This is a draft proposal and may contain errors. Please send comments to
Proposal for in-band Location Signaling
Non-traditional telephony devices create a new class of
problems for establishing location for emergency response
purposes. This document contains a proposal for location
signalling on emergency calls for non-fixed location telephone
numbers which is compatible with existing PSAP equipment.
Traditional 911 signalling relies on out-of-band transmission of a
telephone number. The telephone number is assumed to be associated
with a fixed location. When this assumption fails, so does the ability
for emergency services to locate a caller who cannot speak. For
wireless handsets, a variety of location tricks are now in use. For
example, Verizon wireless is deploying handsets which can approximate
their position while on a call. Newer Nextel handsets have GPS
receivers. It's not clear how/if this location information is
transmitted to the PSAP, as the signalling protocol used is of 1960's
In Massachusetts, the State Police PSAP in Framingham handles calls
from wireless telephone subscribers (Cellular, GSM, PCS, etc). It has
the ability to transfer calls or request dispatch via PSAP's state
wide. The PSAP premesis equipment is equipped with both DTMF and
Baudot TTY decoders. Massachusetts PSAP's use the DTMF decoder and a
procedure called "silent call" to allow a dispatcher to obtain
information about the nature of an emergency from a caller who can
hear but not speak. My proposal is to extend the silent call procedure
at the PSAP to include receiving and decoding location information.
DTMF Location Signalling
- Must maintain compatibility with existing equipment. Any PSAP with
the silent call capability will have a DTMF decoder with display.
- Non-interferance with manual end-user's use of the silent call
feature. Customer equipment must buffer any DTMF tones sent by caller
during the location tone burst and send them after the tone
burst. Because the # tone is used as a delimiter, manual silent call
response tones can be differentiated from automated location tones.
- Location information must with call if call is transferred. By
transmitting the tone burst every 30 seconds, location information
becomes available again within 30 seconds if the call is transferred
between PSAP's without the ability to relay the previously received
- Location information for a moving handset must update during
call. By transmitting location tones every 30 seconds, the handset
device is able to provide updated latitude / longitude
information. Even if heading and speed are not provided, they can be
computed approximately by the PSAP after two or more tone bursts are
- Must have method for providing altitude information - avoids a
ground search if caller is in airplane.
- Must have method for providing or calculating speed and heading -
helps approximate location for moving emergency, especially on
Interstate highway or other controlled traffic area.
- Must have method for indicating accuracy of GPS lock, including
whether or not location data was user-entered. Allows emergency
personnel to adjust response size and scope of search accordingly.
- Must have method for deterring location fraud. By using existing
ANI information, PSAP should be able to determine either directly or
indirectly the reposible person for a given telephone number and
assess penalties or require equipment repair when calls are received
with incorrect location information.
The location should be transmitted in a burst in the following
form. As appropriate, one or more forms may be sent. As an example, in
the case of a cordless phone which has a fixed range from a fixed
dwelling, it may send the zip code / house number form for the
location of the base station, followed by the longitude/latitude form
for the precise location of the handset if available. A mobile device
such as an automobile mounted phone should send always the longitude
and lattitude form, preferably with heading and speed.
The equipment SHOULD make a best effort to minimize the length of
the tone burst and the time interval during which the line's transmit
audio is muted. The equipment should SHOULD provide the caller with an
indication that the audio path is muted or that tones are being
sent. The dispatcher should be trained to ask the caller to repeat any
information that might have been obscured by the tone burst.
Data format: ##NN#location_data#
Where location_data is a value that varies with N.
N is a variable length field. The following values for N are proposed:
- 0 ICBM Address: Location Quality / Longitude in Degrees / Lattitude in Degrees / Heading / Speed. * used to indicate negative, decimal point, and delimiter.
Where the following are defined for lock quality
- *0 - User provided
- *1 - Past GPS lock
- any positive number - radius of uncertainty circle in meters
The lock quality, long, and lat are mandatory. The speed, heading, altitude, and altitude_ref values are optional, but if one is specified those before it must be also. Heading is in degrees from true north, altitude is in meters, speed is in meters/sec. The only value proposed for altitude_ref is 3, which shall mean meters above Earth's sea level. Since it is not possible
to differentiate an empty field from a leading negative sign on the next field, the special value 00 shall mean unknown/unavialble.
- United States 9 Nine Digit Zip Code, Street Number, apartment data, apartment data... * as delimiter.
Format is zip*steet_num*apt_data*apt_data*apt_data...
Apartment data field is optional and may be repeated as many times as needed. For non-numeric apartment numbers, the following map is proposed:
0XX, where XX is the zero padded ordinal rank of the desired letter in the set of 26 [abcde...xyz].
018761149*30 - Building number 30 in zip code 01876-1149
018760000*67*002*27 - Building number 67, apartment B-27 in zip code 01876-0000
Last modified: Wed Apr 27 12:16:13 EDT 2005