Date: Tue, 18 Aug 92 00:50 PDT
From: john@zygot.ati.com (John Higdon)
Organization: Green Hills and Cows
Subject: Re: Switching Systems

sbrack@jupiter.cse.UTOLEDO.edu (Steven S. Brack) writes:

> I have, over the year I've read the Digest, become fairly telecom
> literate.  Recently, people have talked about a type of swiotch called
> a "panel" switch.  Could someone tell me what panel is, and how it
> differs from crossbar or SxS switches?

The panel switch was (for its day) an amazing monstrosity. It was also
the first major attempt at "common control", where a pool of
components would complete calls on a non-dedicated basis. Unlike the
SXS switch which operated in lock-step with the pulses from the
subscriber's dial, the panel system recorded the dial pulses and then
operated under its own timing and rudimentary programming to complete
the call.

And unlike the crossbar switch which uses a complex system of relays
known as a "marker" to control relatively simple mechanical crosspoint
mechanisms (crossbar), the panel switch used gigantic panels of
contacts over which carbon brushes would travel. The size of these
panels was responsible for the high ceilings found in old,
metropolitan central offices.

The carbon brushes were mounted at the end of rods which were
propelled by an archane system of motors and rollers. The
common-control equipment would signal a rod to start moving over a
column of contacts and then would count the pulses returned from the
panel. When the count was correct, the rod would be halted. Common
control equipment in one office could control the sliders in another
office. This was accomplished by passing the "revertive" pulses back
to the controlling office.

Due to the mechanical complexity, as well as the sheer size of the
actual call-connecting mechanism, maintenance was a nightmare.
Contamination of the carbon brushes and the contacts would cause noise
in the connections and would be noticable particularly during call
setup. Indeed, PAT has referred to an old Chicago panel office as "the
Wabash Cannonball" as the sound was similar to constant gunfire.

For a number of reasons, panel switching was only found in major
cities of the thirties. This included New York, Chicago, other areas
of the Midwest, and San Francisco/Oakland. They were also only found
in the Bell System. To my knowledge, GTE never used them, probably
because in their day they were very advanced and we all know about GTE
and innovation. Los Angeles never had panel, even in the Bell areas.

Panel switches were completely incompatible with SXS, and sending a
call from one to the other involved much headache. On the other hand,
early #1 crossbar was intentionally designed to mimic revertive
pulses, making the integration of crossbar into panel areas very easy.

Some of these switches have been maintained in working condition by
members of the Telephone Pioneers. One such switch remains in San
Francisco.


        John Higdon         |   P. O. Box 7648   |   +1 408 264 4115
    john@zygot.ati.com      | San Jose, CA 95150 |       M o o !

Date: Sat, 22 Aug 92 22:34 PDT
From: john@zygot.ati.com (John Higdon)
Organization: Green Hills and Cows
Subject: Re: Switching Systems

On Aug 22 at 22:26, TELECOM Moderator writes:

> [Moderator's Note: So with panel and SXS being incompatible, how were
> calls handled between those offices?   PAT]

There were several hacks that were used. Early on, there were
mechanical translators to convert revertive to rotary and visa versa.
While it was not much trouble sending a call from SXS to Panel the
other direction was problematic. In the former, a switch level was
connected to special trunks from the Panel office equipped with the
same type of rotary receivers that the subscribers used.

However, the only output language spoken by Panel is revertive and
this had to be converted to rotary pulses. Due to the handshaking in
revertive signalling, conversion to rotary was not a trivial task. And
the native tongue of SXS IS rotary.

Later on, calls between SXS and Panel were routed through the local
tandem switch. The first of these were crossbar, and as you will
recall crossbar speaks all traditional signalling languages fluently.
This was how calls between San Mateo and San Francisco were handled
decades ago, when San Mateo had a major amount of SXS and San
Francisco still had some Panel offices.


        John Higdon         |   P. O. Box 7648   |   +1 408 264 4115
    john@zygot.ati.com      | San Jose, CA 95150 |       M o o !