Quantcast Digital signaling - hdbk232a0045

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4.4.6  Filtering.  Filtering and isolation are used to ensure that only the
intelligence intentionally placed on a line egresses the facility and that
extraneous signals do not upset an operation.  In the past, when most
communications used analog transmission techniques, passive LC bandpass
filters were used at the point of egress from the facility.  This was known
as shield point Isolation.  Such filtering can still be used for analog
signals.  However, with the advent of digital transmission techniques and
multilevel multiplexing, passive filters cannot he used for the mission bit
streams.  Filtering may not be indicated if: (1) TEMPEST approved equipment
is used, (2) the line is encrypted, (3) proper RED/BLACK separation has been
maintained, and (4) proper installation procedures have been used.  Nonsecure
lines supporting unclassified circuits and telephone lines
may require filters.  Where a facility has been designed to survive EMP/HEMP,
all signal lines are equipped with surge arrestors, transient suppressors,
filters, and other measures to prevent upset/burnout of equipment.  Where
passive filters cannot be used because of line speed/format, optical
isolators can be used to provide isolation at the point of egress.  Such
devices typically function like repeaters, using opto-electronic coupling to
provide the isolation.  Some optical isolators, however, operate
asynchronously, repeating any signal on the line within the electrical
parameters of the device.  This can be overcome by using clocking signals to
gate the isolator.  Clock signals should originate at the same point as the
signal of interest; i.e., if the signal originates in the RED area, the clock
should be RED.  Such isolators may be used for all signal lines to aid in EMP
or TEMPEST isolation if such devices use fiber optics between stages.  In a
shielded facility, the fiber optics would egress the LEA through
waveguides-beyond-cutoff.  In certain instances, signal lines originating in
the REA must enter the BEA.  These may be control lines or signal lines for
nonsecure circuits in a switching system.  In some cases, a RED/BLACK
boundary needs to be established.  Optical isolation inserted into all
circuits crossing that boundary satisfies that need.
4.4.7  Special considerations.  Because of the density of signal lines in a
patch and test facility (PTF) and the unsecure nature of the administrative
telephone system, the designer must consider the hazards associated with
these areas.  Paragraphs through present the special
considerations that should be included in a facility design.  Patch and test facilities (PTFs).  Most facilities will use patching
equipment to allow swapping equipment and lines in the event of failure, or
to provide alternate routing.  Larger facilities also include provisions to
manually or electrically configure testing equipment into circuits to monitor
or test the circuits.  Many of these facilities were designed using equipment
and materials for technical control facilities.  Some of these materials and
equipment are satisfactory in an unbalanced environment, but are less than
satisfactory in a balanced environment.  Interconnect and distribution frames
also present problems in properly maintaining shielding of signals.
Crossconnecting in such frames also presents a hazard of creating antennas
capable of radiating or receiving at higher frequencies.  When designing and
installing such facilities, the following guidance is given:
Provide separate patching facilities for RED and BLACK signals, and for
BLACK digital and BLACK analog.
Provide separate DFs for each kind of signal group.
If RED communities of interest include nonsecure, collateral, and
compartmented communities. separate patching and DF facilities are
required.  If this situation exists in a small facility, unique wiring


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