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Clos networks are named after Bell Labs researcher Charles Clos, who proposed the model in 1952 as a way to overcome the performance- and cost-related challenges of electromechanical switches then used in telephone networks. Clos used mathematical theory to prove that achieving non-blocking performance in a "switching array" (now known as a fabric) was possible if the switches were organized in a hierarchy. Prior to Clos' discovery, engineers trying to achieve any-to-any connectivity with non-blocking performance required them to design networks in which the number of crosspoints -- electromechanical relay mechanisms in cross-bar switches -- had to equal the number of inputs multiplied by the number of outputs (known as n-squared).
Although electromechanical switching has given way to newer switching technologies, Clos networks have resurfaced in the design of high-performance switches in data center fabrics because of the efficiency gains they offer. In a modern context, a Clos network provides non-blocking performance in an interconnected Ethernet switch fabric without the need for n-squared ports.
The efficiency of a Clos network is proportional to the size of the array, with larger networks that contain thousands or tens of thousands of ports gaining the most benefits.