The Info List - Crossbar Switch

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In electronics , 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 semiconductor chips. The cross-point switch is one of the principal switch architectures, together with a rotary switch , memory switch, and a crossover switch .


* 1 General properties * 2 Applications

* 3 Implementations

* 3.1 Mechanical * 3.2 Electromechanical switching in telephony * 3.3 Instrumentation * 3.4 Telephone
exchange * 3.5 Semiconductor

* 4 Arbitration * 5 See also * 6 References * 7 Further reading * 8 External links


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 the connections cross. 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. Non-blocking means that other concurrent connections do not prevent connecting other inputs to other outputs. Collections of crossbars can be used to implement multiple layer and blocking switches. A crossbar switching system is also called a coordinate switching system.


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 semiconductor memory devices. Here the "bars" are extremely thin metal "wires", and the "switches" are fusible links . The fuses are blown or opened using high voltage and read using low voltage. Such devices are called programmable read-only memory . At the 2008 NSTI Nanotechnology Conference a paper was presented which discussed a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation.

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 sports bar , where numerous programs are displayed simultaneously. Ordinarily, a sports bar would install a separate desk top box for each display for which independent control is desired. The matrix switch enables the operator to route signals at will, so that only enough set top boxes are needed to cover the total number of unique programs to be viewed, while making it easier to control sound from any program in the overall sound system.

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 Ethernet
or RS-232 connection by a whole-house automation controller, such as those made by AMX , Crestron , or Control4 , which provides the user interface that enables the user in each room to select which appliance to watch. The actual user interface varies by system brand, and might include a combination of on-screen menus, touch-screens, and handheld remote controls. The system is necessary to enable the user to select the program they wish to watch from the same room they will watch it from, otherwise it would be necessary for them to walk to the equipment rack.

The special crossbar switches used in distributing satellite TV signals are called multiswitches .


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. In the later part of the 20th century, these literal crossbar switches declined and the term came to be used figuratively for rectangular array switches in general. Modern "crossbar switches" are usually implemented with semiconductor technology. An important emerging class of optical crossbars is being implemented with MEMS technology.


A type of middle 19th-century telegraph exchange consisted of a grid of vertical and horizontal brass bars with a hole at each intersection. The operator inserted a brass pin to connect one telegraph line to another.


A telephony crossbar switch is an electromechanical device for switching telephone calls. The first design of what is now called a crossbar switch was the Bell company Western Electric
Western Electric
's "coordinate selector" of 1915. To save money on control systems, this system was organized on the stepping switch or selector principle rather than the link principle. It was little used in America, but the Televerket Swedish governmental agency manufactured its own design (the Gotthilf Betulander design from 1919, inspired by the Western Electric
Western Electric
system), and used it in Sweden from 1926 until the digitalization in the 1980s in small and medium-sized A204 model switches. The system design used in AT"> Western Electric
Western Electric
100-point six-wire Type B crossbar switch

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. Late-model Western Electric
Western Electric
crossbar switch Back of Type C

The majority of Bell System switches were made to connect three wires including the tip and ring of a balanced pair circuit and a sleeve lead for control. Many connected six wires, either for two distinct circuits or for a four wire circuit or other complex connection. The Bell System Type C miniature crossbar of the 1970s was similar, but the fingers projected forward from the back and the select bars held paddles to move them. The majority of type C had twelve levels; these were the less common ten level ones. The Northern Electric Minibar used in SP1 switch was similar but even smaller. The ITT Pentaconta Multiswitch of the same era had usually 22 verticals, 26 levels, and six to twelve wires. Ericsson
crossbar switches sometimes had only five verticals.


For instrumentation use, James Cunningham, Son and Company made high-speed, very-long-life crossbar switches 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 and 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.


Early crossbar exchanges were divided into an originating side and a terminating side, while the later and prominent Canadian and US SP1 switch and 5XB switch were not. When a user picked up the telephone 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 dialed 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 trunk call completion requirements (type of pulsing, resistance of the trunk, etc.) and the called party's details to the sender and released. The sender then relayed this information 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 V AC at 20 Hz to ring the called user's phone bell, and return ringing tone to the caller.

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 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.

Two principles of crossbar switching existed. One early method was based on the selector principle, and used the switches as functional replacement for Strowger or stepping switches . Control was distributed to the switches themselves. Call establishment progressed through the exchange stage by stage, as successive digits were dialed. With the selector principle, each switch could only handle its portion of one call at a time. Each moving contact of the array was multipled to corresponding crosspoints on other switches to a selector in the next bank of switches. 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. "Banjo" wiring of a 100-point six wire Type B Bell System switch

Starting with the 1XB switch , the later and more common method was based on the link principle, and used the switches as crosspoints. Each moving contact was multipled to the other contacts on the same level by simpler "banjo" wires, to a link on one of the inputs of a switch in the next stage. The switch could handle its portion of as many calls as it had levels or verticals. Thus an exchange with forty 10×10 switches in four stages could have a hundred conversations in progress. The link principle was more efficient, but required a more complex control system to find idle links through the switching fabric .

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 TXK 1 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.

In some countries, no crossbar exchanges remain in revenue service. However, crossbar exchanges remain in use, for example in Russia
, where some massive city telephone networks have not yet been fully upgraded to digital technology. Preserved installations are maintained in museums , such as the Museum
of Communications in Seattle, Washington, and the Science Museum
in London

In current American terminology, switch frequently refers to a system which is also called telephone exchange, the usual term in English, which is a large collection of selectors of some sort within a building. For most of the twentieth century the terms Strowger switch and crossbar switch referred to an individual piece of mechanical equipment making up part of an exchange. Hence the pictures above show a "crossbar switch" using the earlier meaning.


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.


A standard problem in using crossbar switches is that of setting the crosspoints. 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 .


* Nonblocking minimal spanning switch - describes how to combine crossbar switches into larger switches. * RF switch matrix


* ^ Yong Chen, Gun-Young Jung, Douglas A A Ohlberg, Xuema Li, Duncan R Stewart, Jan O Jeppesen, Kent A Nielsen, and J Fraser Stoddart, Nanoscale molecular-switch crossbar circuits, 2003 Nanotechnology 14 462-468 * ^ Mouttet, B. (2008-06-02). "Logicless Computational Architectures with Nanoscale Crossbar Arrays". NSTI Nanotech 2008 Conference. Retrieved 2008-06-02. * ^ http://www.obs-us.com/people/karen/cunningham/chapter6.htm#47 * ^ http://www.obs-us.com/people/karen/cunningham/47.htm


* Pacific Telephone
and Telegraph Company. General Administration Engineering Section (1956). Survey of telephone switching. San Francisco, California. OCLC 11376478 . * Scudder, F.J. (January 1939). "Crossbar Dial Telephone
Switching System". Bell System Technical Journal. 8 (1): 76–118. Retrieved 23 April 2015.