TELECOM Digest OnLine - Sorted: Re: Unlisted Phone Number

Re: Unlisted Phone Number
26 Mar 2007 09:21:30 -0700

On Mar 25, 9:47 am, wrote:

> Another requirement for incoming DDD was that trunk groups be
> reinforced to provide at least P=.01 (not more than one call out of
> 100 would fail because of all-trunks-busy) on the final route. This
> required a considerable increase in trunking capabilities in many
> areas.

At considerable cost. Before WW II, most long distance calls were
made on a "delayed" basis, that is, the caller made his request and
would then get called back when his call was set up. There was a wait
for trunks. After WW II the increase in microwave and carrier allowed
more capacity. Further, the overall growth in telephone use generated
a higher economy of scale -- the new expensive trunks being installed
would get used. So after the war more and more calls would be
completed on a "demand" basis, and many dialed by the operator.

> Calls over the direct trunks were XB to XB and set up virtually
> instantaneously. Calls on the final route had a noticeable delay,
> since Ponca City was a step office and had to send the number to the
> Blackwell office by dial pulses.)

What is amazing is that all the routing and alternate route "logic"
was done by relays in a very sophisticated array. The Number 4
crossbar toll switch must have been an incredibly beautiful machine.
(Some of the routing info was stored on punched metal cards which were
selected out of box.)

Although these machines were not modifiable stored-program in the
modern sense of computers, they were essentially computers just the
same, with input, output, logic, and memory. Pretty head stuff for
the early 1940s!

The first No 4 crossbar was installed in Philadelphia for toll service
around 1943-1944. I've been unable to find any newspaper reference to
it, perhaps it was kept secret for the war to avoid sabotage. The
effect would be transparent to end subscribers who would continue to
place calls in the same manner, though many would be completed
quickly. In any event, at that time they didn't want to encourage
people to make any more calls since circuits were overloaded as it was
and Bell was short of staff. (Wartime newspapers have large display
ads from Bell targeted to retired or married operators to come back to
work. They also hired teens.)

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