Strowger or Step by Step Exchanges: The Principles

Consider a small automatic exchange built on the Strowger model - the type generally known as step-by-step, because each successive dialling of a digit set up another step of the call.

The uniselector

Invented by American engineer Alexander Keith, the sole task of the uniselector was to hunt for a free bi-motional selector - a task which it carried out with a sweep like a clock arm along a single row of electrical contacts.

While simple in concept, uniselectors gave automatic exchanges a new economic edge: first selectors - complex and expensive pieces of electro-mechanical equipment - could be placed in a common pool, instead of one being permanently attached to a single line.

Widely used in the early days of automatic telephony, such an exchange - using telephone numbers of only four digits - could handle up to 10,000 lines. And, as early as 1904, switching in such exchanges had been streamlined by the new uniselector.

To understand these early exchanges, let us follow, step by step, the setting up of a call.

The step-by-step process

The call begins when the caller lifts a telephone handset. This action closes a spring-operated switch in the telephone, completing an electrical circuit. The exchange receives the signal that a caller wishes to dial a number.

The Step by Step Process

On receiving the signal, the arm on that line's uniselector begins wiping across a row of contacts. It passes those linked to busy first selectors, stopping only when it finds a contact whose first selector is free.

The caller's line is now connected, through the uniselector, to a selector from the common pool - Selector 1 . Special equipment then swings into action, transmitting dial tone down the line. The caller can now dial a number - for example, 4388.

When the caller dials 4, the dial, flipping back to rest, interrupts the current four times. The signal, "four", is transmitted to the exchange.

When these pulses reach Selector 1, its shaft steps up a block of contacts, stopping (in this case) at row 4. Its arm then sweeps across that row, testing each contact in turn. When it finds one with a free second selector, it stops. The caller's line is now connected - through the uniselector and Selector 1 - to Selector 2.

Selector 2 deals with the block of 100 contacts devoted to numbers beginning with 4. When the signal "three" arrives, the shaft of Selector 2 moves down to row 3, and the arm sweeps across it, hunting for a contact whose selector is free. When that selector - Selector 3 - is found, the hunt ends.

Selector 3 deals with the 100 contacts devoted to numbers beginning with 43. When the signal "eight" arrives, the shaft of Selector 3 steps up to row 8 and sits there, awaiting the last signal. When "eight" arrives, the arm wipes across to contact 8. The call is now connected to the required number - 4388.

The exchange then tests the status of the line. If it is free, equipment to generate the ring tone is connected to the caller's line. At the same time, a current to ring the bell of the called number is transmitted along line 4388. If the receiver is picked up, all equipment excpet the selectors is disconnected from both lines, and the conversation can begin.

If the called line is engaged, the busy tone is transmitted along the caller's line. Indeed, the same happens at earlier stages during the call, whenever a free selector cannot be found - although exchanges typically have enough selectors to make such events rare.

Quick as a flash

While the process of setting up a call takes a long time to describe, it happens in milliseconds. To the caller, there is usually no perceptible delay between picking up the handset and hearing dial tone. Similarly, the second selector is chosen in only milliseconds - before the caller begins to dial the second digit.

Modern step-by-step exchanges

To deal with 8 or 10-digit numbers, step-by-step exchanges need many more stages and many more selectors. Nor are calls limited to selectors in the same exchange: long-distance calls may need to pass through several exchanges.

How are such calls connected? Say the number is 9555 5678. When 9555 (the exchange prefix) is dialled, the caller's line is connected to an outgoing line in another exchange. To make this possible, exchanges are linked by trunk and tandem exchanges.

Adapted, with acknowledgement, from Telstra educational materials.

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