AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 369

About Telecommunications
Electromechanical Switching. Electrically operated devices with
mechanical parts and motion. Electromechanical switching automated
the manual labor and allowed telephone service to be universally
affordable, but the technology was inflexible since changes in service
required changes in the device itself. It also required high maintenance
due to wear and tear on parts, and did little more than switch calls.
Electronic Switching. Electronic, computer-controlled equipment.
Electronic switching reduced the size, power consumption, and cost. At
the same time, it increased operating speeds, ruggedness, and
reliability. Computer control provides flexibility because changes and
enhancements are made to the switching system's software rather than
to the hardware.
Manual switching was used for the first few decades of telephone service.
Switching was performed by human operators who made the actual
connections of circuits at a switchboard by using cords that had plugs at each
end. Each of the plugs had a tip and a ring which completed the electrical
circuit over which the signals traveled. The operator plugged one end of the
cord into the caller's jack, and then completed a call (that is, completed a
circuit) by plugging in the other end of the cord to the called party's jack, one of
perhaps 10,000 subscriber jacks within reach
Approximately 120 lines terminated at answering jacks on the operator's
switchboard. In turn, each operator had 18 cords that could be used to make
connections.
The first automatic switch was invented in 1892 by Almon B. Strowger, an
undertaker, whose competitor was getting all the undertaking business in the
town referred by the other undertaker's wife, who was the town's telephone
operator. The Strowger switch was an electromechanical device controlled by
the caller's telephone.
Strowger's switch was adapted for use in the Bell System starting in 1919. It was
slow, noisy, and not very flexible with respect to offering new services but,
because it was more cost-effective than human operators, it was directly
responsible for making telephone service affordable and universal.
The next innovation in electromechanical switching was the Bell System's
crossbar switch, first installed in 1938, and still in use in some areas today. It
had fewer switches, a sophisticated control mechanism, and lower maintenance
but, like its predecessor, was not flexible because it couldn't be programmed.
It was, therefore, a natural progression to the idea of using a computer, with its
inherent programmable flexibility, to control the operation of the switching
network that resulted in the new generation of switching technology called an
electronic switching system (ESS).
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System Manager's Guide