Telephone switching

The invention: 



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

system development

Wallace Andrew Depp (1914- ), director of Electronic

Switching

Merton Brown Purvis (1923- ), designer of switching

matrices











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

pay telephones.

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

were electronic.

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

required rewiring.





Consequences





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”

(ISDN).



See also : Cell phone; Long-distance telephone; Rotary dial telephone;