Editing Nonblocking minimal spanning switch (section)

===The crossbar switch===
The [[crossbar switch]] has the property of being able to connect N inputs to N outputs in any [[bijection|one-to-one]] combination, so it can connect any caller to any non-busy receiver, a property given the technical term "nonblocking".  Being nonblocking it could always complete a call (to a non-busy receiver), which would maximize service availability.

However, the crossbar switch does so at the expense of using N<sup>2</sup> (N squared) simple [[Switch#Contact terminology|SPST switches]].  For large N (and the practical requirements of a phone switch are considered large) this growth was too expensive.  Further, large crossbar switches had physical problems.  Not only did the switch require too much space, but the metal bars containing the switch contacts would become so long that they would sag and become unreliable.  Engineers also noticed that at any time, each bar of a crossbar switch was only making a single connection.  The other contacts on the two bars were unused.  This seemed to imply that most of the switching fabric of a crossbar switch was wasted.

The obvious way to emulate a crossbar switch was to find some way to build it from smaller crossbar switches.  If a crossbar switch could be emulated by some arrangement of smaller crossbar switches, then these smaller crossbar switches could also, in turn be emulated by even smaller crossbar switches.  The switching fabric could become very efficient, and possibly even be created from standardized parts.  This is called a [[Clos network]].