Wireline Enhanced 911
When a call is placed to 911, the source of the call is recorded (this is allowed by special privacy legislation). The source number is used to look up the Emergency Service Number (phone number) of the appropriate call center (Public-safety answering point, or PSAP) in a database and connect the call.
Address information is not passed along by the public phone network; only the calling party's phone number is passed. The PSAP uses the calling party number to look up the address in the Automatic Location Identification (ALI) database. The ALI database is secured and separate from the public phone network by design. It 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. When a new account is created, the address is looked up in the Master Street Address Guide to find the appropriate Emergency Service Number (ESN) that 911 calls from that phone number should be routed to. 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 populate the database using the ILEC MSAG as a guide.
If the phone number is not passed or the phone number is not in the ALI database, this is known as ALI Failure; the call is then passed to the trunk group's default ESN, which is a PSAP designated for this function. The PSAP operator must then ask the incoming caller 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.
Read more about this topic: Enhanced 9-1-1
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