Transmission
Verification
As you might expect,
DTMF
is quite a bit faster than pulsed
formats because a single tone – rather than a series of pulses
– is used to convey each digit. As a result,
DTMF
transmissions take less time and therefore can be used to
report additional information (such as the current condition of
all
of the system's zones), not just the condition that caused
the alarm. When used with the
Contact I.D.
formats,
DTMF
can provide some very specialized reporting, involving not
only alarms, but the specific location of troubles and
supervisory conditions, bypasses, tests, and system faults.
Frequency Shift Keying
(FSK and BFSK)
Somewhat less common than the pulsed and the
DTMF
formats is the format known as
FSK
(Frequency Shift
Keying)
. With
FSK
, two distinct but very brief tones
provide the "0s" and the "1s" to represent the binary digits
(bits) used to form the hexadecimal characters that
ultimately make up the message. Although for a given
message, about four times the number of tones are used in
FSK
when compared to
DTMF,
FSK
is actually faster
because the
FSK
tones are of extremely short duration.
NOTE: Binary numbering is the heart of the hexadecimal
system. Some familiarity with it is recommended to
better follow this discussion. Consult
Appendix A
for
additional information.
Before any message can be acted upon, it must first be verified
that it was accurately received at the Central Station.
Although the percentages are high that subscriber
transmissions will arrive as they're intended, it's possible that
random "noise" or disturbances on the phone lines may alter a
digit and cause a disparity between how a message left the
subscriber's premises and the way it was received by the
Central Station. Were the potential for errors not accounted
for, authorities could be dispatched to the wrong account, or
the police could be sent where the fire department was
required – both dangerous situations that could result in the
loss of life. Fortunately, before the receiver displays any
message or sends it to an Automation System for processing,
its validity has already been determined through one of several
error-checking methods, described below.
Page B-6 Appendix B:
CP-220A Central Station Receiver
Communication Formats
Hook-Up and Installation Manual