Automatic Telephone Exchange System

Automatic Telephone Exchange System Points : automatic telephone exchange system definition Telephone Exchange is a central place throughout which one telephone can be coupled to any other telephone not directly. Large number of telephone lines are interconnected by every other all the way through the exchange. An auto telephone exchange contain a number of electromechanical switches, controlled by dialled pulses. Exchange is at all times located at the centre of population. In Strowger kind of automatic exchange, there are two unusual methods, on answering side of exchange.
1. Pre-selection
2. Line finding 1. Pre-selection In pre-selection kind of automatic exchange each subscriber’s line is complete on the input side of the uniselector call pre-selector. Wipers of uniselector are joined to subscriber line and fixed contacts are attached to the contact arms of a group of two-motion selectors, called first group selectors. The 1st group selector deal with the first digit in the number dialled. The is group selector is connected to the 2 group selector, which deal with the second digit in the number. The 2 group selector in turn is connected to the final selector, which deal with the last two digits in the number. Final selector is connected to the lines of the various subscribers, thus making the connection.

There are as many uniselectors as the number of subscribers. The respective fixed contacts of all the uniselectors are connected together. So that all the subscribers have access to each of the available 1st group selectors.

When the subscriber lifts the handset, his d.c. loop is completed and the vipers of the pre-selector start moving to find a free l group selector. This is known as hunting of the uniselector, for a free 1st group selector. When it find a free l group selector, it seizes it and the subscriber hears a dial tone. This dial tone is the signal to start dialling.

When the subscriber dials the first digit, the l group selector’s wipers step up to the desired row of contacts, corresponding to the first digit dialled and then rotates on this row till it finds a contact connected to a free 2 group selector. When the second digit is dialled, the 2nd group selector steps up to the desired row and then hunt for a contact connected to a free final selector. The subscriber controls the vertical and rotary motion of the final selector, by dialling the last two digits. When the second last digit is dialled, the wipers of the final selector steps up to the row corresponding to the digit and when the last digit is dialled, the wipers of the final selector rotates to the desired contact, without hunting. So making the connection to the wanted four- digit number dialled.

The exchange then sends, a ringing current to the called party and a ringing tone to the calling party, to let him know that the connection has been is shown. The block diagram of the pre-selection type of automatic exchange.

In the pre-selection type of exchange a separate uniselector is used for switching, to connect each subscriber’s line to a free group selector. This means that each subscriber has a separate uniselector. In such type of exchanges, if the calling rate is low, then the uniselectors remain idle for most of the time. This situation is improved by placing the switching of the subscriber’s line to a free group selector on a common switching basis. In this the subscriber’s line is connected to the switching equipment through a linefinder switch.

2. Line finding Each subscriber’s lines are connected to the bank contacts of linefinder switches and the wipers are connected to first group selectors; The linefinder switch associated with a first group selector hunts over the subscriber’s lines to find the line of the calling subscriber. The number of linefinders required is equal to the number of first group selectors.

When the subscriber lifts his handset, the wipers of the line finder starts moving on fixed bank contacts until its wipers reach the bank contact of the calling subscriber. All further switching stages following on the line finder are controlled in a similar manner as that of the step-by-step exchange.

In practice the number of linefinders and first group selectors is small compared with the number of lines served, because a linefinder is more expansive than a uniselector. And by doing so, greatest saving is obtained. However, in case of heavy telephone traffic, the economic advantages of linefinder operation does not matter. The block diagram of the line finder type of exchange.