Protected distribution system
In U.S. military communications, a protected distribution system (PDS) is a communications system, of copper wires or optical fiber, that has sufficient physical protection that it is trusted to carry unencrypted classified information. A PDS is compliant with what the military calls RED/BLACK engineering criteria: RED media carry classified material, while BLACK media carry either unclassified or encrypted material. There must never be a direct connection between RED and BLACK, except through an approved security device, typically an encryptor, or, when a PDS is used, an optoisolator or other device that prevents RED signals from "leaking" around the PDS connection.  That protection includes approved acoustical, electrical, electromagnetic, and physical safeguards have been applied to permit the transmission of unencrypted classified information.
A PDS has two parts:
- Distribution system: The metallic wire paths or fiber optic transmission paths that provide interconnection between components of the protected system. The distribution system may be an internal PDS within the controlled space or an external PDS traversing an uncontrolled access area.
- Subscriber sets and terminal equipment: The complete assembly of equipment, exclusive of interconnecting signal lines, located on the end user's or customer's premises. This includes such items as telephones, teletypewriters, facsimile data sets, input/output devices, switchboards, patch boards, consoles, or any other device which processes classified information.
When the distribution system is totally confined to an area where open storage of the highest classification level is authorized, there is no specific need for protection. Otherwise, site-specific protection is needed. For example, the wires or fibers may run through thick concrete ducts or steel pipes, positioned such that human security personnel would see any attempt to cut through the protective shell, much less wiretap the line. Other approaches include putting the RED transmission facility inside a pipe that is pressurized with gas, such that cutting into it would cause a drop in pressure that would immediately trigger an alarm.
Some end and interconnection equipment may be placed inside a "bubble" of clear plastic or wire mesh, so the lines and other devices are under constant visual observation.
- U.S. Army Information Systems Engineering and Integration Center (24 October 2000), Military Handbook: RED/BLACK Engineering Installation Guidelines, MIL-HDBK-232A