Telephone Exchange invention-by Alexander Graham Bell

Telephone Exchange

“One day there will be a telephone in every major town in America.” By Alexander Graham Bell, inventor

The invention of the telephone was gigantic leap forward. However, early models were connected by a wire in pairs, meaning that you could only connect to one other telephone. This was not useful for keeping in contact with lots of different people. Early phones did not even use a ringing bell-you had to whistle into the speaker to get the other person’s attention.

On March 10, 1891, Almon Brown Strowger (1839-1902), an undertaker in Kansas City, Missouri, patented the stepping switch, a device which led to the automation of telephone circuit switching. His local exchange operator was the wife of a rival undertaker and would divert calls away from Strowger’s business.

The motivated Strowger to invent an automated exchange that recognized the number dialed in by the caller. The earliest model was a hollow cylinder with a shaft in the middle capable of moving up and down and also rotating. A connector in the shaft could make contact with many different contacts on the inside of the cylinder and so send this down the line as a pulse, hence pulse dialing and rotary dial telephones. This system ended the need for manually connected calls until it too was superseded by digital systems.

Exchanges based on the Strowger switch were eventually challenged by other exchange types and later by crossbar technology. These exchange designs promised faster switching and would accept pulses faster than the Strowger’s typical 10 pps—typically about 20 pps. At a later date many also accepted DTMF “touch tones” or other tone signaling systems.

A transitional technology (from pulse to DTMF) had DTMF link finders which converted DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid-2002.


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