AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 50

Background
Today, a local area within which there is a single uniform set of charges for
telephone service is called a local exchange area . A number of COs may serve
a local exchange area, and a call between any two points within an exchange
area is a local call . A toll call is a call made to a point outside the local
exchange area and includes service through the switching office hierarchy.
Switching Methods
For the first few decades of telephone service, human operators manually
switched calls and made the actual connections of circuits. They made the
connections at switchboards by using cords that had plugs at each end.
Approximately 120 lines terminated at answering jacks on an operator's
switchboard. In turn, each operator had 18 cords for making connections.
When a telephone service subscriber made a call, a lamp lit at his or her jack,
telling the operator that the person on that line desired service. The operator
connected to the subscriber's jack and the calling party would then give the
name (and later, the telephone number) of the party he or she was calling. Then
the operator completed the call (that is, completed the circuit) by connecting
the cord to one of perhaps 10,000 subscriber jacks within reach. When the call
was over and the parties had hung up, the lamp associated with each
connecting cord would go out and the operator knew that the call was complete
and the cord could be removed.
The first automatic switch was invented in 1892 by Almon B. Strowger, an
undertaker who realized that his competitor was getting all the undertaking
business in the town, referred by the town telephone operator—who was also
the competing undertaker's wife! The Strowger switch was an
electromechanical device controlled by the caller's telephone ( station
switching ).
Strowger's switch was adapted for use in the Bell System in 1919. It was noisy
and not very flexible at offering new services but, because it was more cost-
effective than human operators, it was directly responsible for making telephone
service affordable and universal.
In 1938, the Bell System developed and installed the next innovation in
electromechanical switching, and it is still in use in some areas today. It had
fewer switches, a sophisticated control mechanism, and lower maintenance.
However, like its predecessor, it was not flexible because it couldn't be
programmed.
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About the System