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List of Definitions regarding Area Codes


[Available in PDF]


Administrative Numbers: Numbers that perform specific administrative functions with the qualification that these numbers cannot be assigned to customers. These numbers can be employee/official numbers, Location Routing Numbers, test numbers, Temporary Local Directory Numbers (TLDN), wireless E911 emergency service routing digits/key (ESRD/ESRK) numbers, and soft-dial tone numbers. These can also be numbers used by carriers to perform internal administrative or operational functions necessary to maintain reasonable quality of service standards.

Aging Numbers: Disconnected numbers that are not available for assignment to another end-user or customer for a specified period of time

Area Code: The first set of three digits of a telephone number. Also referred to as a numbering plan area (NPA).

Area Code Exhaust: Exhaust occurs when there are no longer any NXX codes available for assignment within an area code.

Area Code Split: A form of an area code change where by a geographical region is divided into multiple regions with different area codes. Only one area code will be associated with a particular region.

Area Code Overlay: A form of an area code change where by a geographical region will have multiple area codes co-existing in the same region.

Assigned Numbers: Numbers working in the Public Switched Telephone Network under an agreement such as a contract or tariff at the request of specific customers for their use, or as numbers not yet working but having a customer service order pending.

Available Numbers: Numbers within existing central office codes (NXX) or thousand-blocks (NXX-X) that are available for assignment to subscriber access lines or their equivalents within a switching entity/point of interconnection (POI) and are not categorized as assigned, intermediate, administrative, aging or reserved.

Central Office Code (CO Code): The second set of three digits of a telephone number. Central office codes are in the form "NXX." Central office codes are assigned to rate centers.

Contamination: Contamination occurs when a telephone number is no longer considered as being available for assignment to end-users.

Contamination Level: The level at which a thousand-block or NXX code has telephone numbers that are no longer considered as being assignment to end-users. It is calculated by dividing the sum of assigned, intermediate, reserved, aging, and administrative numbers by the total numbering resources within the thousand-block or NXX code, and multiplying the result by 100.

Contamination Threshold: The contamination level upon which eligibility of thousand-block donations or returns are based. The contamination threshold is currently at 10% except for the 310 area code where it is at 25%.

Footprint Thousand-Block: A telecommunications carrier's initial thousand-block in a rate center.

Intermediate Numbers: Numbers that are made available for use by another carrier or non-carrier entity for the purpose of providing telecommunications service to an end-user or customer. Numbers provided for use by resellers, numbers in dealer numbering pools, numbers preprogrammed into customer premises equipment offered for retail sale, and numbers assigned to messaging telecommunications companies. Numbers controlled or made available to an end-user or customer by a carrier or non-carrier entity other than the code or block holder, and exclude all numbers assigned to end-user customers of code or block holders.

Local Routing Number: A unique 10-digit number that serves as a network address and is assigned to each central office code to identify each switch or point of interconnection in the network.

NeuStar Number Pool Administration: The administration that is responsible for the administration and assignment of thousand-blocks to Local Number Portability (LNP)-capable telecommunications companies in rate centers where thousand-block number pooling occurs. It processes thousand-block applications, returns, and donations.

North American Number Plan (NANP): A numbering architecture in which every station in the NANP area is identified by a unique 10-digit address consisting of an area code, central office code, and subscriber line number.

North American Number Plan Administration (NANPA): The administration that is responsible for coordinating and administrating the North American Numbering/Dialing Plans.

Numbering Categories: Categories in which telephone numbers are identified. There are six categories of telephone numbers. They are assigned, intermediate, reserved, aging, administrative, and available.

Numbering Plan Area (NPA): The first set of three digits of a telephone number, commonly known as the area code.

Numbering Resource Utilization and Forecast Report (NRUF Report): A semi-annual report demonstrating the forecast and utilization information to monitor and project area code and NANP exhaust.

NXX Code: A block of 10,000 telephone numbers; represented by the second set of three digits of a telephone number. Also referred to as a prefix.

Prefix: The second set of three digits of a telephone number. Also referred to as NXX code.

Point of Interconnection (POI): The physical location where a telecommunications carrier's connecting circuits interconnect for the purpose of interchanging traffic on the Public Switched Telephone Network.

Rate Boundary: Border between one rate center and another.

Rate Center: The smallest geographic area used to distinguish rate boundaries. A rate center is a geographic area containing one or more wire centers, used as the basis to define local and toll-calling areas. When communities were smaller, the rate center was the center of each community's greatest concentration of population, such as the post office or other centrally located points. As communities grew and population centers changed, planners connected large population centers by drawing vertical and horizontal lines across a map of the United States. When the vertical and horizontal lines intersected, a rate center was identified. The distance between rate centers was measured in airline miles. The mileage is used for determining local versus ZUM calls, local-toll calls, and long distance toll calls. All local and long distance telephone companies in the United States use rate centers to calculate the rates that are charged for telephone calls. For purposes of central office code assignments and relief planning the rate centers are a major component in an area code.

Reserved Numbers: Numbers held by telecommunications companies at the request of specific end-user customers for their future use. The purpose of having reserved numbers is to give prospective clients some assurance that numbers with the characteristics those customers are seeking will be available to them in the near future.

Stranded Telephone Numbers: Telephone numbers that cannot be shared amongst telecommunications carriers.

Subscriber Line Number: The last four digits of a telephone number.

Thousand-Block: A block of one thousand telephone numbers.

Thousand-Block Donation: A process for donating thousand-blocks that were allocated to telecommunications companies by NANPA.

Thousand-Block Number Pooling: A system that divides a central office code into ten sequential blocks of one thousand numbers and allocates telephone numbers in blocks of one thousand numbers. This system allows multiple telecommunications companies to share the ten thousand-blocks in a central office code.

Thousand-Block Return: A process for returning thousand-blocks that were allocated to telecommunications companies by the NeuStar Number Pool Administration.

Utilization Level: The level at which telecommunications companies are using the telephone numbers allocated to them. The utilization level is calculated by dividing all assigned numbers by the total numbering resources, and multiplying the result by 100.

Utilization Threshold: The utilization level that telecommunications companies must meet before receiving growth numbering resources. Currently, the utilization threshold is at 75%.


Last Modified: 10/9/2007

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