Definition

Clos network

A Clos network is a type of non-blocking, multistage switching architecture that reduces the number of ports required in an interconnected fabric.

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.

Contributor(s): Jessica Scarpati, Tom Nolle
This was last updated in January 2014
Posted by: Margaret Rouse

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