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Number pooling
Number pooling is a method of reallocating telephony numbering space in the North American Numbering Plan, primarily in growth areas in the United States.
Instead of allocating blocks of ten thousand numbers to each carrier in each community, a block of ten thousand numbers is assigned to an individual geographic rate centre. That block is then split into ten blocks of a thousand numbers each, which can be separately assigned to competitive local exchange carriers by a number pooling administrator. This reduces the quantity of wasted numbers in markets which have been fragmented between multiple carriers.
==History==
The North American Numbering Plan is based on fixed-length telephone numbers; when area codes (1947) and direct distance dialling (1951) were first introduced, North American numbers were gradually extended to a fixed length as 1 + three-digit area code + three-digit exchange prefix + four-digit subscriber number.〔(【引用サイトリンク】title=Why Not 8-digit Phone Numbers? )〕 Each central office code (exchange prefix) contained 10,000 possible local numbers - enough for a village or small town telephone exchange. A mid-size city would have multiple exchanges with multiple CO codes assigned to each.
North American mobile telephones, from their introduction in 1983, have used local numbers from the same geographic area codes as wireline services. Unlike the system in nations where mobiles have their own special area code, the recipient of a mobile call in the US or Canada pays the airtime charges.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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