larry@uunet.uu.net (Larry Lippman) (09/10/89)
TELECOM Digest Sat, 9 Sep 89 17:10:00 CDT Special: SxS Office Tour
Today's Topics: Moderator: Patrick Townson
Musings on Step-by-Step Central Offices (Larry Lippman)
[Moderator's Note: I received this essay several days ago, but decided to
hold it until the weekend and send it as a special edition, since you may
want to print it out and keep it in your files for future reference. I also
want to take this opportunity to say thanks to Mr. Lippman for submitting
this to the Digest, and note that his submissions are among the best and
most interesting of the many items received each day. Enjoy it! PT]
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Subject: Musings on Step-by-Step Central Offices
Date: 4 Sep 89 13:52:13 EDT (Mon)
From: Larry Lippman <kitty!larry@uunet.uu.net>
In article <telecom-v09i0336m04@vector.dallas.tx.us> bmk@mvuxi.att.com
(Bernard Mckeever) writes:
> I have to second, or maybe third or forth by now, the
> recommendation to visit a Step By Step [SXS] office while they
> are still around. The stepper is a wonderful machine that was
> actually fun to work on and the descriptions that were provided
> bring back fond memories. I can't provide details of how to
> arrange a visit, but, if you know someone at the local Telco ask
> them to take you around the office, this always works for me.
I certainly agree with Bernard Mckeever - a SxS CO would be a great
place to visit. Unfortunately, SxS offices are becoming a thing of the
past, and are non-existent in many areas - even if you could manage to
find a "cooperative" telephone company employee.
Some of what I am about comment on will be lost on many people, but
it should bring a real smile to those readers who have had any firsthand
experience in an SxS CO.
> If your lucky enough to go through at night when the traffic is
> light, you can hear calls as they proceed from line finder to
> selectors to connectors. One of the best trouble shooting tools
> of the stepper switchman is a good ear.
When an SxS office is relatively quiet, a good ear can really be
a useful diagnostic tool. A few examples:
1. A faulty selector, trunk or open wire in a Tip-Ring-Sleeve triple
will cause the switch train to abruptly collapse and immediately
seize the next available linefinder-first selector, all occurring
with a rather characteristic sound.
2. A faulty preference chain circuit in a linefinder frame, or a fault
in a subscriber line/cutoff relay circuit will cause the linefinders
in a given linefinder frame to "thrash" wildly with a truly
unforgettable sound.
3. A group of outgoing trunks that are out of service, say due to a
carrier system failure will result in an ATB (All Trunks Busy)
condition having a characteristic "BZZZZZT!" sound as a selector
runs around into the 11th ATB position while trying to find an
idle trunk. In those CO's equipped with ROTS (Rotary Out Trunk
Switch), the same sound, but slightly different, will emanate
as the 22-position ROTS step switch runs around like a ferris wheel
trying to find an idle trunk.
> It also helps to have good eye sight, SXS offices tend to be very dark.
An abyssmal job is fixing a bank multiple wiring problem, while
sandwiched between the bank wiring sides of two adjacent rows of switch
frames. It's cramped and dark, with the primary illumination coming
from a portable 48-volt spotlight. And just when you found the problem,
you pull the spotlight cord too far and an alligator clip comes off the
battery cord, plunging the work area into twilight again. :-)
> Today most remaining SXS offices have evolved far enough to
> provide Touchtone service, and use digital transmission systems
> for interoffice communications.
During the early 1960's the Bell System developed grandiose plans
for conversion of SxS CO's to permit offering DTMF service. The basic
scheme involved placing a simple trunk circuit between each linefinder
and its associated first selector. When this trunk circuit detected a
service request from a linefinder it would signal a simple arbitration
circuit (I hesitate to use the term "marker") which would select an idle
register-sender and connect it to the linefinder through a crossbar switch.
The register-sender circuit would decode the DTMF tones using a receiver,
store the decoded digits, and then outpulse them to the first selector.
The register-sender would release and be available for another call upon
decoding and outpulsing a predetermined number of digits, or upon a timeout.
From a human factors standpoint converting SxS to DTMF presents a
problem in that it is REALLY NO FASTER than rotary dialing. While a
subscriber may lift their handset and DTMF-dial 7-digits in less than
2 seconds, they may have to wait for at least 7 seconds of *silence* while
the DTMF digits are outpulsed as rotary digits.
As public "demand" for DTMF service grew in the 1970's, most Bell
System operating companies elected NOT to convert SxS offices for DTMF
service, but to simply wait for eventual total replacement with 2ESS or 3ESS.
The reasons for the lack of conversion effort were many, and included:
(1) "poor" financial investment in already-obsolete apparatus; (2) less than
optimal human factors acceptance; (3) lack of other features that the
public was demanding which could NEVER be provided in a SxS CO, like
call forwarding, call waiting, etc.; (4) desire to implement message unit
timing for all subscribers, whose implementation through a CDA (Call Data
Accumulator) was very expensive; etc.
As a result, during the 1960's and 1970's comparatively few SxS CO's
were ever converted to DTMF. While WECO had designed conversion apparatus,
very little was manufactured, and the Bell System elected to do something
which I always felt was unusual: they used conversion apparatus manufactured
by ITT in many small SxS CO's. ITT makes pretty strange apparatus as
compared to WECO, and I could never understand why the Bell System chose to
give WECO installers and BOC switchmen the culture shock of dealing with this
apparatus. The only Bell System SxS CO's that I have personally seen (during
the late 1960's, early 1970's) converted to DTMF used the ITT "Tel-Touch"
apparatus.
The only WECO apparatus that I have personally seen for DTMF
conversion in SxS used a clever, but bizarre scheme of "magnetic core logic"
(really!), and was intended for conversion of 701-type SxS PBX's. The
operation of this apparatus is similar to that described above, except that
DTMF matrix-to-digit translation was performed using magnetic cores, and the
digits to be outpulsed were stored in magnetic cores (really!). Such
upgraded 701's, which were relatively few and usually a Centrex CU switch,
represented a unique blend of SxS, crossbar, wire-spring relay, magnetic
core logic and solid-state circuitry never again found anywhere else!
It is more common to see independent operating telephone companies
still running SxS apparatus that is converted to DTMF. The technology
most common in this marketplace uses solid-state register-sender circuits
which are dedicated to one linefider-first selector, thereby eliminating
any need for crossbar switches or any common control apparatus. With
today's integrated circuits, one can manufacture such a register-sender
on a single printed circuit card for a cost of less than $ 35.00 - but
25 years ago such an attempt would have required at least six cubic
feet of space PER CIRCUIT with an apparatus cost 100 times that amount!
> Way back when, the only modern
> features you could get with a stepper were number hunting,
> [rotary or level] and toll diversion if you were in a PBX
> selector group. Of course you could always have a 1, 2, or 4
> party line and in some cases 8 party rural service, not to
> mention message rate service.
There were also emergency alerting systems for use in small towns
with volunteer fire departments which would ring the telephones of
pre-determined firemen in a distinctive fashion when an alarm switch was
operated at the fire station.
> Yes indeed, bending relays, oiling switches, and adjusting wipers, what
> fun.
Adjusting relays using an assortment of bending tools, tension
gages and a 35-type relay test set is an unforgettable skill - sort of
like learning to ride a bicycle. Also like learning to ride a bicycle,
one thoroughly screws up a lot of relays before learning the art. :-)
Words alone cannot do justice to a 35-type test set. This test set, whose
principal design evolved BEFORE 1920, has been virtually unchanged for
almost 70 years - and is STILL IN USE TODAY. The 35-type test set has
four miniature telegraph-style keys, plus 8 resistance sliders which move
across a tubular ceramic resistance coil, and is used to set the various
operate/non-operate/release current values for the windings on a relay.
A SxS maintenance job that is the real pits was cleaning switch
bank contacts using "circular" toothbrushes dipped in trichloroethylene;
this was before the days when this solvent was recognized as hazardous,
and extensive cleaning in a poorly-ventilated CO would result in the
switchman getting a good "buzz" from the vapors.
The most complex circuit I have ever seen in a "traditional" SxS
CO was the SD-31592 Prepay Coin Control Trunk. Any switchman who could
understand and troubleshoot this circuit was a real craftsman! A bizarre
bend of old and new technology resulted was when LCOT (Local Coin Overtime)
circuits were added to these trunks. LCOT used integrated circuits and
optoisolators for interface to the 48 volt SxS apparatus. Considering
that these SxS coin control trunks were designed in the 1930's, LCOT was
something to behold.
> And who could ever forget digit absorbing selectors, rotary out trunk
> selectors [ROTS], E-2 repeaters, level hunt connectors, and the brush
> replacement routines for the ringing machines and motor generators. No
> rectifiers for that tough switch.
Digit-absorbing selectors were rather clever. The digit levels
to be absorbed were stored in memories which used BMF technology.
That stands for Bent Metal Fingers. :-) There was this U-shaped piece
of metal with 10 metal fingers on each side, which were bent with a
needle-nose pliers. This metal part (called a "normal post cam") was
attached to the selector switch, and rose vertically as a digit was dialed,
with the bent fingers actuating a roller switch at a given digit position.
Another interesting facit of digit-absorbing selectors is that depending
upon the digit sequence to be absorbed, in many case ONLY the LAST digit
had to be dialed. This resulted in a dialing "short-cut". For example,
if you were served by a CO with a prefix of say, 778, you could usually dial
other subscribers within the same CO by dialing 81234 instead of 7781234.
Dialing 777777777777781234 would also work. :-)
Well, there was the 100A and similar power plants which used
mercury vapor rectifiers that gave off a pleasant purple glow. Also
interesting is that in smaller CO's the +110 V and -110 V collect and
return supplies for coin telephones were furnished by large dry cell
batteries. No rechargeable batteries, M-G sets or DC/DC converters -
just dry batteries which were periodically replaced.
SxS design philosophy before 1960 was dead set against anything
electronic. Even a simple rectifier diode - which in some cases could
eliminate a whole relay - was taboo, despite the reliability and use
of such diodes for over ten years. Timing circuits were implemented in
five ways: (1) thermal time-delay relay; (2) Adlake pneumatic dash-pot
relay; (3) motor-driven timer; (4) common two-pulse interrupter timing
(where the resultant timing value could vary by a factor of 2); and
(5) the three-element cold cathode electron tube - which required 130
volts to operate, however.
> And what a marvel the power room is. For the life of me I don't
> know why some people were afraid to work in one. Just remember to
> use only brass tools around the counter cells, we wouldn't want
> one of those things blowing up and sending glass all over the place.
In view of today's technology, counter-EMF cells are unique - to say
the least. Most of them used potassium hydroxide as an electrolyte, on
top of which was poured mineral oil to retard evaporation. 1960's vintage
counter-EMF cells finally got around to solid-state diodes.
> And last of all, don't forget the rubber gloves aprons and your face
> shield when measuring the specific gravity of the batteries.
While I have only seen them in pictures, until the 1960's there
were some large central offices which had battery rooms with open-top
batteries. That's right, open-top glass or hard rubber containers with
lead plate arrays just hanging over the side walls of the container.
And these were BIG cells, with capacities of 2-3,000 ampere-hours.
> As for all them copper bars and 600 amp fuses, don't worry it's only 48
> volts.
ONLY 48 volts? Believe me, on a hot, sticky summer day 48 volts
can feel like 480 volts! While the primary battery voltage was only
48, there was serious current behind it which could result in real-life
pyrotechnics that are worthy of a hokey sci-fi movie. Power cable was
primarily of the RHW-type, which used a rubber insulation and deteriorates
over time. There is nothing more harrowing than pulling a new cable in a
cable rack and watching a section of old 750 MCM insulation disintegrate
into dust, exposing the bare conductor. 750 MCM carrying 48 volts will arc
and burn through 1/4 inch cable rack steel long before it blows a fuse!
> For anybody that does not understand what I'm talking about, or
> who remembers parts of SXS switching but not the rest, take
> heart. If desired I will provide all the detail you ever needed
> in 10 or 15 short easy lessons, about one every few weeks.
I'll reminisce some more, if you will. :-)
> REMEMBER ESS MEANS EVENTUALLY STEP X STEP
I used to think that way too, except that advances in parallel
processing may someday make us both wrong. :-)
<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"
[Moderator's Note: And my thanks to Mr. McKeever, whose original article
sparked the reply by Mr. Lippman. I hope both of you will post again. PT]
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End of TELECOM Digest V9 #362
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