Crossbar switch

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A Crossbar switch is one of the principal architectures used to construct switches of many types. Crossbar switches are sometimes refered to as "Cross-point switches" or "Crosspoint switches". The other principal switch architectures are that of a Memory switch or a crossover switch. A Banyan switch is an important type of crossover switch.


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General properties of crossbar switches

Crossbar switches have a characteristic matrix of switches between the inputs to the switch an the output of the switch. If the switch has M inputs and N outputs, then a crossbar has a matrix with M x N cross-points or places where there the "bars" "cross". A given crossbar is a single layer, non-blocking switch. Collections of crossbars can be used to implement multiple layer and/or blocking switches.


Applications of crossbar switches

Crossbar switches are most famously used in information processing applications such as telephony and packet switching, but they are also used in applications such as mechanical sorting machines with multiple inputs.


Implementations of crossbar switches

Historically, a crossbar consisted of metal bars connected to each input and metal connected to each output with switches at each cross-point. Modern crossbar switches are usually implemented with semiconductor technolgy. An important emerging class of optical crossbars are being implemented with MEMS technology.


Semiconductor crossbar switches

Semiconductor implementations of crossbar switches typically consist of a set of input amplifiers or retimers connected to a series of metalizations or "bars" within a semiconductor device. A similar set of metalizations or "bars" are connected to output amplifiers or retimers. At each cross-point where the "bars" cross, a pass transistor is implemented which connects the bars. When the pass transistor is enabled, the input is connected to the output.


Electromechanical telephony crossbar switch

A telephony crossbar switch is an electromechanical device for switching telephone calls. They were first widely installed in the 1950s in both the United States and England, and from there quickly spread to the rest of the world. They replaced most earlier designs like the Strowger switch in larger installations.

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Crossbar2.jpg
Crossbar switch

Crossbar switches use switching matrices made from a two-dimensional array of contacts arranged in an x-y format. These switching matrices are operated by a series of bars arranged over the contacts. These bars can be rocked from side to side by electromagnets. A second set of bars is set at right angles to the first (hence the name, "crossbar") and also operated by electromagnets. One set of bars carries spring-loaded wire fingers that operate the contacts beneath the bars. By operating the electromagnets that move the bars, it is possible to close the contacts beneath the point where two bars cross. This then makes the connection through the switch to connect the telephone call. The crossbar switching interface was referred to as the TXC switch (Telephone eXchange Crossbar).

The crossbar system could control phone switching by watching the voltage on the lines connected to the user's phones. When the user picked up the handset, the voltage dropped from about 48 V to about 10 V, so the crossbar knew that person wanted to place a call. It would then play a dial tone and wait for the user to dial. It could also tell when the user had hung up when it saw the voltage increase back to about 48 V again. When a call was received, the crossbar would switch to an intermittent ring voltage of about 90 VAC at 20 Hz to make the hammer repeatedly strike the bell inside the phone and cause the phone to ring.

Although the crossbar was technically more advanced than the earlier systems it replaced, it was mechanically much simpler. The design criterion was to have two hours of "downtime" for service every forty years, which was a huge improvement. The crossbar was also smaller for any given number of lines, which is why it was primarily used in high-use areas in larger cities. For this reason they were also typically the first switches to be replaced with digital systems, which were even smaller and more reliable. There are probably no operating crossbar exchanges left, except in museums like the Science Museum in London.


Crossbar switch arbitration

A standard problem in using crossbar switches is that of setting the cross-points. In the classic telephony application of cross-bars, the crosspoints are closed and open as the telephone calls come and go. In Asynchronous Transfer Mode or packet switching applications, the crosspoints must be made and broken at each decision interval. In high-speed switches, the settings of all of the cross-points must be determined and then set millions or billions of times per second. One approach for making these decisions quickly is through the use of a Wavefront arbiter.de:Koppelfeld

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