The Unisys fiber optic data transfer system (DTS), which is a compatible implementation of the lower four layers of the US Navy SAFENET standard, is described. SAFENET (survivable, adaptable, fiber optic, embedded network) is a real-time information transfer system. It combines standard LAN protocols with widely accepted ISO OSI (open systems interconnection) protocols and a fiber-optic transmission system to: (1) provide an open system environment which enhances interoperability and communicability; (2) provide improved connectivity and flexibility; and (3) allow for graceful evolution to fully distributed system architectures.