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 7.2.2 Proprietary Installations. The wholly owned and maintained wire
networks within a facility or group of facilities are considered to be
proprietary. These networks, when properly maintained, represent the
greatest amount of system control and responsibility to the user. The
electronic on/off or voltage pulse communications are sent by the detector
to the control unit. Control unit communications are sent to the reporting
and display components of the IDS (see Section 8). All of these
communications will require some proprietary wire networks.
7.2.2.1 Loops. Loop communication networks consist of a pair of wires
laced through an area or building with devices connected at intervals.
These devices or subsystems report on this signal line pair to the
controller and, subsequently, the display. A simple example of this scheme
is depicted in Figure 42. There is a coding technique to differentiate
devices on the single line pair. The party-line or McCulloh coding methods
permit each device to send a specific signal, at any time, along the wire
pair loop to the controller. These discrete signals are interpreted by a
receiver and subsequently displayed. The disadvantages in using this type
of coding technique are that any fault in the line cuts off all
"downstream" reporting devices, and that two simultaneous reports on the
line will be indecipherable. The single most significant fault is the
"clash" or simultaneous report which can serve to frustrate monitoring
personnel who know that there are alarm conditions but do not know which
devices are reporting. Loop device reporting and communications are not
acceptable for Navy IDS system designs.
7.2.2.2 Point-to-Point. Communications conducted on one wire pair per
device are point-to-point circuits. Typically, this circuit costs more to
install than a loop system because of the additional wiring used to complete
the system. One sensor to each of the circuits permits specific reporting
by device and enhances the maintenance of the system. A single fault on the
circuit disables only one sensor. Typical applications require low voltage
wiring on 500 mA current or less wire, which is in pairs and which will
require shielding if data is communicated (usually from control or data
gathering units). This wiring scheme requires that the knowledgeable
intruder bypass several diverse types and locations of wires in order to
defeat the protection-in-depth schemes.
7.2.2.3 Multiplexed. Multiplexed is a term used to designate a
communication technique that permits multiple communications on the same
line to be differentiated and not interfere with each other. Usually more
cost-effective because of the greater number of communicating units able to
use the same pathway, the multiplexed scheme uses either frequency division
multiplex or time division multiplex to separate the signals. Unique
frequency or time designations are allotted to each reporting subsystem.
The use of dual pair communications paths permits the loop to be maintained
even in the event of one pair failure by allowing reverse of direction to
continue communication on both sides of a fault. Multiplexing and
demultiplexing equipment is a more costly technique than direct point wiring
except where several devices could use the same pathway or loop.
Frequently, access control systems provide up to eight multiplexed points
for door alarms, etc. to be transmitted on the door control/reader lines as
standard equipment. Multiplexed communications
13.02-107
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