From: http://www.cisco.com/en/US/products/hw/ ... 1.shtml#n3
Q. What is a Switch Fabric Card (SFC) and a Clock and Scheduler Card (CSC)?
A. The SFC and CSC provide the physical switch fabric for the system as well as the clocking for the Cisco cells that carry data and control packets among the line cards and route processors.
On the 12008, 12012, and 12016, you must have at least one CSC for the router to run. Having only one CSC and no SFCs is called quarter bandwidth, and only works with Engine 0 Line Cards (LCs). If other LCs are in the system, they are automatically shut down. If you require LCs other than Engine 0, full bandwidth (three SFCs and one CSC) must be installed in the router. If redundancy is required, a second CSC is necessary. This redundant CSC only functions if either the CSC or an SFC goes bad. The redundant CSC can function as either the CSC or SFC.