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1.2 NUMBERING PLANS 9Local exchange, local office, central office, end officeIntermediate exchange, tandem exchange, toll exchange, transit exchangeInternational switching center, gateway, international exchangeTrunk, junction, circuitTelecom, administration, carrier, operating company, telephone company, telco,service providerExchange, switchSwitchblock, switch fabric1.2 NUMBERING PLANSThis section explores the formats of the numbers (sometimes called addresses) thatidentify the subscribers of telecommunication networks.Subscriber Numbers (Directory Numbers). The geographical area of a nationis divided into several numbering areas, and subscriber numbers (SNs) identifysubscriber lines within a particular numbering area. A SN consists of an exchangecode (EC) that identifies an exchange within a numbering area, followed by a linenumber (LN):SN ¼ EC-LNNational Numbers. Within a country, a subscriber is identified by a nationalnumber (NN), consisting of an area code (AC), which identifies the numberingarea, followed by a subscriber number:NN ¼ AC-SN ¼ AC-EC-LNInternational Numbers. Worldwide, a subscriber is known by an internationalnumber (IN) that consists of a country code (CC), followed by a national number:IN ¼ CC-NNThe generic format for the international numbering plan is specified by ITU-T inRec. E.164 [2]. Three types of numbering schemes are supported by E.164, all withthe CC component having up to three digits and the total IN number having amaximum of 15 digits:1. Numbering plan for geographic areas (subscriber numbers)2. Numbering plan for global services (e.g., Freephone numbers)3. Numbering plan for networks (other than the telephone network)