TELECOM Digest OnLine - Sorted: Re: Bell Usage of IBM Computers For Switching

Re: Bell Usage of IBM Computers For Switching
7 Jul 2005 07:34:09 -0700

Tony P. wrote:

> 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

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