An ISDN-based VTC system supporting secure (classified) and non-secure (unclassified) conferences must be cabled to maintain a minimum of 5 or 15 centimeters RED/BLACK separation on either side of any Type 1 encryptor and any dial isolator (depending on the TEMPEST zone).

Information leakage is the intentional or unintentional release of information to an untrusted environment from electromagnetic signals emanations. Security categories or classifications of information systems (with respect to confidentiality) and organizational security policies guide the selection of security controls employed to protect systems against information leakage due to electromagnetic signals emanations. The Committee on National Security Systems Advisory Memorandum (CNSSAM) TEMPEST/01-13, RED/BLACK Installation Guidance, provides criteria for the installation of electronic equipment, cabling, and facility support for the processing of secure information.

The TEMPEST/01-13 requires separation between RED and BLACK equipment and cables, to include cable routing inside equipment cabinets. Depending on TEMPEST ZONE, the separation requirements are:
- Minimum equipment separation - 50 cm or 1m
- Minimum cable separation - 5 cm or 15 cm

The unencrypted information, wiring, and processing equipment are considered RED while the encrypted information, wiring, and processing equipment are considered BLACK.