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MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About Telecommunications
Page B-6Switching Equipment
B
Three basic types of technology have been used in switching:
■ Manually-Operated Switching. Human operators made the connections
of circuits by placing plug-ended wires into jacks on a switchboard. Manual
switching was slow, labor-intensive and, therefore, costly, but it afforded
some special functionality: calls could be forwarded, messages taken, and
calls interrupted.
■ 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
townreferred 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 could not be programmed.