This is a draft proposal and may contain errors. Please send comments to brianr-voip911@osiris.978.org

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

Requirements:

• 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 information.
• 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 received.
• 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.

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:

• 0 ICBM Address: Location Quality / Longitude in Degrees / Lattitude in Degrees / Heading / Speed. * used to indicate negative, decimal point, and delimiter.
  LQ*long*lat*heading*speed*altitude*altitude_ref
  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.
  0*42*5115*72*2279
• 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].
  Examples:
  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