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 MIL-HDBK-232A
telephone lines used for data communications. Telephone lines used for
dedicated, full-period data communications would be treated as analog lines,
From the facility entrance plate, all cables are run in conduit to the areas
where the end instruments are located. The conduit should be extended to the
desk, rack, or table on which the instrument is located. Overall shielded
cable should be used. For other requirements see paragraph 5.8.
5.4.1.1.4 Secure voice. Analog extensions of secure voice systems should be
run in separate conduit from the security equipment to the end instrument.
Cables should be shielded. Analog runs on the BLACK side of the security
equipment should be as described in paragraph 5.4.1.1.1.
5.4.1.1.5 Video. Video systems in an LEA display the most diverse
conditions requiring treatment. Two categories of video equipment are
discussed here -- video display units (VDUs) used as terminal devices and
televisions as part of briefing and display systems. Video equipment, used
as terminal devices in the generic sense, are directly connected to the
terminal device controller and require no installation criteria. Some
units., however, will use an rf modulator in the controller for interfacing
to the display unit. Such connection would be made with coaxial cable, with
the VDU in the immediate vicinity of the controller. In some cases, the
VDU/keyboard is remoted from the controller. The video cable should be run
in conduit. If conduit cannot be used, triaxial cable should be used instead
of coaxial cable. Specialized facilities may use a closed circuit television
system for briefing and display schemes. This system would be interconnected
with coaxial cable or fiber optic schemes. Distribution is in dedicated
conduit due to the high frequencies involved. Triaxial cable should be
considered for additional shielding. Because of the inherent radiation of
the rf stage in most TVs, the system should operate at baseband instead of
broadband, with external channel switching. If baseband cannot be used, then
the use of triaxial cable or a filter optic scheme is indicated.
5.4.1.1.6 Local area networks (LANs). Broadband and baseband LANs are
implemented using coaxial cable to interconnect the nodes. Fiber optic cable
(FOC) may be substituted for coaxial cable. Since most LANs will come from
commercial sources, additional treatments are necessary. Ethernet, for
example, requires a specific type of coaxial cable which is normally
installed in overhead plenums. Connection of nodes is accomplished using
vampire taps. This design allows easy reconfiguration. In a secure
environment, this feature is lost since the cable should be run in conduit
for physical security, with pull boxes installed at the points required for
taps. Where possible, fiber optics should be used instead of metallic
schemes.
5.4.1.2 Digital signals. Digital signals are found between processing
equipment in a facility. These signals may be interfaced in either a
balanced or unbalanced mode. Balanced interfaces are always distributed with
twisted pair, while unbalanced interfaces, by equipment design, may be
distributed with single wire or twisted pair.
5.4.1.2.1 Balanced signals. Balanced voltage digital signaling requires a
dedicated current return for each signal line from the receiver back to the
transmitter. The arrangement depends upon a differential between the two
lines to determine signal state. Of concern for the designer and installer
is the treatment of all pairs in DFs and patching facilities. DFS should not
be wired for bused returns, but should include provision for all pairs.
Likewise, patching facilities also should include provisions to patch all
pairs. (See figure 29.)
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