A Protected distribution system
(PDS) is a wireline or fiber-optics telecommunication system
that includes terminals and adequate acoustical
, and physical safeguards to permit its use for the unencrypted transmission
of classified information
Note: A complete protected distribution system includes the subscriber and terminal equipment and the interconnecting lines. Deprecated synonym - "approved circuit".
Source: from Federal Standard 1037C and from the National Information Systems Security Glossary