Telephone Exchange is a central place through which one telephone can be connected to any other telephone indirectly. A large number of telephone lines are interconnected with each other through the exchange. An auto telephone exchange contains a number of electromechanical switches, controlled by dialed pulses. The exchange is always located at the centre of population.
In the Strowger type of automatic exchange, there are two alternative methods, on the answering side of the exchange:
- Line finding.
In the pre-selection type of automatic telephone exchange each subscriber's line is terminated on the input side of the uniselector called pre-selector. The wipers of the uniselector are connected to the subscriber line and the fixed contacts are connected to the contact arms of a group of two-motion selectors, called first group selectors. The lsl group selector deal with the first digit in the number dialed. The 1st group selector is connected to the 2nd group selector, which deal with the second digit in the number. The 2nd group selector in turn is connected to the final selector, which deal with the last two digits in the number. The final selector is connected to the lines of the various subscribers, thus making the connection.
There are as many uni selectors as the number of subscribers. The respective fixed contacts of all the uni selectors 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 wipers of the pre-selector start moving to find a free 1st group selector. This is known as hunting of the uniselector, for a free 1st group selector. When it find a free 1st group selector, it seizes it and the subscriber hears a dial tone. This dial tone is the signal to start dialing.
When the subscriber dials the first digit, the 1st group selector's wipers step up to the desired row of contacts, corresponding to the first digit dialed and then rotates on this row till it finds a contact connected to a free 2nd group selector. When the second digit is dialed, 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 dialing the last two digits. When the second last digit is dialed, the wipers of the final selector steps up to the row corresponding to the digit and when the last digit is dialed, the wipers of the final selector rotates to the desired contact, without hunting. So making the connection to the wanted four-digit number dialed.
Block diagram of the pre-selection type automatic exchange.
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 in the figure, established. 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 uni selectors 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 line finder switch.
Each subscriber's lines are connected to the bank contacts of line finder switches and the wipers are connected to first group selectors. The line finder switch associated with a first group selector hunts over the subscriber's lines to find the line of the calling subscriber. The number of line finders 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 line finders and first group selectors is small compared with the number of lines served, because a line finder 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 line finder operation does not matter.
Merits & Demerits of Automatic Telephone Exchange
The merits of an automatic telephone exchanges given below:
- In the auto telephone exchange the subscriber himself is responsible for making connection with other subscribers, without the help of human operator, by dialing the number.
- In this type of exchange the possibility of getting a wrong number is very much less.
- The quality of service is good. The service is uniform throughout the day and night.
- The provision for privacy is high. As the conversation can not be overheard or monitored without introducing additional circuits.
- The traffic handling capacity in the auto exchange is high, so the connections between subscriber are established much faster. In case the switching equipment is inadequate or busy, busy tone is returned to the calling party, to give him the impression that the called party is engaged.
- The task of fault finding is very, much simplified due to the provision of automatic routines.
The demerits of an auto telephone exchange are given below:
- The cost of installation of an auto telephone exchange is very high.
- The maintenance of an auto telephone exchange is costlier. As it requires more skilled staff.
- The switching equipment of an auto telephone exchange is more sensitive to dust.
- The switching equipment of an auto telephone exchange is noisier.