A critical part of telephone switching is routing a call--the 'road map'--from the calling exchange to the called exchange. The further distance of the call, the more routes the call may use.
When setting up the path of a call, the switching machines need to know what route to use, and alternate routes in case the main routes are busy. Before the days of computers, the routing 'knowledge' was stored in varous ways. In the days when trunks were very expensive, it was critical to make good use of them--to have enough to meet busy hour demand, but not waste them. A great deal of effort went into efficient trunk utilization over the years.
For long distance routing, Bell Labs devised an ingenius steel punched card memory. For each area code, and sometimes for exchanges within an area code, a primary and alternate routing code was punched into thin steel cards. Magnets would pull up the appropriate cards using notched tangs, and then photocells would read off the information (an early application of transistors). That information would then guide the switching equipment. The box could hold and read 1,000 cards. Each switch had at least once box. DDD couldn't have existed without this component.
Remember, in 1950, electronic computers were still essentially laboratory curiosities. Random access memory of the size needed for routing information did not yet exist. (The disk drive wouldn't be invented by IBM until several years later and the magnetic drum wasn't big enough).
I was wondering: This random access metal card storage device seems like a very powerful tool. Would anyone know if IBM ever considered it as a read-only memory store for its early machines? It seems like a useful way to store machine accessible information.
Thanks for your help. Any comments about toll routing then or today would be appreciated. [public replies, please]
[Information taken from the Bell System Eng & Sci history, Switching, 1925-1975).