| The
Step by Step switching system was invented in 1888 and patented
in 1891 by Almon Strowger, an undertaker in Kansas City, MO
USA. Rumor has it that the local switchboard operator was
diverting calls from his business to others. He wanted to
have some way to have calls automatically routed to the destination
without the intervention of an operator.
His invention had changed throughout the years,
but the basic concept has remained the same. The system operates
by using rotating "selectors" that can select a
"level" (or step) by the number of pulses that are
interrupted on a line (using a "rotary" telephone).
This instructed a solenoid to move this wiper
(or selector) to a particular position based on the number
of pulses it had received. This then put the caller on another
level and again pulse dialed to get to another level and so
on until the caller had reached the subscriber they were calling
on the last level (or digit they dialed).
The addition of a line finder selector to
reduce the number of switching selectors that were needed
(one for every 10 or more subscribers, instead of one for
each subscriber). Additional circuitry was later added to
add a “tone plant” to provide progress and informational
tones such as dial tones, ringback tones and the like.
Strowger founded the Strowger Automatic Telephone
Exchange Company in 1892. Strowger sold his patents in 1896
and sold Strowger Automatic Electric in 1898. Strowger died
in 1902 of a brain aneurism.
The Bell System eventually began to manufacture
Step-by-Step switches. However, their first installation of
a Step-by-Step switch was NOT made by Western Electric, but
by Automatic Electric! This first Bell System owned Step-by-Step
switch was installed in 1919 in Norfolk, VA. This Automatic
Electric-Western Electric arrangement lasted for five years
until Western Electric could start manufacturer of their own
switches.
There were many variants to the step by step.
One was called the "XY" switch made by Stromberg-Carlson.
This was a two-axis (horizontal & vertical). It was a
flat mechanism, and the moving contacts moved both back and
forth and side to side. This was considered to be a simpler
and cheaper version of the Strowger Step by Step, and was
commonly used in rural areas of the mid-western United States.
Step-by-Step systems of various styles and
manufacturers were in widespread use throughout the entire
world. In the United States, the last Step-by-Step switch
was removed from service and replaced with a digital switching
system in 1999. The last Step-by-Step switch in the United
Kingdom was replaced in 1995.
In Canada, the last Step-by-Step switch in
the North American public switched telephone network was replaced
in 2002. Outside of North America and Western Europe, there
may still be in use in some small third-world countries and
former Soviet controlled countries in Europe that may still
be using these systems.
What
is telephone exchange ? | Step-by-Step
Telephone Switching Systems |
Western
Electric Telephone | Western
Electric Modern Telephone
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