Q. How numbering plan is achieved in modern telephony?
Ans:
The purpose of numbering plan is to uniquely identify every subscriber connected to a telecommunication network. A numbering plan may be closed, semi open or open. An open-numbering plan 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 nations equipped extensively with non-Director Strowger switching systems. In such cases numbering scheme is generally an exact image of network structure changes. A semi-open plan permits number lengths to differ by almost one or two digits. Today, this technique is the most common and is used in many countries. In closed numbering plan or Uniform numbering scheme, number of digits in a subscriber number is fixed. A world numbering plan or international numbering plan has been defined by CCITT in its recommendations E.160-E.163. For numbering purposes, world is divided into zones as displayed in figure. Every zone is given a single digit code. For European zone two codes have been allotted due to large number of countries within this zone. Each international telephone number comprises two parts as demonstrated in figure. Country code contains one, two or three digits, first digit being the zone code in which the country lies. In cases where an integrated numbering plan already covers an entire zone, countries in that zone are identified by single digit zone code itself.
Existence of world numbering plan places restriction on the national numbering plan of each country. Number of digits in an international subscriber number is limited to an absolute maximum of 12. In practical, with a few exceptions, world numbers are limited to 11 digits. As a result, number of digits available for a national numbering plan is 11-N, where N is number of digits in country code.
Generally a national number comprises three parts as demonstrated in figure. Area or trunk code identify a particular numbering area or multi exchange area of the called subscriber and thus determine routing for a trunk call and a charge for it. According to CCITT international usage, a numbering area is identified as that area in that any two subscriber use identical dialling procedure to reach other subscriber in the network. An exchange code identifies a specific exchange within a numbering area. It determine routing for incoming trunk call from another numbering area or for a call originating from one exchange and destined to another one in same numbering area. Subscriber line number is used to select called subscriber line at terminating exchange. In CCITT terminology, combination of exchange code and subscriber line number is called the subscriber number which is number listed in the telephone directory.