The objective of numbering plan is to uniquely identify every subscriber connected to a telecommunication network. In the early stages of development, a numbering scheme was confined to a single local exchange, and exchanges were identified by the names of the towns in which they were located. This scheme works well as long as there is only one exchange per town. But as the subscriber volume grew, it became necessary to introduce more than one exchange in a town.Generally, a large centrally located exchange called the main exchanges serving the main business center of the town, and a number of smaller exchanges known as satellite exchanges serving different residential localities, were used to cope with the growing traffic in a large area. The area containing the complete network of the main exchange, and the satellites is known as multi-exchange area. A common numbering scheme was then required for the area so that the digits dialed to identify a given terminating exchange do not vary with the exchange originating the call. For calls originating from a location outside the multi-exchange area, there is a need to identify the area by a common code. The common numbering scheme is sometimes called a linked numbering scheme. In this scheme, all exchanges in a town were collectively identified by the name of the town.
The introduction of subscriber trunk dialing (STD) or direct distance dialing for intercity and intertwine long distance connections called for a national numbering plan, where multiexchange areas are identified uniquely by numbers. Subsequent development of international subscriber dialing (ISD) makes it necessary to have an international numbering plan and to have the national numbering plan conform to the international one.
A numbering plan may be open, semi-open or closed. An open numbering plan, also known as nonuniform numbering scheme, permits wide variation in the number of digits to be used to identify a subscriber within a multi-exchange area or within a country. This plan is used in countries equipped extensively with non-director Strowger switching countries equipped extensively with non-director Strowger switching network structure .In such cases, the numbering schemes usually an exact image of the network structure and requires to be changed if the network structure changes. This scheme is now almost extinct. A Semi-open plan permits number lengths to differ by almost one or two digits.Today; this scheme is the most common and is used in many countries including India, Sweden, Switzerland and the United Kingdom. In closed numbering plan or the uniform numbering scheme, the number of digits in a subscriber numbers fixed. This scheme is used by a few countries which include France, Belgium and the countries in the North America (USA, Canada, Hawaii etc).
An international numbering plan or world numbering plan has been defined by CCITT in its recommendations E.160-E.163.For numbering purposes, the world is divided into zones .Each zone is given a single digit code. For the European zone, two codes have been allotted because of the large number of countries within this zone. Every international telephone number consists of two parts. The country code contains one, two or three digits, the first digit being the zone code in which the country lies. For example, France has the country code 33 and Albania 355 and India has the country code 91 and Maldives 960.In cases where an integrated numbering plan already covers an entire zone, the ,the countries in that zone are identified by the single digit zone code itself. All the countries in the North American zone have the code as 1 and all the countries in the USSR have the code as 7.It is not clear if this code would change on account of the recent changes in the political setup of the USSR and the formation of the new Commonwealth of Independent States (CIS).The standard country codes are listed in CCITT Rec.E.163.
The existence of a world numbering plan places restrictions on the national numbering plan of each country. The number of digits in an international subscriber number is limited to an absolute maximum of 12.In practice with a few exceptions; world numbers are limited to 11 digits. As a result the number, the number of digits available for a national numbering plan is 11-N,where N is the number of digits in the country code.
In general a national number consists of three parts. The area or the trunk code identifies a particular numbering area or the multiexchange area of the called subscriber. According to CCITT international usage, a numbering area is defined as that area in which any two subscribers use identical dialing procedure to reach any other subscriber in the network. An exchange code identifies a particular exchange within a numbering area. It determines the routing for incoming trunk call from another numbering area or for a call originating from one exchange and destined to another in the same numbering area. Subscriber line number is used to select the called subscriber line at the terminating exchange. In CCITT terminology, the combination of the exchange code and the subscriber line number is known as the subscriber number which is the number listed in the telephone directory.
In closed numbering scheme of North America, and area code consists of three digits ,an exchange code three digits, and a subscriber line number four digits.Thus,a fixed 10-digit number forms the national number. In India where a semi -open numbering scheme is used, a national number has 7-9 digits. But the apportionment between the STD code and the subscriber number varies widely.STD codes have 2-6 digits and the subscriber numbers 3-7 digits. The exchange codes are 1-3 digits long and the subscriber number 2-4 digits.Broadly, the country is divided into eight regions with each region being identified by a single-digit code. A numbering area in a region always has the region code as the first digit of the STD code.
In addition to dealing with the numbering structure, a national numbering plan must also deal with appropriate dialing procedures for local, national and international calls. The term “local call” here implies a call within a numbering area and the term “national call” a trunk call between two different numbering areas within the same country.
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