Crossbar switch
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In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of interconnecting lines between which a connection may be established by closing a switch located at each intersection, the elements of the matrix. Originally, a crossbar switch consisted literally of crossing metal bars that provided the input and output paths. Later implementations achieved the same switching topology in solid-state electronics. The crossbar switch is one of the principal telephone exchange architectures, together with a rotary switch, memory switch,[2] and a crossover switch.
General properties
A crossbar switch is an assembly of individual switches between a set of inputs and a set of outputs. The switches are arranged in a matrix. If the crossbar switch has M inputs and N outputs, then a crossbar has a matrix with M × N cross-points or places where connections can be made. At each crosspoint is a switch; when closed, it connects one of the inputs to one of the outputs. A given crossbar is a single layer, non-blocking switch. A crossbar switching system is also called a coordinate switching system.
Collections of crossbars can be used to implement multiple layer and blocking switches. A blocking switch prevents connecting more than one input. A non-blocking switch allows other concurrent connections from inputs to other outputs.
Applications
Crossbar switches are commonly used in information processing applications such as telephony and circuit switching, but they are also used in applications such as mechanical sorting machines.
The matrix layout of a crossbar switch is also used in some
Matrix arrays are fundamental to modern flat-panel displays. Thin-film-transistor LCDs have a transistor at each crosspoint, so they could be considered to include a crossbar switch as part of their structure.
For video switching in home and professional theater applications, a crossbar switch (or a matrix switch, as it is more commonly called in this application) is used to distribute the output of multiple video appliances simultaneously to every monitor or every room throughout a building. In a typical installation, all the video sources are located on an equipment rack, and are connected as inputs to the matrix switch.
Where central control of the matrix is practical, a typical rack-mount matrix switch offers front-panel buttons to allow manual connection of inputs to outputs. An example of such a usage might be a
Such switches are used in high-end home theater applications. Video sources typically shared include set-top receivers or DVD changers; the same concept applies to audio. The outputs are wired to televisions in individual rooms. The matrix switch is controlled via an
The special crossbar switches used in distributing satellite TV signals are called multiswitches.
Implementations
Historically, a crossbar switch consisted of metal bars associated with each input and output, together with some means of controlling movable contacts at each cross-point. The first switches used metal pins or plugs to bridge a vertical and horizontal bar. In the later part of the 20th century, the use of mechanical crossbar switches declined and the term described any rectangular array of switches in general. Modern crossbar switches are usually implemented with semiconductor technology. An important emerging class of optical crossbars is implemented with
Mechanical
A type of mid-20th-century telegraph exchange consisted of a grid of vertical and horizontal brass bars with a hole at each intersection (c.f. top picture). The operator inserted a metal pin to connect one telegraph line to another.
Electromechanical switching in telephony
A telephony crossbar switch is an
In 1950, the Swedish
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 horizontal bars arranged over the contacts. Each such select bar can be rocked up or down by electromagnets to provide access to two levels of the matrix. A second set of vertical hold bars is set at right angles to the first (hence the name, "crossbar") and also operated by electromagnets. The select bars carry spring-loaded wire fingers that enable the hold bars to operate the contacts beneath the bars. When the select and then the hold electromagnets operate in sequence to move the bars, they trap one of the spring fingers to close the contacts beneath the point where two bars cross. This then makes the connection through the switch as part of setting up a calling path through the exchange. Once connected, the select magnet is then released so it can use its other fingers for other connections, while the hold magnet remains energized for the duration of the call to maintain the connection. The crossbar switching interface was referred to as the TXK or TXC (telephone exchange crossbar) switch in the UK.
However, the Bell System Type B crossbar switch of the 1960s was made in the largest quantity. The majority were 200-point switches, with twenty verticals and ten levels of three wires. Each select bar carries ten fingers so that any of the ten circuits assigned to the ten verticals can connect to either of two levels. Five select bars, each able to rotate up or down, mean a choice of ten links to the next stage of switching. Each crosspoint in this particular model connected six wires. The vertical off-normal contacts next to the hold magnets are lined up along the bottom of the switch. They perform logic and memory functions, and the hold bar keeps them in the active position as long as the connection is up. The horizontal off-normals on the sides of the switch are activated by the horizontal bars when the butterfly magnets rotate them. This only happens while the connection is being set up, since the butterflies are only energized then.
The majority of Bell System switches were made to connect three wires including the
Instrumentation
For instrumentation use, James Cunningham, Son and Company[5] made high-speed, very-long-life crossbar switches[6] with physically small mechanical parts which permitted faster operation than telephone-type crossbar switches. Many of their switches had the mechanical Boolean AND function of telephony crossbar switches, but other models had individual relays (one coil per crosspoint) in matrix arrays, connecting the relay contacts to [x] and [y] buses. These latter types were equivalent to separate relays; there was no logical AND function built in. Cunningham crossbar switches had precious-metal contacts capable of handling millivolt signals.
Telephone exchange
Early crossbar exchanges were divided into an originating side and a terminating side, while the later and prominent Canadian and US
The crossbar switch itself was simple: exchange design moved all the logical decision-making to the common control elements, which were very reliable as relay sets. The design criteria specified only two hours of downtime for service every forty years, which was a large improvement over 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.
Two principles of crossbar switching existed. An early method was based on the selector principle, which used crossbar switches to implement the same switching fabric used with Strowger switches. In this principle, each crossbar switch would receive one dialed digit, corresponding to one of several groups of switches or trunks. The switch would then find an idle switch or trunk among those selected and connect to it. Each crossbar switch could only handle one call at a time; thus, an exchange with a hundred 10×10 switches in five stages could only have twenty conversations in progress. 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 degrades the traffic capacity of a switch.[citation needed]
Starting with the
This meant 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. 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.
The Plessey TXK1 or 5005 design used an intermediate form, in which a clear path was marked through the switching fabric by distributed logic, and then closed through all at once.
Crossbar exchanges remain in revenue service only in a few telephone networks. Preserved installations are maintained in
Semiconductor
Semiconductor implementations of crossbar switches typically consist of a set of input amplifiers or retimers connected to a series of interconnects within a semiconductor device. A similar set of interconnects 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.
Arbitration
A standard problem in using crossbar switches is that of setting the crosspoints.[citation needed] In the classic telephony application of crossbars, 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 crosspoints 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
- Matrix mixer
- Nonblocking minimal spanning switch - describes how to combine crossbar switches into larger switches.
- RF switch matrix
References
- ^ Kennedy, Rankin (1903 edition (five volumes) of pre-1903 four volume edition.) Electrical Installations, vol. V, London: Caxton
- ^ "Crossbar Systems – Telecommunications Heritage Group". Retrieved 2023-05-03.
- S2CID 250853934.
- ^ Mouttet, B. (2008-06-02). "Logicless Computational Architectures with Nanoscale Crossbar Arrays". NSTI Nanotech 2008 Conference. Archived from the original on 2016-03-04. Retrieved 2008-06-02.
- ^ Hinrichs, Noël (1964). "6. The Era of Automation". The Pursuit of Excellence. James Cunningham, Son & Co.
- ^ Hinrichs 1964, Crossbar Switch
- ^ The Western Electric Engineer: Volumes 5-7. Western Electric. 1961. p. 23.
Further reading
- Pacific Telephone and Telegraph Company. General Administration Engineering Section (1956). Survey of telephone switching. San Francisco, California. )
- Scudder, F.J.; Reynolds, J.N. (January 1939). "Crossbar Dial Telephone Switching System". Bell System Technical Journal. 8 (1): 76–118. S2CID 51659407. Retrieved 23 April 2015.