Editing Nonblocking minimal spanning switch (section)

==Digital switches==

A practical implementation of a switch can be created from an '''odd''' number of layers of smaller subswitches.  Conceptually, the crossbar switches of the three-stage switch can each be further decomposed into smaller crossbar switches.  Although each subswitch has limited multiplexing capability, working together they synthesize the effect of a larger ''N''×''N'' crossbar switch.

In a modern digital telephone switch, application of two different multiplexer approaches in alternate layers further reduces the cost of the switching fabric:

#space-division [[multiplexer]]s are something like the [[crossbar switch]]es already described, or some arrangement of [[crossover switch]]es or [[banyan switch]]es. Any single output can select from any input.  In digital switches, this is usually an arrangement of [[AND gate]]s.  8000 times per second, the connection is reprogrammed to connect particular wires for the duration of a [[time-division multiplexing|time slot]]. '''Design advantage:''' In space-division systems the number of space-division connections is divided by the number of time slots in the time-division multiplexing system.  This dramatically reduces the size and expense of the switching fabric.  It also increases the reliability, because there are far fewer physical connections to fail.
#[[Time-slot interchange|time-division multiplexer]]s each have a memory which is read in a fixed order and written in a programmable order (or ''vice versa'').  This type of switch permutes time-slots in a [[time-division multiplexing|time-division multiplexed signal]] that goes to the space-division multiplexers in its adjacent layers. '''Design advantage:'''  Time-division switches have only one input and output wire.  Since they have far fewer electrical connections to fail, they are far more reliable than space-division switches, and are therefore the preferred switches for the outer (input and output) layers of modern telephone switches.

Practical digital telephonic switches minimize the size and expense of the electronics. First, it is typical to "fold" the switch, so that both the input and output connections to a subscriber-line are handled by the same control logic.  Then, a time-division switch is used in the outer layer. The outer layer is implemented  in subscriber-line interface cards (SLICs) in the local presence street-side boxes. Under remote control from the central switch, the cards connect to timing-slots in a time-multiplexed line to a central switch. In the U.S. the multiplexed line is a multiple of a [[T-carrier|T-1 line]].  In Europe and many other countries it is a multiple of an [[E-carrier|E-1 line]].

The scarce resources in a [[telephone switch]] are the connections between layers of subswitches.  These connections can be either time slots or wires, depending on the type of multiplexing. The [[control logic]] has to allocate these connections, and the basic method is the algorithm already discussed.  The subswitches are logically arranged so that they synthesize larger subswitches.  Each subswitch, and synthesized subswitch is controlled ([[recursion|recursively]]) by logic derived from Clos's mathematics. The computer code decomposes larger multiplexers into smaller multiplexers.

If the recursion is taken to the limit, breaking down the crossbar to the minimum possible number of switching elements, the resulting device is sometimes called a [[crossover switch]] or a [[banyan switch]] depending on its topology.

Switches typically interface to other switches and fiber optic networks via fast multiplexed data lines such as [[SONET]].

Each line of a switch may be periodically tested by the computer, by sending test data through it. If a switch's line fails, all lines of a switch are marked as in use. Multiplexer lines are allocated in a first-in-first out way, so that new connections find new switch elements. When all connections are gone from a defective switch, the defective switch can be avoided, and later replaced.

As of 2018, such switches are no longer made. They are being replaced by high-speed [[Internet Protocol]] routers.