Your telephone calls and mine, those of your parents, the President, and the town drunk, were all built upon paths those relays selected in a flash of a second..

Myriad telephone wires strung from home to city to town were gathered up, unseen, into secure buildings called 'Central Offices' in every community across the nation.

Inside, electromechanical 'brains' made of thousands of relays connected those wires together creating a temporary voice path a voice path built just for the duration of your call.


Switch fabric of the network

From the late 1930's through well into the 1970's, the fabric on which those paths were switched into existence centered on one very elegant device: the crossbar switch.

Diagram of a 200-crosspoint crossbar switch. Partial cutaway view shows detail of one of the verticals. Click image for larger view.

 


Inside a Crossbar exchange

Installed in Central Offices all over the nation, this equipment was kept secure, and almost entirely hidden from the general public..

The crossbars were arranged in huge equipment racks and formed a physical matrix of 'x' and 'y' coordinates, the crosspoints of which created all voice paths in the network.

View along an equipment aisle inside a Crossbar #1 office in Brooklyn NY. The gold-colored (anodized) devices at the right are crossbar switches. Click image for a larger view. Photo courtesy of Steph Kerman.

Various functional assemblies of relays (markers, registers, senders; collectively termed 'common control') would be engaged momentarily, make routing decisions, and operate the relevant magnets on the correct crossbars to close the crosspoints necessary to create the desired talk path. In a flash.

These 'common control' units would then release and handle other call requests, leaving the established connection under the supervision of circuit-dedicated relays.


A basic call - diagrammed

These crossbars were arranged in huge equipment racks and formed a physical matrix of 'x' and 'y' coordinates, the crosspoints of which created all voice paths in the network.

Various functional assemblies of relays (markers, registers, senders; collectively termed 'common control') would be engaged momentarily, make routing decisions, and operate the relevant magnets on the correct crossbars to close the crosspoints necessary to create the desired talk path. In a flash.

These 'common control' units would then release and handle other call requests, leaving the established connection under the supervision of circuit-dedicated relays. 

The path of a single simple intra-office call (e.g., to a next door neighbor) within a Crossbar #1 office is diagrammed above.

Once your dialed digits were collected, your voice path might be built in a third of a second, and involve the split-second operation of more than ten thousand relays. 

In practice this was quite a complex process. To better appreciate this, click on the diagram above, and from there select 'view descriptive text'.

To create a voice path across a State or across the country required that many additional offices all work in concert to build a specific call path from among the incalculable possibilities; an almost unbelievable achievement of electromechanical computing, executed in fractions of seconds..

The following pages contain views of a unique museum of working central office systems in Seattle: the Vintage Telephone Equipment Museum.

 

 
       
 

TO MORE CROSSBAR VIEWS