The Cisco Clock and Scheduler Card (CSC) accepts transmission requests from line cards, issues grants to access the fabric, and provides a reference clock to all the cards in the system to synchronize data transfer across the crossbar. Only one Cisco Clock and Scheduler Card is active at any time.The Cisco Clock and Scheduler Card can be removed and replaced, without disrupting normal system operations, only if a second (redundant) CSC is installed in the system. One CSC must be present and operational at all times to maintain normal system operations. A second CSC provides data path, scheduler, and reference clock redundancy. The interfaces between the line cards and the switch fabric are monitored constantly. If the system detects a Loss of Synchronization (LoS), it automatically activates the data paths of the redundant CSC, and data flows across the redundant path. The switch to the redundant CSC occurs within microseconds, with little or no loss of data.