At the heart of the Cisco 12000 GSR is a multigigabit crossbar switch fabric that is optimized to provide high-capacity switching at gigabit rates. The crossbar switch enables high performance for two reasons: connections from the LCs to a centralized fabric are point-to-point links that can operate at very high speeds and multiple bus transactions can be supported simultaneously, increasing the aggregate bandwidth of the system. A GSR system can be configured as 40 Gbps for the Cisco 12008, 60 Gbps for the Cisco 120012, and 80 Gbps for the Cisco 12016.The switch fabric includes two card types: switch-fabric cards (SFCs) and clock and scheduler cards (CSC). Each GSR must have at least one CSC in the chassis. The CSC handles requests from LCs, 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. The SFCs receive the scheduling information and clocking reference from the CSC cards and perform the switching functions.