[TUHS] Known Specimens of Pre-5ESS UNIX Telephone Switching Software?

Jon Steinhart jon at fourwinds.com
Tue Sep 26 11:37:54 AEST 2023

segaloco via TUHS writes:
> Hello, my studies lately bring me to the question: Are there any extant
> examples of telephone switching software, built on UNIX, from the various
> parts of the Bell System prior to the introduction of the 5ESS and 3B20D?
> My focus veers earlier as some 5ESS/3B20D/DMERT technology is still in
> active use, that sleeping dragon can lie.
> What's gotten me curious is reading about 1ESS in a BSTJ volume I
> picked up, noting the particulars on how previous concerns of manual and
> electro-mechanical systems were abstracted into software.  Even without
> surviving examples, were previous systems such as the 1ESS central
> control ever ported to or considered for porting to UNIX, or was the
> hardware interface to the telco lines too specific to consider a future
> swap-out with, say, a PDP11 running arbitrary software?  Columbus's SCCS
> (switching, not source code) also comes to mind, although all I know that
> survives of that is the CB-UNIX 2.3 manual descriptions of bits and pieces.
> By the way, it's funny, I have UNIX to thank for my current experiments
> with telephones and other signalling stuff, what with making me study the
> Bell System more generally.  It's starting to come full circle in that I
> want to take a crack at reading dialing, at least pulse, into some sort
> of software abstraction on a SBC that can, among other things, provide a
> switching service on top of a UNIX-like kernel.  I don't know what I'd do
> with such a thing other than assign work conference call rooms their own
> phone numbers to dial with a telephone on a serial line...but if I can even
> get that far I'd call it a success.  One less dependency on the mobile...
> - Matt G.

Heinz might know something about this.  If I remember correctly, one of the
projects in his group was SS1, an all-digital exchange.  I have some vague
memory of him and Carl poring over some gigantic switch statement looking
for a bug - the long distance code wasn't sending the ST pulse and as a
result all of the key pulse senders at the Berkeley Heights telephone
exchange were taken off line and needed a technician to go in and manually
reset them.  They were not amused.  Fortunately, they and BTL were both
children of Ma Bell.

If my memory serves me correctly, the system had a pair of PDP-11/10s that
ran Hal Alles's digital filter code, a PDP11/70 behind the whole thing,
Harry Breece's active replacement circuitry for the hybrid transformers,
and some huge insanely fast wire-wrapped boards designed by John Sheets
that did TDM switching.


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