Re: Bell Usage of IBM Computers For Switching

From what I'm to gather the phone switches themselves had their own > processors.

Yes, they did and do now.

As to processor requirements, I don't know but in the case of a switch > the more critical component is the t/d matrix. All the computer does > is keep track of call store which is nothing but a table.

A computer must break down every function into tiny little steps handled one by one. For instance, IIRC, on a PC when you depress key you're actually sending two signals to the processor -- one that you depressed a particular key, and a subsequent signal that you released that particular key. The processor must respond appropriately to the key or combination of keys you depressed, then the operating system and/or application program is passed the information.

Likewise with a telephone, when you lift the receiver the processor must detect that, connect you to a dial-pulse receiver and send you an audible dial tone and then interpret your dialing -- all this before it even actually 'switches' your call. And there's the optional "flash" signal which calls in special routines.

All of this work can either be done by the central processor (which eats up cycles) or by sub-processors to take the load off. There are cost and performance issues with each approach. Generally, cheaper computers (phone or digital) do it all in the processor while more sophisticated ones offload to give more speed.

(It was like adding a math co-processor in PC early days to get more heavy math speed. The regular processor could do math, but the co-processor did it faster.)

In the early days of ESS Bell Labs came out with a sub-processor to take some load off the main one. Doing this offered more capacity to handle calls at a modest cost. This unit was later discontinued when faster processors didn't need it.

Another use of this concept was with outside loop concentrators. Some concentrators in the field had sophisticated logic in them which freed up the central office from doing certain chores (I think a fancy concentrator could even connect calls within itself without help from the central). But such field units were expensive and not worth the cost. There were always tradeoffs to be made.

But I have seen references to DEC PDP series computers being used to > write the code, etc. for the switches.

Getting back to the original question: Message Accounting is something that can be done by the processor or a separate machine. In #5 Crossbar it was done separately. (AMA machines were critical to customer DDD). Anyway, Bell used electronic computers instead of its own electro-mechanical AMA machines to time and record phone calls. Originally it used an IBM System/7, but then switched to PDP.

My impression is that Bell tended to favor PDP gear over IBM for many applications. Also, then tended to home-build pretty much everything else.

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