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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 vintage.

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


Implementation Details

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:

Brian Ristuccia
Last modified: Wed Apr 27 12:16:13 EDT 2005