Numbering Plan

The objective of the numbering plan is uniquely identify every subscriber connected to telecommunication network.

A large centrally located exchange called the main exchange serving the main business centre of the town and number of small exchanges known as satellite exchanges serving different residential localities.

The area containing the complete network of the main exchange and the satellites is known as multiexchange area.

A common numbering scheme was then required for the area so that the digits dialed to identify the terminating exchange do not vary with the exchange originating the call. The common numbering scheme is called a linked numbering scheme.

Subscriber Trunk Dialing or Direct Distance Dialing(STD or DDD)International Subscriber Dialing (ISD)

A numbering plan may be Open (Nonuniform Numbering Plan)Semi openClosed numbering plan(Uniform Numbering Plan)

There are four possible approaches to dialing procedures

1. Use a single uniform procedure for all calls, viz. local, national and international calls.

2. Use two different procedures, one for international calls and the other for local, national calls.

3. Use three different procedures, one for international calls second for national trunk calls third for local.

4. Use four different procedures, three procedures same as above and a fourth procedure for calls in the adjacent numbering areas.

Another important aspect of the numbering plan is the direct inward dialing (DID) facility for PABXs.

With the advent of electronic switching , most of the PABXs provide direct dialing access to the public networks i.e., direct outward dialing (DOD).

There are basically two approaches to providing DID facility.1. Use a set of suffix digits to national number to

identify PABX extensions.

2. Allot a national number to each PABX extension.

Charging Plan

A charging plan for a telecommunication service levies three different charges on a subscriber:

1. An initial charge for providing a network connection

2. A rental or leasing charge

3. Charges for individual calls made.Certain operational costs are incurred even if the network carries no traffic. These are covered by rental.

The technical progress in trunk transmission has resulted in significant cost reduction in trunk networks whereas the local network service still continue to be expensive.

By feeding revenue from one service to another, the subscribers are given reasonable tariff structures for both local and long distance services.

Charging for individual calls is accounted for by using either a metering instrument connected to each subscriber line or a metering register assigned to each subscriber in the case of electronic exchanges.

The count in the meter represents the number of charging units. A bill is raised by assigning a rate to the charging unit. The count is incremented by sending a pulse to the meter.

Methods for individual calls fall under two broad categories:

Duration independent charging

Duration dependent charging.

Local calls with in a numbering area are usually charged on a duration independent basis. The charging meter is incremented once for every successful call, i.e. whenever the called party answers.

The scheme of sending more than one pulse for a call is known as multimetering.

In the multiexchange case the control system in the originating exchange sends as many pulses as the number of exchanges in the connection, to the calling subscriber meter.

To avoid the capital cost of providing meters and the operating costs of reading them at regular intervals and preparing the bills, some administrations have adopted a flat rate tariff system.

flat rate tariff system is advantageous to subscribers who make a large number of calls but unfair to sparing users. To reduce the disparity, business subscribers are charged a higher flat rate compared to domestic subscribers.

Some administrations combine the features of flat rate and call rate charging.

In the case of duration dependent charging, a periodic train of pulses from a common pulse generator operates the calling subscriber’s meter at appropriate intervals. This method is called periodic pulse metering.

The traffic carried by a telecommunication network varies throughout the day. The quantum of switching equipment and junction plant provided in the network is based on the estimated busy hour traffic.

Trunk calls are almost invariably charged on a duration dependent basis.

In addition, the charges also depend on the radial distance between the calling and the called stations. That is, the trunk call charging is based on the distance-time product.

When DDD or STD facility is used to establish a long distance call, charging is usually accounted for by pulsing the meter at an appropriate rate.

Depending on the time of day and the distance involved between the stations, the meter pulsing frequency varies.

In countries that employ a flat rate tariff for local calls, subscribers do not have meters and an automatic method known as automatic toll ticketing or automatic message accounting is used for trunk call charging.

When ISD is used, call charging is carried out by the same methods as for STD. However, pulse metering may encounter two difficulties.

For international connections, it is necessary to associate call metering with incoming and outgoing international circuits in addition charges for calls passing through more than one country and to collect charges from other administrations as required.

In countries where electronic exchanges are not yet prevalent, operator assistance is required to obtain special trunk services like person-to-person calls and reverse charging. These services are expensive.

Public telephone booths use coin operated boxes which are two types: prepayment and post payment type.