Clos network

In the field of telecommunications, a Clos network is a kind of multistage circuit switching network, first formalized by Charles Clos in 1952, which represents a theoretical idealization of practical multi-stage telephone switching systems. Clos networks are required when the physical circuit switching needs to exceed the capacity of the largest feasible single crossbar switch. The key advantage of Clos networks is that the number of crosspoints (which make up each crossbar switch) required can be far fewer than would be the case if the entire switching system were implemented with one large crossbar switch. When the Clos network was first devised, the number of crosspoints was a reasonable approximate indication of the total cost of the switching system. While this was important for electromechanical crossbars, it became less relevant with the advent of VLSI, wherein the interconnects often could be implemented either directly in silicon, or within a relatively small cluster of boards. However, the advent of complex data centers, with huge interconnect structures, each based on optical fiber links, means that they are again important.