The U.S. government has been concerned about the tapping threat for many years. In the context of classified information on United States Department of Defense (DOD) networks, protective distribution system (PDS) is a set of military instructions and guidelines for network physical protection. PDS is defined as a system of carriers (e.g. raceways, conduits, or ducts) used to distribute military and national security information between two or more controlled areas, or from a controlled area through an area of lesser classification, i.e., outside the Sensitive Compartmented Information Facility (SCIF) or other similar area. PDS provides guidance for the protection of SIPRNet wire line and optical fiber to transmit unencrypted classified information.