The Operational Control Center represents the largest component in the Ariane 5 ground segment. It handles all interface management between the Ariane launcher and ground facilities during launch preparation phases. It ensures information exchange between on-board equipment and the ground, and controls the launch count-down. The control center is a real-time system distributed on four sites and linked with an optical fiber network. For safety and availability reasons, redundancy has been applied to most of the control center's subsystems: front-end equipment, processing units, networks. The design and development of the Ariane 5 control center was a challenge for several reasons: the safety and operational constraints; the compatibility with the test benches without redundancy used at earlier stages of the development cycle; the fully distributed architecture; the need for online repair and re-insertion of failed redundant units without interrupting the launch countdown.