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Short note: Telephone numbering in PSTN

Short answer

The PSTN numbering plan follows ITU-T E.164, limiting international numbers to 15 digits. It comprises an international prefix (00), country code (e.g., 977 for Nepal), area code, and subscriber number. Nepal uses an open plan where the trunk prefix 0 is dialled only for long-distance calls; locally, just the subscriber number is used.

Write a short note on telephone numbering plan in PSTN (land line telephone exchange).

A telephone numbering plan in the PSTN is the scheme that gives every landline subscriber a unique number, so that exchanges can route and charge calls from the dialled digits. National plans follow ITU-T E.164, which limits an international number to 15 digits.

Structure of a PSTN number

 00   977   1    44    12345
 |     |    |    |       |
 IP   CC   area  exch.  line
          code   code   number
      <-- NSN = area+subscriber -->
  • International prefix (00): to dial out of the country.
  • Country code (CC): e.g. 977 Nepal, 91 India.
  • Trunk prefix (0): dialled before the area code for a national long distance (STD) call.
  • Area (trunk) code: identifies the charging/numbering area, e.g. 01 Kathmandu, 061 Pokhara, 021 Biratnagar.
  • Subscriber number: exchange code + line number; the exchange code selects the local exchange, the line number selects the subscriber's line.

Types of numbering

  1. Open plan: a trunk prefix and area code are dialled only for calls outside the area; number lengths may differ (Nepal, UK).
  2. Closed plan: the full number, of fixed length, is always dialled (USA 10 digits).

Requirements

  • Unique and easy to remember.
  • Number length planned for future growth.
  • Digits allow routing and charging.
  • Special short codes for emergency and services (Nepal: 100 police, 101 fire, 102 ambulance, 197 directory).

Example (Nepal): a Kathmandu subscriber 4412345 is dialled as 4412345 locally, 01-4412345 from another district, and +977-1-4412345 from abroad.

In a large network the digits are analysed step by step: the first digit 0 sends the call to the trunk exchange, the area code selects the route, and the remaining digits select the local exchange and line.

✓ Answer checked by Claude Opus 5.5 (high effort)

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