Defense Switched Network

The Defense Switched Network (DSN) is a primary information transfer network for the Defense Information Systems Network (DISN). The DSN provides the worldwide non-secure voice, secure voice, data, facsimile, and video teleconferencing services for DOD Command and Control (C2) elements, their supporting activities engaged in logistics, personnel, engineering, and intelligence, as well as other Federal agencies.

In 1982, the DSN was designated by the Office of the Secretary of Defense (OSD) and the Joint Chiefs of Staff (JCS) as the provider of long-distance communications service for the DOD. The DSN is designated as a primary system of communication during peacetime, periods of crisis, preattack, non-nuclear, and post-attack phases of war. The network assures nonblocking service for users with "flash" and "flash override" precedence capabilities. Key users include the National Command Authorities, Commanders of the Combatant Commands, and subordinate component commanders. DSN replaced the older Autovon system.

The DSN consists of four subsystems:

The DSN Switching Subsystem consists of multifunction, stand-alone tandem, end office, and remote switching units. Using the transmission, timing, and control elements of the DISN, they interconnect all military locations worldwide and provide end-to-end long-distance common user and dedicated voice, secure voice, data, and video services worldwide.

...
Wikipedia