The Cisco 12012 requires that one clock and scheduler card be installed in the lower card cage. You can add one additional clock and scheduler card for redundancy and to increase the amount of data able to be transferred between line cards. The clock and scheduler card contains the following functionality. System clock is sent to all line cards, RP, and switch fabric cards. The system clock synchronizes data transfers between line cards or line cards and the RP through the switch fabric. In systems with redundant clock and scheduler cards, the two system clocks are synchronized. Should one system clock fail, the other clock takes over. Scheduler handles requests from the line cards for access to the switch fabric. When the scheduler receives a request from a line card for switch fabric access, the scheduler determines when to allow the line card access to the switch fabric. Switch fabric is the circuitry that carries the user traffic between line cards or between the RP and a line card. The switch fabric on the clock and scheduler card is identical to the switch fabric on the switch fabric card.