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A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of administrative regions of the public switched telephone network (PSTN) and they are also present in private telephone networks. Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements. A broad division is commonly recognized, distinguishing open numbering plans and closed numbering plans. A closed numbering plan imposes a fixed number of digits to every telephone number, while an open numbering plan allows variance in the numbers of digits. Many numbering plans subdivide their territory of service into geographic regions designated by an area code, which is a fixed-length or variable-length set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber. The North American Numbering Plan is a closed numbering plan which prescribes ten digits for each complete destination routing code that is divided into three parts. The most significant part is a three-digit Numbering Plan Area (NPA) code (area code). Within each plan area central offices are numbered with a three-digit central office (CO) code, the second part. The remaining four digits number the specific line assigned to each telephone. Other countries with open numbering plans may use variable-length numbers; in some, such as Finland, subscriber numbers may vary in length even within a local exchange. The International Telecommunication Union (ITU) has established a comprehensive numbering plan, designated E.164, for uniform interoperability of the networks of its member state or regional administrations. It is an open numbering plan, however, imposing a maximum length of 15 digits to telephone numbers. The standard defines a country calling code (''country code'') for each state or region which is prefixed to each national numbering plan telephone number for international destination routing. Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus. Such systems may be supported by a private branch exchange (PBX) which controls internal communications between telephone extensions. In contrast to numbering plans, which determine telephone numbers assigned to subscriber stations, a dial plan establishes the customer dialing procedures, i.e. the sequence of digits users are required to dial to reach a destination. Even in closed numbering plans, it is not always necessary to dial all digits of a number. For example, an area code may often be omitted when the destination is in the same area as the calling station. ==History== In early telephone systems, connections were made in the central office by switchboard operators using patch cords to connect one party to another. To make a telephone call, a person would wind a crank to generate a ring signal to the central office operator, either before or after the user took the telephone handset off-hook. At the central office a gong or later an electric light indicated the need to respond to the customer, upon which the operator inserted a patch cord into a socket and assisted the customer with the call by voice. Another patch cord connected the caller to the destination telephone line. If the destination party belonged to another exchange, the operator used a patch cord to connect to that exchange where an operator would complete the call setup. As technology advanced, automatic electro-mechanical switches were introduced and telephones were equipped initially with rotary dials for pulse-dialing and then Touch-Tone key pads in the 1960s, which increased the speed of dialing and enabled other vertical telephone features. Initial use of area codes in the United States and Canada began in 1947 in large cities for connecting long-distance telephone calls between toll switching centers.〔(【引用サイトリンク】title=Area Code History )〕 The first customer-dialed long-distance calls were possible in Englewood, NJ in 1951. By 1966, the system was implemented fully in both countries.〔(【引用サイトリンク】title=LincMad's Area Codes of the 1970s )〕 The Bell System organized the numbering plan to minimize the cost of providing automatic dialing to large population centers. Electromechanical switching systems, the technology used until the 1960s, imposed limitation on the speed of dialing a digit. Dialing a complete 10-digit telephone number could take up to ten seconds, during which hardware resources had to be dedicated to a circuit. The time to dial a digit was directly proportional to the digit, with the exception of the 0, which required ten pulses or one second. The area codes for a few large cities in the US were initially assigned based on the volume of telephone calls made in each area. The most populous areas received codes that required the least time for dialing using a rotary dial telephone. The densely populated areas of New York City, Chicago, Los Angeles, and Detroit had large incoming call volume and were assigned the shortest area codes, 212, 312, 213, and 313, respectively. Areas that covered an entire province or state had ''0'' as the middle digit. The first area code installed was 201 for New Jersey, while the District of Columbia received the second code (202). The second digit of all original area codes was 0 or 1, while the second digit of the three-digit exchange code was never 0 or 1, thus facilitating the recognition of whether a user was dialing a full 10-digit number or merely dialing within the local area code. Toll operators were able to differentiate between the two types of areas from the middle digit of the area code when a routing operator had to be consulted. By the 1990s, the electromechanical central office switches were replaced with electronic switching system (ESS) equipment and the previous area code logic was no longer necessary. The demand for telephone numbers was increasing rapidly, and the remaining n0n and n1n combinations were insufficient to sustain growth. This area code scheme was abandoned, with the result that area codes and central office codes could not necessarily be automatically distinguished by the switching equipment. The solution was to require the dialing of a preceding 1 for calls across area codes, in which case the equipment expected 10 more digits. If the first digit dialed was not a "1", only 7 digits were expected and the area code was inferred from the originating subscriber's area code. For a short while, in some area codes, one could enter the full 11 digits for a call within their own neighborhood or just enter the last 7 digits, and the call would be routed and billed identically. The rising popularity of fax machines and pagers required far more telephone numbers than were anticipated in the design of the numbering system. As a remedy, the restrictions on the format of area codes were eased. Since 1995, over 380 new area codes were added to the North American Numbering Plan. Some areas used area code splits, by which an existing numbering plan area (NPA) was split into multiple divisions each assigned a new area code. Thus, many businesses were required to reprint business stationery, catalogs, and directories. Area code splits were often contested as to which area could keep the existing code, which usually fell to the largest city. For example, 305 was split in 1995, and had served both the Miami and Fort Lauderdale area. Dade County (Miami-Dade) kept 305 and Broward County (Fort Lauderdale area) had to change to 954. Another method was using area code overlays, which avoided renumbering existing stations. An overlay is a new area code that covers the same geographical area as an existing code. Over 75 overlays have been introduced since 1995.〔(【引用サイトリンク】url=http://greatdata.com )〕 Area code overlays typically require that the full ten-digit number is dialed. As a result of Internet-based telephony services, area codes often no longer correspond to the actual geographic locations of telephone equipment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Telephone numbering plan」の詳細全文を読む スポンサード リンク
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