23 January 2014
The first completely automatic electronic system
for switching telephone calls.
The people behind the invention:
Almon B. Strowger (1839-1902), an American inventor
Charles Wilson Hoover, Jr. (1925- ), supervisor of memory
Wallace Andrew Depp (1914- ), director of Electronic
Merton Brown Purvis (1923- ), designer of switching
Electromechanical Switching Systems
The introduction of electronic switching technology into the telephone
network was motivated by the desire to improve the quality
of the telephone system, add new features, and reduce the cost of
switching technology. Telephone switching systems have three features:
signaling, control, and switching functions. There were several
generations of telephone switching equipment before the first
fully electronic switching “office” (device) was designed.
The first automatic electromechanical (partly electronic and partly
mechanical) switching office was the Strowger step-by-step switch.
Strowger switches relied upon the dial pulses generated by rotary
dial telephones to move their switching elements to the proper positions
to connect one telephone with another. In the step-by-step process,
the first digit dialed moved the first mechanical switch into position,
the second digit moved the second mechanical switch into
position, and so forth, until the proper telephone connection was established.
These Strowger switching offices were quite large, and
they lacked flexibility and calling features.
The second generation of automatic electromechanical telephone
switching offices was of the “crossbar” type. Initially, crossbar
switches relied upon a specialized electromechanical controller called
a “marker” to establish call connections. Electromechanical telephone
switching offices had difficulty implementing additional features
and were unable to handle large numbers of incoming calls.
Electronic Switching Systems
In the early 1940’s, research into the programmed control of
switching offices began at the American Telephone and Telegraph
Company’s Bell Labs. This early research resulted in a trial office being
put into service in Morris, Illinois, in 1960. The Morris switch
used a unique memory called the “flying spot store.” It used a photographic
plate as a program memory, and the memory was accessed
optically. In order to change the memory, one had to scratch
out or cover parts of the photographic plate.
Before the development of the Morris switch, gas tubes had been
used to establish voice connections. This was accomplished by applying
a voltage difference across the end points of the conversation.
When this voltage difference was applied, the gas tubes would
conduct electricity, thus establishing the voice connection. The Morris
trial showed that gas tubes could not support the voltages that
the new technology required to make telephones ring or to operate
The knowledge gained from the Morris trial led to the development
of the first full-scale, commercial, computer-controlled
electronic switch, the electronic switching system 1 (ESS-1). The
first ESS-1 went into service in New Jersey in 1965. In the ESS-1,
electromechanical switching elements, or relays, were controlled
by computer software. A centralized computer handled call processing.
Because the telephone service of an entire community
depends on the reliability of the telephone switching office, the
ESS-1 had two central processors, so that one would be available
if the other broke down. The switching system of the ESS-1 was
composed of electromechanical relays; the control of the switching
system was electronic, but the switching itself remained mechanical.
Bell Labs developed models to demonstrate the concept of integrating
digital transmission and switching systems. Unfortunately,
the solid state electronics necessary for such an undertaking had not
developed sufficiently at that time, so the commercial development
of digital switching was not pursued. New versions of the ESS continued
to employ electromechanical technology, although mechanical
switching elements can cause impulse noise in voice signals and
are larger and more difficult to maintain than electronic switching
elements. Ten years later, however, Bell Labs began to develop a digital
toll switch, the ESS-4, in which both switching and control functions
Although the ESS-1 was the first electronically controlled switching
system, it did not switch voices electronically. The ESS-1 used
computer control to move mechanical contacts in order to establish
a conversation. In a fully electronic switching system, the voices are
digitized before switching is performed. This technique, which is
called “digital switching,” is still used.
The advent of electronically controlled switching systems made
possible features such as call forwarding, call waiting, and detailed
billing for long-distance calls. Changing these services became a
matter of simply changing tables in computer programs. Telephone
maintenance personnel could communicate with the central processor
of the ESS-1 by using a teletype, and they could change numbers
simply by typing commands on the teletype. In electromechanically
controlled telephone switching systems, however, changing numbers
Electronic switching has greatly decreased the size of switching
offices. Digitization of the voice prior to transmission improves
voice quality. When telephone switches were electromechanical, a
large area was needed to house the many mechanical switches that
were required. In the era of electronic switching, voices are switched
digitally by computer. In this method, voice samples are read into a
computer memory and then read out of the memory when it is time
to connect a caller with a desired number. Basically, electronic telephone
systems are specialized computer systems that move digitized
voice samples between customers.
Telephone networks are moving toward complete digitization.
Digitization was first applied to the transmission of voice signals.
This made it possible for a single pair of copper wires to be shared
by a number of telephone users. Currently, voices are digitized
upon their arrival at the switching office. If the final destination of
the telephone call is not connected to the particular switching office,
the voice is sent to the remote office by means of digital circuits.
Currently, voice signals are sent between the switching office and
homes or businesses. In the future, digitization of the voice signal
will occur in the telephone sets themselves. Digital voice signals
will be sent directly from one telephone to another. This will provide
homes with direct digital communication. Anetwork that provides
such services is called the “integrated services digital network”
See also : Cell phone; Long-distance telephone; Rotary dial telephone;