toll charges for long distance
calls. With advances in technology, it was eventually
employed to aid in relaying needed information to the PSAP
for 911 response.
How does it work? Within each call, information
containing eight digits is embedded in the signal. These
eight digits contain the seven digits of the caller's local
number. The eighth digit is called a Numbering Plan Digit (NPD).
NPD is basically shorthand for the area code of the
originating call. Since most 911 tandems rarely dealt with
more than two or three area codes, this was an economical
way to relay information with one digit instead of three.
With special equipment, the 911 tandem can read the ANI
information and route the callback number to a digital
display at the appropriate PSAP. Armed with this ANI
information, the PSAP has equipment allowing it to request
and receive the caller's physical address or Automatic
Location Information (ALI).
Below is a diagram of How
A screen at the 911
center displays the telephone number as soon as the call is
received. Within seconds the display will indicate the exact
address the call is originating from. It will also display
the proper fire, police and ambulance service for the
address. Coin telephones will provide access to the 911
system free of charge.
accidentally dial 911 or hear the communicator answer:
Do you need Police, Fire, or Ambulance?"
Please do not hang up. All you need to do is explain you have
dialed the wrong number. The communicator will confirm this
with you by asking a few questions, to ensure you are safe
and that there is no problem. If you do hang up without
talking to a communicator, you will have company stopping
in. It is this Sheriff's Office policy to send Deputies
when a 911 caller hangs up.
Our 911 center is equipped with a
Telecommunications Device for the Deaf (T.D.D./T.Y.Y.). This
device will detect an in coming B.A.U.D.O.T or ASCII call
and allow hearing impaired persons to report an emergency
via a keyboard device directly to a 911 dispatcher.
device is interfaced with the telephone system in our 911
center and will automatically alert the dispatcher of an
incoming T.D.D./T.Y.Y. call. Our 911 communicators have been
trained on the operation and use of the T.D.D./T.Y.Y.
Hearing impaired customers will not have to call a
translation service in an emergency; they too can dial 911.