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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 eachsuccessive dialling of a digit set up another step of the call.

The uniselector

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

While simple in concept, uniselectors gave automatic exchanges anew economic edge: first selectors - complex and expensivepieces of electro-mechanical equipment - could be placed in acommon pool, instead of one being permanently attached to asingle line.

Widely used in the early days of automatic telephony, such anexchange - using telephone numbers of only four digits - could

handle up to 10,000 lines. And, as early as 1904, switching insuch 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. Thisaction closes a spring-operated switch in the telephone,completing an electrical circuit. The exchange receives the signalthat a caller wishes to dial a number.

The Step by Step Process

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

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

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

When these pulses reach Selector 1, its shaft steps up a block ofcontacts, stopping (in this case) at row 4. Its arm then sweepsacross that row, testing each contact in turn. When it finds onewith a free second selector, it stops. The caller's line is nowconnected - through the uniselector and Selector 1 - to Selector2.

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Selector 2 deals with the block of 100 contacts devoted tonumbers beginning with 4. When the signal "three" arrives, theshaft of Selector 2 moves down to row 3, and the arm sweepsacross 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 numbersbeginning with 43. When the signal "eight" arrives, the shaft ofSelector 3 steps up to row 8 and sits there, awaiting the lastsignal. 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'sline. At the same time, a current to ring the bell of the callednumber is transmitted along line 4388. If the receiver is pickedup, all equipment excpet the selectors is disconnected from bothlines, and the conversation can begin.

If the called line is engaged, the busy tone is transmitted alongthe caller's line. Indeed, the same happens at earlier stagesduring 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 todescribe, it happens in milliseconds. To the caller, there is usuallyno perceptible delay between picking up the handset and hearingdial tone. Similarly, the second selector is chosen in onlymilliseconds - 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 needmany more stages and many more selectors. Nor are calls limitedto selectors in the same exchange: long-distance calls may needto pass through several exchanges.

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

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