September 1961 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
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Call forwarding, call waiting, call holding, speed dialing, conference
calling, all of these features are taken for granted with mobile
phone and VOIP phone service and are included in the base service
package. It will cost you extra if you subscribe to a local legacy
POTS (Plain Old Telephone Service)
provider. What is standard now was considered ground-breaking technology
in the early1960s. When phone calls were processed via human operators
manipulating patch cords and then electromechanical relays, it was
enough to simply place a successful call and not be interrupted
or disconnected. Once transistorized circuits entered the scene,
much more was possible, and phone system engineers were quick to
exploit the technology. Sophisticated decision making requires both
logical circuits and a form of memory. Logic could be provided using
hard-wired diode steering, but a fast and efficient means of storage
was needed that did not involve vacuum tubes. That is where the
photo-optic based 'flying
spot store' came in. It 'wrote' digital data onto a strip of
fim that was scanned by a light beam to read back information.
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Operation Telephone 1965
By Ken Gilmore
Within the next four years, American's telephones will
undergo a major and far-reaching innovation when the new,
all-electronic "centrals" take over.
The time: a day in 1965. You're planning to spend the afternoon
at a friend's house. But you're also expecting an important telephone
call. You pick up your phone, dial first a special code number,
then your friend's number. This done, you leave for his house, knowing
that all calls to your number will be automatically switched to
his. When you return home that evening, you dial another code number
and incoming calls are once again routed to your own phone.
This special service - and dozens of others just as advanced
- will soon be available to you. Already, a prototype all-electronic
telephone central office is in operation in Morris, Illinois. And
it's delighting subscribers with services which make present-day
systems seem as obsolete as a hand-crank on an old-fashioned wall
Special Services. Within a few years - as versatile
all-electronic equipment replaces the present relay-switching systems
- your phone will perform such tricks as these:
- You're talking to a friend about a new stereo amplifier you're
planning to buy. But you need more information. So without either
of you hanging up, you simply dial your dealer's number. A few seconds
later he is connected into the circuit, and all three of you can
discuss the amplifier at will. You can even continue calling additional
numbers - as many as you like - and all will be connected so that
everybody can talk to everyone else.
- There are several numbers you call regularly. A word to the central
office, and each of these "regulars" is assigned a special two-number
code. Then, instead of having to dial the usual seven-digit number
each time, you simply dial "12" when you want your office, "13"
for the corner drugstore, "14" for a friend you call often, and
Control center (three photos, above) of world's first
all-electronic telephone central office, now serving customers
in Morris. Illinois, is but a portion of overall network
shown in block form below. The system was developed by Bell
- You run a small business and don't want to miss any incoming
calls. You make the proper arrangements, and if your office line
is busy when someone dials it, your home phone rings automatically.
If your home phone is busy, too, a third number - perhaps an answering
service - will ring, and so on for as many alternate numbers as
These are just a few of the scores of special services you'll
enjoy when electronics takes over completely. With the new system,
switching and routing of calls - now done by relatively slow-moving
relays - will be accomplished with no moving parts at all. Hordes
of electrons rushing through transistors, diodes, and gas tubes
will do the job, and they'll do it within millionths of a second.
Thus, the all-electronic system will be able to perform thousands
of different operations, carrying out extremely complex switching
operations impossible with present equipment.
Electronic "Central." To see how the new system
works, let's take a look at what will happen to the central office
- the heart of any telephone system. At this giant terminal, the
wires from your phone, thousands of others in your area, and trunk
lines from communities all over the country are brought together.
The sole purpose of all the complicated gear at the office is to
connect the line from your phone to that of any other phone you
want to reach.
In the old days, this was a simple job. An operator simply took
a plug connected to your line and pushed it into a jack, connecting
you with the number you wanted. Then she pressed down a lever to
ring the bell.
A few years later, the dial system came along and substituted
automatic relays for the plugs. Every time your dial clicks, a number
of relays move. When you have finished dialing, the clicking relays
have selected a single phone and connected your line to it.
In the new electronic system, a giant computer with a special
scanner checks over every line coming into the central office to
see whether it is in use. It does this job so quickly that it takes
just one-tenth of a second to check all of the thousands of lines
terminated at the central office. As soon as one check is over,
it starts another. Thus, every line is checked to see whether it
is idle or busy ten times every second, twenty-four hours a day.
Scanner-Computer Circuit. Most of the time any
given line will be idle - the phone will be "on the hook." But when
you pick up your telephone to make a call, the scanner notices not
more than one-tenth of a second later that your phone is no longer
idle, and notifies the computer. In the next few millionths of a
second, this electronic brain performs a complex series of operations.
First, it checks its memory to see if a change was made when
you picked up your telephone. It finds that there is no record of
your phone having been in use a tenth of a second before. It then
checks to see if your line is ringing. If it is, your picking up
the phone would be in answer to the ring. If there is no ringing,
the system concludes that you picked up the phone because you want
to make a call.
Having reached this conclusion, the computer switches the dial
tone onto your line to notify you that it is ready for you to dial.
At the same time, it writes your phone number on what engineers
call "an electronic scratch pad" - a temporary memory circuit. It
also reserves a space on the "scratch pad" to record the number
you dial. Finally, it steps up the number of times your line is
being scanned from the regular 10 per second to 100 per second,
so that it won't miss any of the pulses your dial sends out as it
All this began when you lifted the phone from its cradle, and
was completed long before you got it to your ear. In addition, the
scanner went on sampling several thousand other lines, and signaling
the computer to take whatever action was necessary in each case.
In this way, one scanner-computer circuit operates fast enough to
handle all the business on all of the lines coming into the central
office, moving from one to the other with lightning speed.
As you dial, the scanner is looking at your line 100 times a second.
Every time your dial generates a pulse, the scanner notes the event
and records it in its temporary memory. When you finish dialing,
the computer hooks a ringing connection to the line you dialed.
It also sets up the ringing connection on your line, to assure you
the line you want is being rung. Simultaneously, of course, the
scanner is checking the line you're calling. When someone answers,
the "brain" is notified, and it then sets up a talking circuit between
the two lines.
After your conversation, you hang up.
The scanner notes that your line is now idle, but just to make
sure, it waits until your line reads idle for three consecutive
checks. Satisfied that you are now through talking, the computer
disconnects both phones.
Automatic Switching. Why set up such a complex electronic system
when the present-day relays seem to do the job pretty well? There
are several reasons, but by far the most important is the fact that
the electronic "central" can do things no other setup can even approach.
The present relay system can be connected so that another phone
will ring when your line is busy. But to do this, the phone company
has to wire in separate circuits, including special relays at the
central office. Once the circuit is in, it is permanent. And since
extra labor and equipment are involved, it is relatively expensive.
When diode fires, the neon glows, setting up a low-resistance
path from cathode to anode.
Switching network in the all-electronic telephone system
uses tiny gas diodes in place of conventional relays to
connect one line to another.
Network of wires can easily be connected by means of
diodes. As long as diodes do not fire, wires are not connected.
But if diode 3 fires, for example, input 1 and output 3
are connected; if diode 4 fires, input 2 is connected to
Cathode-ray tube in electronic central's "flying spot
store" is a photographic "memory" device capable of storing
over two million "bits" of information.
Tube sweeps spots on film - over 30,000 to each 1 1/2"
square - which are either clear or opaque and which pass
or withhold the beam of light accordingly.
With the electronic system, no wiring changes at all are required.
The computer which controls the system has memory circuits. Instruct
it to ring another phone when your phone is busy, and it complies
without so much as a single wiring change.
Other arrangements which are now completely impossible will be
a snap with the electronic system. For example, our present setup
cannot let you reach regularly called numbers by dialing only two
digits instead of the usual seven. But the computer finds this chore
simple. And since the electronic giant acts with such tremendous
speed, it can take care of thousands of such special requests without
interrupting its normal service.
The new system even diagnoses its own troubles, and in some cases
repairs them. If a certain circuit goes out of order, the computer
automatically switches in a spare. Then it runs a number of checks
on the bad unit, diagnoses the trouble, and writes instructions
for replacing the faulty part on a teletype-writer. It also periodically
checks some 800 critical voltages throughout the system and lists
them on the teletype. If any voltages are off, technicians can cure
the developing trouble before it becomes serious.
The system's teletype, by the way, is a vehicle for two-way communications-technicians
also use it for "talking" to the computer. Let's say, for example,
that you want all incoming calls to your office switched to your
home phone from five o'clock every afternoon to nine each morning.
You simply call the telephone company, and an operator- using the
teletype - "tells" the computer what you want. Your phone service
is then automatically switched as you directed, without your having
to worry about it again.
If you move, technicians can use the teletype to instruct the
computer to take your line out of service. Or they can call on it
to add additional services to a particular telephone, run special
checks, and so on. You can even ask the computer what time it is,
and it will respond with the month, day, hour, and minute.
Experimental Systems. The first experimental
electronic "central" mentioned earlier went into regular commercial
operation for the first time only a few months ago. But Bell Laboratories
scientists actually began working toward such a system in the early
1930's. Even at that time, they saw that electronic switching would
offer many advantages which could be achieved in no other way. Experimental
systems were built and tested - and they worked. But they were not
practical for regular use.
In the first place, the number of vacuum tubes required for a
full-scale system was enormous - and enormously expensive, since
the tubes gobbled up a lot of power. Then, too, the power generated
tremendous amounts of heat, and the heat created additional problems
of its own. Furthermore, building a memory section for the computer
would require millions of tubes. And with that many of the little
bottles, reliability would become an overwhelmingly difficult problem.
In use, it was calculated, tubes would "pop" faster than technicians
could replace them.
The first big breakthrough came in the late 1940's when Bell
scientists invented the transistor. This solved the problem of the
switching circuits, but a practical, inexpensive, large-scale memory
was still not available.
In 1954, Bell executives decided to launch a multi-million dollar
research program to develop such a memory, and to incorporate it
into a full-scale, practical electronic-switching system. Before
the project was over, scientists assigned to it had designed and
built two "memory" devices. One was for semi-permanent information
which would be stored in the computer, such as a list of which telephones
are connected to which lines. The other, with a "temporary" memory,
"remembers" information the system must retain for only a few minutes,
hours, or days - such as the number you are calling, or where you
can be reached for the next few hours if you've left instructions
for your call to be transferred.
Future Possibilities. Although the electronic
system now in operation in Illinois performs many unusual services,
the range of possibilities has hardly been touched. When R. W. Ketchledge,
director of Bell's electronic central office development project,
was asked just what the system could do, he leaned back in his chair
"There's only one way we can answer that," he said. "And that's,
'What did you have in mind?'"
He went on to explain that the computer can be instructed to
make virtually any kind of interconnection you can dream up, and
all without changing a single wiring connection. A couple of the
possibilities Bell officials think might be popular with customers
- The "Baby Sitter." Before you go out for the evening, you dial
a special code, then the number where you can be reached. If the
baby sitter needs you, she'll simply pick up the phone and wait
for five seconds. The computer will recognize this as a special
signal and ring the number you specified before you left. The service
could, of course, be left in operation permanently for any number
you call frequently .
- The "Camp-On." You call a friend and his line is busy. You're
anxious to reach him, but don't want to keep dialing his number
over and over again. Besides, he might complete his call and dial
another number before you get through. With the "camp-on" system
in operation, a pleasant voice notifies you that his line is busy.
But if you hang on, the voice says, the system will ring his line
the instant he puts his phone down.
So impressive is the operation of the prototype electronic "central"
that officials are rushing plans to extend the system to the entire
country. Since it takes a long time to standardize designs, set
up production lines, and install these immensely complex systems,
you won't have an all-electronic phone next month, or even next
year. Officials hope, however, to have electronic central office
equipment in normal operation in some places not later than 1965.
Posted August 14, 2014