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Cuyahoga County: PSAP Assessment
Wireline enhanced 9-1-1
In all North American jurisdictions, special privacy legislation permits emergency
operators to obtain a 9-1-1 caller's telephone number and location information.
This information is gathered by mapping the calling phone number to an address
in a database. This database function is known as Automatic Location
Identification (ALI). The database is generally maintained by the local telephone
company, under a contract with the PSAP. Each telephone company has its 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 address elements including the exact spellings of street names,
and street number ranges.
Each telephone company has at least two redundant telephone trunk lines
connecting each host office telephone switch to each PSAP. These trunks are
either directly connected to the PSAPs, or are connected to a telephone company
central switch that intelligently distributes calls to the PSAPs. These special
switches are often known as 9-1-1 Selective Routers. The use of 9-1-1 Selective
Routers is becoming increasingly more common, as it simplifies the
interconnection between newer office switches and the many older PSAP
systems.
The effectiveness of this technology may sometimes be affected by the type of
telephone infrastructure that the call is routed through. The PSAP may receive
calls from the telephone company on older analog trunks, which are similar to
regular telephone lines but are formatted to pass the calling party number. The
PSAP may also receive calls on older-style digital trunks, which must be specially
formatted to pass Automatic Number Identification (ANI) information only. Some
upgraded PSAPs can receive calls in which the calling party number is already
present. The location of the call is drawn from a computer routine which
supports telephone company service billing, called the Charge Number
Parameter.
With some technologies, the PSAP trunking does not pass address information
along with the call. Instead, only the calling party number is passed, and the
PSAP must use 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. Sometimes, on calls using land lines, the originating
telephone number may not be passed to the PSAP at all, generally because the
number is not in the ALI database. When this happens, the call receiver must
confirm the location of the incoming call, and may have to redirect the call to
another, more appropriate PSAP. ALI Failure occurs when the phone number is
not passed or the phone number passed is not in the ALI database. In most
jurisdictions, when ALI database lookup failure occurs, the telephone company
has a legal mandate to fix the database entry.
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