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.

Clos networks have three stages: the ingress stage, middle stage, and the egress stage. Each stage is made up of a number of crossbar switches (see diagram below), often just called crossbars. Each call entering an ingress crossbar switch can be routed through any of the available middle stage crossbar switches, to the relevant egress crossbar switch. A middle stage crossbar is available for a particular new call if both the link connecting the ingress switch to the middle stage switch, and the link connecting the middle stage switch to the egress switch, are free.

Clos networks are defined by three integers n, m, and r. n represents the number of sources which feed into each of r ingress stage crossbar switches. Each ingress stage crossbar switch has m outlets, and there are m middle stage crossbar switches. There is exactly one connection between each ingress stage switch and each middle stage switch. There are r egress stage switches, each with m inputs and n outputs. Each middle stage switch is connected exactly once to each egress stage switch. Thus, the ingress stage has r switches, each of which has n inputs and m outputs. The middle stage has m switches, each of which has r inputs and r outputs. The egress stage has r switches, each of which has m inputs and n outputs.

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