Custom Search
|
|
|
||
 MIL-HDBK-232A
4.4.1 Signal types. Signal distribution in a facility involves transmission
of two types of signals -- analog and digital.
4.4.1.1 Analog signaling. An analog signal is defined as continuously
variable in some direct correlation to another signal impressed upon It. In
many facilities, quasi-analog signals exist as audio outputs of modems
connecting the facility to the transmission medium, and the administrative
telephone system. Such signals in a facility are in the voice frequency (VF)
range of 300 to 4000 Hz. Other analog signals may exist within equipment
reaching into the higher frequency ranges in specialized equipment such as
magnetic tape transports, magnetic disk units, VDUs, or closed-circuit
television. Specialized facilities may use radio with even higher
frequencies, or use high frequencies in broadband LANS. The designer must
use techniques which prevent cables carrying such signals from acting as
antennas and thus transmitting or receiving signals. Grounding and shielding
are essential in order to contain emanations, along with attention to proper
cable selection, termination, and impedance matching.
4.4.1.2 Digital signaling. Digital signals are normally discontinuous,
changing from one state to another in discrete steps. Digital signals
represent the information being processed in a facility and may be used to
modulate analog signals to transmit such information. Rate changes can
typically occur in a range of 50 to several million bits per second. In the
design of digital signal schemes, it is important to minimize the possibility
of EMI by keeping: (a) voltage levels low, (b) all pairs properly terminated,
and (c) shields properly closed and grounded.
4.4.2 Patching. Patch panels are provided in facilities to allow equipment
and signal paths to be interchanged in the event of failure or alternate
routing. Separate patching is provided for BLACK digital/analog, RED
digital/analog, and the RED signals of special security levels. Panels are
installed with protective schemes that assure patching cannot be accomplished
between different types of signals or communities of interest.
4.4.3 Facility entrance plates. Facility entrance plates provide the
demarcation point between the facility and the external transmission media.
It is at this point that surge, transient, EMI/RFI, and EMP/HEMP protective
measures are applied to signal lines entering and egressing the facility.
4.4.4 Distribution frames (DFs). DFs are points within the facility where
cables are interconnected to equipment or other cables. DFs may be provided
for BLACK analog, BLACK digital, RED analog, or RED digital terminations.
Terminations may be made using connectors and plugs, crimped taper pins, wire
wraps, solder wraps, or insulation displacement techniques.
4.4.5 Distribution planning. Distribution of signal cable in a facility is
designed to ensure tile proper segregation and integrity of signals. It is a
critical part of the RED/BLACK concept. The proper segregation of RED and
BLACK signals is best accomplished by planning each cable run from source to
sink. If the facility is viewed as a series of concentric rings, each
defining a boundary, accounting for each signal run to a boundary before
going to the next boundary should ensure RED/BLACK integrity. All cabling
should be distributed in ducts, conduits, cable trays, or ladders.
Separate
runs are provided for RED and BLACK signals, with special attention to
physical separation when such ducts/conduits must parallel each other. The
use of ducts provides physical protection, ensuring control of separation and
routing, while also providing a degree of shielding. In some instances, RED
duct runs must egress the LEA and traverse a UAA. Such runs require a PDS.
Guidance for a PDS is contained in paragraph 5.7.3.
|
 
|
|
|
||
