A complete protected distribution system includes the subscriber and terminal equipment and the interconnecting lines.
The purpose of a PDS is to deter, detect and/or make difficult physical access to the communication lines carrying national security information.
[1] Approval authority, standards, and guidance for the design, installation, and maintenance for PDS are provided by NSTISSI 7003 to U.S. government departments and agencies and their contractors and vendors.
All of the connections in a Hardened Carrier System are permanently sealed completely around all surfaces with welds, epoxy or other such sealants.
With a hardened carrier system, detection is accomplished via human inspections that are required to be performed periodically.
These periodic visual inspections (PVIs) occur at a frequency dependent upon the level of threat to the environment, the security classification of the data, and the access control to the area.