Wire line or fiber optic system that includes adequate safeguards and/or countermeasures (e.g., acoustic, electric, electromagnetic, and physical) to permit its use for the transmission of unencrypted information through an area of lesser classification or control.
Sources:
CNSSI 4009-2015
from
CNSSAM TEMPEST/01-13, NSA/CSS Policy 3-12
NIST SP 800-53 Rev. 5
under protected distribution system
from
CNSSI 4009-2015