The first servers

Wednesday 29 October 2008This is nearly 15 years old. Be careful.

Doing some research into the theory of load testing and traffic loads, I read about the Poisson distribution, which led to Agner Krarup Erlang, which led to early phone switches.

It’s fascinating to realize that the work we do every day with web servers, which seems like a recent modern technology, was predated by guys like Erlang working with early phone switches over 100 years ago. Phone switches were the first servers: central machines connected to a large number of potential clients. In building these switches, the early engineers had to figure out from scratch how to anticipate the possible work load, so they could build switches large enough but not too large. The whole of queueing theory springs from the theories worked out by telephone switch engineers.

And they were clever guys, even adjusting the UI to lighten the load on the switches. When dialing a rotary phone, the particular digits determined how long the switch was engaged before the call could be routed. So when they allocated area codes,

the biggest population areas [got] the numbers that took the shortest time to dial on rotary phones. That is why New York City was given 212, Los Angeles given 213, Chicago 312, and Detroit 313, while Vermont received 802 (a total of 20 clicks, 8+10+2). Four areas received the then-maximum number of 21 clicks: South Dakota (605), North Carolina (704), South Carolina (803), and Nova Scotia/Prince Edward Island in the Canadian Maritimes (902).


That's funny. I noticed the correlation between population centers and "low click" numbers years ago, but always assumed it was for the benefit of the callers rather than the equipment.
Cororally: 911 is used because it is unlikely to be dialed by relay switching 'noise' that may occur during an electric storm (imagine 111, for example; false switches could occur during the dial time much more easily) . Other emergency numbers around the world, e.g., 999, 000, are used for the same reason. It'd be interesting to know if using 999 instead of 911 makes a noticable difference to the number of accidental 911 calls.
I worked in that industry for many years. The dial scheme was originally more complex than just the area codes. The original plan was NYX-NNX-XXXX where Y = {0,1}, N = {2 .. 9} and X = {0 .. 9}. Thus when the second digit was dialed, it was immediately apparent whether it was an area code (0 or 1) or an office code (never 0 or 1).
This was important if you were parsing the clicks, or tone sequence, as it happened. This was how the relay based "stepper" switched worked... each click moved your selector level.
Due to number shortages, the NNX became NXX, and later area codes became NXX also. The whole stepper process became unnecessary because computer-controlled switching became the norm. Who woulda thunk?

Oh, and Mr. Poisson originally developed his tables as an exercise he developed trying to determine what process he could use to kill his wife that would minimize his chances of being caught. He went ahead and poisoned her, and was caught anyway, or his science may have never been known.

@weshowe: Poisson = fish (in French)

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