Crossbar switch
From Free net encyclopedia
A crossbar switch is one of the principal architectures used to construct switches of many types. Crossbar switches are sometimes referred 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
Crossbar switches have a characteristic matrix of switches between the inputs to the switch and 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
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
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 technology. An important emerging class of optical crossbars are being implemented with MEMS technology.
Semiconductor
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.
As computer technologies have improved, crossbar switches have found uses in systems such as the multistage interconnection networks that connect the various processing units in a uniform memory access parallel processor to the array of memory elements.
Electromechanical / telephony
A telephony crossbar switch is an electromechanical device for switching telephone calls. The first design of what is now called a crossbar switch was Western Electric's "coordinate selector" of 1915. The design as used in AT&T's crossbar exchanges, which entered revenue service from 1938, was developed by Bell Telephone Labs. Delayed by the Second World War, they were widely installed from the 1950s in the United States, and from there quickly spread to the rest of the world. They replaced most earlier designs like the Strowger and Panel systems in larger installations in the U.S., where, graduating from entirely electromechanical control on introduction, they were gradually elaborated to have full electronic control and a large variety of extra services like short-code and speed dialling. In the U.K. the Plessey Company produced a range of crossbar exchanges, but their widespread rollout by the British Post Office was inhibited by the parallel development of reed relay and electronic exchange systems, and they never achieved a large number of customer connections although they did find some success as tandem exchanges.
Image:Crossbar2.jpg 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).
A crossbar exchange was divided into an originating side and a terminating side. When a user picked up their handset, the resulting line loop operating the user's line relay caused the exchange to connect the user's telephone to an originating sender, which returned the user a dial tone. The sender then recorded the dialled digits and passed them to the originating marker, which selected an outgoing trunk and operated the various crossbar switch stages to connect the calling user to it. The originating marker then passed the called party's details to a terminating sender (which could be on either the same or a different exchange). This sender then used a terminating marker to connect the calling user, via the selected incoming trunk, to the called user, and caused the controlling relay set to pass intermittent ring voltage of about 90 VAC at 20 Hz to ring the called user's phone bell, and return ringing tone to the caller.
The crossbar switch itself was very simple: exchange design moved all the logical decision-making to the common control elements, which as relay sets were themselves very reliable. The design criterion was to have two hours of "downtime" for service every forty years, which was a huge improvement on earlier electromechanical systems. The exchange design concept lent itself to incremental upgrades, as the control elements could be replaced separately from the call switching elements. The minimum size of a crossbar exchange was comparatively large, but in city areas with a large installed line capacity the whole exchange occupied less space than other exchange technologies of equivalent capacity. For this reason they were also typically the first switches to be replaced with digital systems, which were even smaller and more reliable.
In some countries, there are no crossbar exchanges left in revenue service. However, crossbar exchanges remain in use in countries like Russia, where some massive city telephone networks have not yet been fully upgraded to digital technology. Preserved installations may be seen in museums like the Science Museum in London.
Two methods of controlling crossbar switches were used. One method used the switches as functional replacement for Strowger switches, and the control was distributed to the switches themselves. Call establishment progressed through the exchange stage by stage, as successive digits were dialled. The more common method used common control, as described above: all the digits were recorded, then passed to the common control equipment - the marker - to establish the call at all the separate switch stages simultaneously. Distributed control meant there was no common point of failure, but also meant that the setup stage lasted for the ten seconds or so the caller took to dial the required number. In control occupancy terms this comparatively long interval seriously degrades the traffic capacity of a switch. By contrast a marker-controlled crossbar system had in the marker a highly vulnerable central control; this was invariably protected by having duplicate markers. The great advantage was that the control occupancy on the switches was of the order of one second or less, representing the operate and release lags of the X-then-Y armatures of the switches. The only downside of common control was the need to provide digit recorders enough to deal with the greatest forecast originating traffic level on the exchange.
Changing nomenclature can confuse: in current terminology a 'switch' now frequently refers to something which was once called a 'telephone exchange' - that is, a large collection of selectors of some sort within a building. Formerly a 'Strowger switch' or a 'crossbar switch' referred to an individual piece of equipment making up part of an exchange. Hence the picture above shows a 'crossbar switch' using the second meaning.
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.
See also
- nonblocking minimal spanning switch - describes how to combine crossbar switches into larger switches.de:Koppelfeld