September 1961 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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 this article appeared in Popular
Electronics magazine. 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 film that was scanned
by a light beam to read back information.
Operation Telephone 1965
Within the next four years, American's telephones will undergo
a major and far-reaching innovation when the new, all-electronic "centrals" take
By Ken Gilmore
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 telephone.
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.
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 Telephone
- 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 so on.
- 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 you wish.
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 clicks around.
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.
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 output 1.
Cathode-ray tube in electronic central's "flying spot store"
is a photographic "memory" device capable of storing over two million "bits" of
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
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.
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
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 and smiled.
"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 are:
- 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 December 24, 2021
(updated from original post on 8/14/2014)