| The
"Bell System" (Western Electric) never liked the
idea of allowing customers to dial their own calls since they
specialized in "customer service" and had operators
complete all calls. By the late 1910's, they started to realize
that this may not be a good idea and started to use step by
step systems (as described on the Step by Step page).
The first Panel switch was installed in the
early 1920's in Omaha, Nebraska and others were installed
throughout the 20's and 30's in most metropolitan areas in
the USA (except for Los Angeles, which was step by step to
be compatible with surrounding independent telephone companies).
Step by Step switches could work well in small
communities, but the Bell System found that it would not work
well for large metropolitan areas where there were a large
number of calls in progress at an one time. The reason was
that the step-by-step switch had limitations due to its direct
control design and small amount of connecting circuits that
could connect to other subscribers in the same switch or to
another switch..
The Panel switch was the first telephone switch
to use a concept of “store and forward” by use
of a device called a “sender” which can store
a number and then act upon the dialed number to route the
call the best way possible.
Their first switch was called a Panel switch
because of the large vertical panels. It followed the same
basic concept of the Step by Step but used a method of where
the selector system operated on a concept of "ladders"
where the selector would rise up on a panel by the number
of pulses it had received.
A Panel switch uses a series of “frames”
and each of these frames has a different type of function.
A brief summary is as follows:
Line Finder Frame - The line finder frame
provides means for associating a subscriber’s line with
the switching equipment and establishing connections.
District Frame - The district frame provides
outgoing routes for the originating call. At this frame are
circuits to other switches, to operator switchbaords, or to
other frames.
Office Frame - Office frames provide additional
outgoing circuit capacity when the requirements exceed the
capacity of the district frame.
Incoming Frame - The incoming frame is the
terminating point for incoming circuits to make connection
between and provides a path to the final frames.
Final Frame - The final frame is the last
step in the connection of a call to the destination subsbriber.
Panel systems had a unique system of "revertive
pulsing" when a call was connected to or from a Panel
switch (or a switch which supported revertice pulsing) by
the receiving switch pulsing BACK to the originating switch!
Instead of sending dial pulses out TO the terminating switch
(as in Step by Step), it received them FROM the terminating
switch.
Most Panel offices were removed from service
by the late 1970's, though there is a rumor that the last
Panel office in service was located in northern New Jersey
and was removed in the early 1980's.
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is telephone exchange ? | Step-by-Step
Telephone Switching Systems |
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