Double-Round Comparison
One of the earliest means for checking the accuracy of a
transmission was to allow the Digital Communicator at the
protected area to send the message up to four times during any
communication attempt. Each message would then be
compared by the Central Station receiver with the one
preceding it. As soon as a successful comparison was achieved,
the message was considered valid, and a verification or
kissoff
signal was sent back to the communicator, letting it know that
its work was done, and instructing it to "shut down."
Under these conditions, if two successive messages were
identical (
Example 1,
below), it's a practical certainty that
they were received as intended because the likelihood of two
such messages being
both
incorrect
and
identical is
extremely rare.
Because of this, the CP-220A uses a
Double-Round
Comparison
method, which considers a message valid if it
is the same as
any
message that preceded it during the
same phone call – even if the two messages are not
successive (see
Example 3,
below). This saves time and is of
benefit to a busy Central Station that monitors lots of
accounts and processes
opening and closing
reports.
The following table first illustrates a "normal"
communication using the
Double-Round Comparison
method, and then the two variations of this technique. In
all cases, [1 2 3 1] is the correct message.
CP-220A Central Station Receiver Appendix B:
Page B-7
Hook-Up and Installation Manual
Communication Formats
