 | Enhanced 911: Encyclopedia II - Enhanced 911 - Wireline Enhanced 911
Enhanced 911 - Wireline Enhanced 911
There is special privacy legislation that permits emergency operators to obtain the caller's information. This information is gathered by mapping the calling phone number to an address and ESN in a database. This database function is known as ALI, Automatic Line Identification. The database is generally maintained by the Incumbent Local Exchange Carrier (ILEC) under contract by the PSAP. Each ILEC has their own standards for the formatting of the database. Most ALI databases have a companion database known as the MSAG, Master Street Address Guide. The MSAG describes the exact spelling of streets, street number ranges, and other address elements.
Each telephone company (LEC) has at least two redundant DS0-level (that is, 64 kbit/s, or voice quality) trunks connecting each host office telephone switch to each PSAP. These trunks are either directly connected to the PSAPs or they are connected to a telephone company central switch that intelligently distributes calls to the PSAPs. These special switches are often known as 911 Selective Routers. Their use is becomming increasingly more common as it simplifies the interconnection between newer ISUP/SS7-based host office switches and the many older PSAP systems.
If the PSAP receives calls from the telephone company on older analog trunks, they are usually CAMA circuits. These circuits are similar to regular telephone lines, but are formatted to pass the calling party number, ANI, rather than caller ID.
If the PSAP receives calls on older-style digital trunks, they are specially formatted Multi-Frequency (MF) trunks that pass calling party number, ANI, rather than Caller ID during call establishment.
Some of the upgraded PSAPs can receive calls on ISUP trunks controlled by the SS7 protocol. In that case, the calling party number, ANI, is already present in the SS7 setup message.
The PSAP trunking does not pass address information along with the call. Instead, only the calling party number, ANI, is passed. The PSAP uses the calling party number to look up the address in the ALI database. The ALI database is secured and separate from the public phone network by design.
ALI Failure is when the phone number is not passed or that the phone number is not in the ALI database. If this happens, the call is passed to the trunk group's default ESN, which is a PSAP designated for this function. The PSAP operator must then ask the incoming call for their location and redirect them to the correct PSAP. The legal penalty in most states for ALI database lookup failure is limited to a requirement that the telephone company fix the database entry.
Competitive local exchange carriers (CLEC) and other competing wireline carriers negotiate for access to the ALI database in their respective Interconnect Agreement with the ILEC. They typically populate the database using the ILEC MSAG as a guide.
Other related archives9-1-1, 911, ANI, CDMA, Canada, Cingular, Competitive local exchange carriers, Craig Neidorf, DS0, Federal Communications Commission, GPS, GSM, Global Positioning System, Hybrid, ILEC, Multi-Frequency (MF), NANP, North American, PBXs, Public Safety Answering Point, Sprint PCS, T-Mobile, TDMA, TV, United States, Verizon Wireless, Voice over IP, assisted GPS, base stations, call, caller, calling for help, cellular network, cellular telephony, computer-assisted dispatch, data, default, emergency, emergency dispatch, emergency responders, emergency services, emergency telephone number, exchange, fingerprinting, handset, intersect, line of sight, location-based services, map, metropolitan areas, mobile telephone, mountainous, multilateration, multipath, network, operator, patterns, phone numbers, privacy, radio towers, radiolocation, regular telephone lines, seismometers, skyscrapers, telephone, telephone company, telephone company (LEC), telephone directory, telephone number, telephone switch, terrain, triangulation, wireless
 Adapted from the Wikipedia article "Wireline Enhanced 911", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
