PSTN: Numbering Plan

Numbering Plan
A numbering plan is a system that identifies communication points within a communications network through the structured use of numbers. The structure of the numbers is divided to indicate specific regions or groups of users. It is important that all users connected to a telephone network agree on a specific numbering plan to be able to identify and route calls from one point to another.

Telephone numbering plans throughout the world and systems vary dramatically. In some countries, it is possible to dial using 5 digits and others require 10 digits. To uniquely identify every device that is connected to public telephone networks, the Comite Consultatif Internationale de Telegraphique et Telehonique (CCITT) devised a world numbering plan that provides codes for telephone access to each country. These are called country codes. Coupled with the national telephone number assigned to each subscriber in a country, the country code telephone makes that subscribers number unique worldwide. The International Telecommunications Union (ITU) administers the World Numbering Plan standard E.164 publishes any new standards or modifications to existing standards on the Internet.

Each country defines its public telephone network numbering plans. The United States and Canada adopted the North American Numbering Plan (NANP) that allows the two countries to appear as one when dialing internally. Each country has a country code prescribed by the World

Numbering Plan so they are accessed internationally as separate entities. The NANP is based on 10 digit numbering (NXX-NXX-XXXX). The number consists of a 3-digit area code, a 3-digit central office code, and a 4-digit line number. The first three digits (NXX) are the Numbering Plan Area (NPA) or area code. It is this 3-digit code that designates one of the numbering plan areas in the North American Numbering Plan for direct distance dialing. Originally, the format was N0/1X, where N is any digit 2 through 9 and X is any digit. From 1995 on, the acceptable format is NXX.

With the massive requirement for telephone numbers generated by Internet access, fax machines, and cellular telephones, new area codes are being placed in service at an all time high rate. This is causing the telecommunications industry and standards bodies in North America to consider the implementation of “number portability”. When this occurs each subscriber will be assigned telephone numbers permanently (e.g., all subscribers in North America will dial ten digits to make a local call and take their number with them when they move.).