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 MIL-HDBK-232A
d.
Provide patching and distribution facilities that can accommodate every
signal and return line. Past practices typically did not include
patching return lines.
e.
Design the crossconnects to be as short as possible.
f.
If automatic line quality monitoring is incorporated, provide separate
monitoring equipment for RED and BLACK lines.
4.4.7.2 Administrative telephones. The treatment of administrative
telephones is discussed in other portions of this handbook (see 4.8). This
paragraph emphasizes certain installation criteria. First, telephone cable
is installed in completely separate distribution facilities. Second, if
party lines or shared lines are used within the LEA, such lines will not be
shared with users outside the LEA. Third, all telephone lines may require
filters or isolators. See appendix D for treatment of special features.
4.4.7.3 Fiber optics. Many facilities are using FOC to interface equipment.
Because FOC does not use an electrical medium, it is relatively immune to the
effects of EMI/RFI. Further, its radiation characteristics are negligible.
Therefore, it is ideal for signals caressing an LEA and for interconnecting
LEAs separated by a UAA. However, the designer and installer must provide
physical protection and security to the cable. The designer must also
include EMP protection at facility penetrations such as
waveguides-beyond-cutoff for FOC that penetrates the facility entrance plate
or other EMP barrier. The designer must also be aware that FOC is
susceptible to fogging during an EMP and must be protected.
4.5 General guidance for the use of filters and isolators. The function of
filters and isolators is similar to that of shields -- the attenuation of
undesirable signals which attempt to pass through. Filters and isolators are
applied to conducted signals, while shields are used against free space
radiated signals. Filters attempt to block signals by shunting to a return
path, thus reflecting the unwanted signals back to the source. Isolators
attempt to present an open circuit to unwanted signals. Engineering
considerations for the use of filters and isolators are somewhat dissimilar
because of the differing mechanisms used to perform these functions. Because
a filter operates by shunting the interfering or compromising energy to a
return path and reflecting it back to the source, the path provided to the
return must (a) be able to carry the amount of current which may be
delivered, and (b) present minimal impedance to ground to the shunted current
at all frequencies of interest. When a filter is used at the point where a
conductor passes through a shield, the desired effect is accomplished by
directly bonding the filter return (usually its chassis) to the shield.
Isolators, conversely, shunt no current, but must he able to withstand
whatever voltage may develop across the internal open circuit. Isolators
connected to lines which may carry lightning or EMP transients will be
subjected to considerable stress (see 4.6). In general, the external barrier
of a facility should use filters (preceded by surge arrestors) in preference
to isolators, because of the difficulty of preventing arcing when a large
incoming voltage transient encounters an open circuit. Figures 9, 10, 11,
12, and 13 provide general information on filter function.
4.6 General guidance for grounding, bonding, and shielding (GBS). Control
of compromising emanations, EMP/HEMP protection, and RED/BLACK isolation
depends fundamentally on proper CBS. MIL-HDBK-419 provides a detailed
discussion of GBS theory and practice. MIL-STD-188-124 mandates the
requirements for long-haul and tactical communications facilities. The
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