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7.2  Types of Communications Links.  Communications of intrusion detection
systems involves the third step of the sense, control, and signal process.
This process requires further input by personnel for the mission of
security.  The knowledge of intrusions and intrusion attempts must be
communicated from the sensor to the monitoring/display in a clear, speedy
manner which is resistant to compromise and conducive to rapid fault
detection and repair.  Wire, microwave, radio frequency and, most recently,
fiber optics may carry the electronic, voice, or video communications with
the protection required for security operations.
7.2.1  Hardwire/Landline.  Hardwire or landline communications is typically
a physical wire connection from point-to-point.  The size, type, and number
of conductors vary according to the sense devices, control instrument, and
reporting display.  The type of signal transmitted and the susceptibility of
that signal to compromise will dictate the need for protection from the
environmental and man-initiated degradations of the information signal.  Sensor Data.  Communication from the detection sensor is most
commonly carried on wire.  The two principal types of this data are:  the
interruption of a stream of current through a loop normally closed (N.C.)
circuit and the passage of a stream of current (or detection of continuity)
in a loop-normally open (N.O.) current.  The normally closed (N.C.) circuit
is depicted in Figure 40; the normally open (N.O.) circuit is depicted in
Figure 41.  The indication of detection results from a change of state in
the line.  There are limitations to the circuit designs, particularly if
more than one contact device is on a single line.  The approach of more than
one device on the line is termed "daisy chain."  The Figure 42 local
hardwire depiction is indicative of the unique problem of identification of
device(s) in alarm.  Any one of the detectors in alarm is indicative of a
single general alarm condition, requiring additional investigation to
determine the device in alarm.  The daisy-chain loop approach does not
direct immediate attention to the point of intrusion and is not recommended
for IDS implementation at Department of Navy facilities.  The recommended
wiring technique is point-to-point, single line from detection device to
control unit.  Wire is the most commonly used media for the transfer of
point-to-point information


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