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electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
and
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
, a crossbar switch (cross-point switch, matrix switch) is a collection of
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
es arranged in a
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
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 Solid-state electronics means semiconductor electronics: electronic equipment using semiconductor devices such as transistors, diodes and integrated circuits (ICs). The term is also used as an adjective for devices in which semiconductor electr ...
. The crossbar switch is one of the principal
telephone exchange telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...
architectures, together with a rotary switch, memory switch, 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 Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
and
circuit switching Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full ...
, 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 Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a si ...
devices which enables the data transmission. 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 A programmable read-only memory (PROM) is a form of digital memory where the contents can be changed once after manufacture of the device. The data is then permanent and cannot be changed. It is one type of read-only memory (ROM). PROMs are used ...
. At the 2008 NSTI Nanotechnology Conference a paper was presented that 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 Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
or
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' ('' data terminal equipment'') suc ...
connection by a whole-house automation controller, such as those made by AMX, Crestron, or
Control4 Control4 is a provider of automation and networking systems for homes and businesses, offering a personalized and unified smart home system to automate and control connected devices including lighting, audio, video, climate control, intercom, and ...
, 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.


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
microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
(MEMS) technology.


Mechanical

A type of mid-19th-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
electromechanical In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
device for switching
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
calls. The first design of what is now called a crossbar switch was the Bell company
Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
's coordinate selector of 1915. To save money on control systems, this system was organized on the
stepping switch In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical puls ...
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 Gotthilf is a male given name. Notable people with this name include: * Gotthilf Christoph Wilhelm Busolt (1771–1831), German scholar * Gotthilf Fischer (1928–2020), German choir and orchestra director * Gotthilf Hagen (1797–1884), German phy ...
design from 1919, inspired by the Western Electric system), and used it in Sweden from 1926 until the digitization in the 1980s in small and medium-sized A204 model switches. The system design used in
AT&T Corporation AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agen ...
's 1XB crossbar exchanges, which entered revenue service from 1938, developed by Bell Telephone Labs, was inspired by the Swedish design but was based on the rediscovered link principle. In 1945, a similar design by Swedish Televerket was installed in Sweden, making it possible to increase the capacity of the A204 model switch. Delayed by the Second World War, several millions of urban 1XB lines were installed from the 1950s in the United States. In 1950, the
Ericsson (lit. "Telephone Stock Company of LM Ericsson"), commonly known as Ericsson, is a Swedish multinational networking and telecommunications company headquartered in Stockholm. The company sells infrastructure, software, and services in inform ...
Swedish company developed their own versions of the 1XB and A204 systems for the international market. In the early 1960s, the company's sales of crossbar switches exceeded those of their rotating 500-switching system, as measured in the number of lines. Crossbar switching quickly spread to the rest of the world, replacing most earlier designs like the Strowger (step-by-step) and
Panel Panel may refer to: Arts and media Visual arts *Panel (comics), a single image in a comic book, comic strip or cartoon; also, a comic strip containing one such image *Panel painting, in art, either one element of a multi-element piece of art, ...
systems in larger installations in the U.S. Graduating from entirely electromechanical control on introduction, they were gradually elaborated to have full electronic control and a variety of
calling feature A vertical service code (VSC) is a sequence of digits and the signals star (*) and number sign (#) dialed on a telephone keypad or rotary dial to enable or disable certain telephone service features. Some vertical service codes require dialing of ...
s including short-code and speed-dialing. In the UK the
Plessey The Plessey Company plc was a British electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after World War II by acquisition of companies and formed overseas compan ...
Company produced a range of TXK crossbar exchanges, but their widespread rollout by the British Post Office began later than in other countries, and then was inhibited by the parallel development of TXE
reed relay A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narro ...
and electronic exchange systems, so they never achieved a large number of customer connections although they did find some success as tandem switch exchanges. 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
electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...
s 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 Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is co ...
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 The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
''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
tip and ring Tip and ring are the two conductors or sides of a telephone line. Their names are derived from the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal ''tip'' of the plug, and ...
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 In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its ...
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 Northern Electric was an electricity supply and distribution company serving north east England. History It had its origins as the North Eastern Electricity Board, formed as part of the nationalisation of the electricity industry by the Elec ...
''Minibar'' used in
SP1 switch SP-1 (Stored Program 1) was the name of a computerized telephone exchange (a so-called switching office) manufactured by Northern Electric (later Northern Telecom and now Nortel Networks beginning in 1972) in Canada. It was introduced in 1971 (as ...
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.


Instrumentation

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.


Telephone exchange

Early crossbar exchanges were divided into an originating side and a terminating side, while the later and prominent Canadian and US
SP1 switch SP-1 (Stored Program 1) was the name of a computerized telephone exchange (a so-called switching office) manufactured by Northern Electric (later Northern Telecom and now Nortel Networks beginning in 1972) in Canada. It was introduced in 1971 (as ...
and 5XB switch were not. When a user picked up the
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
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 send the ring signal to the called user's phone, 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 In telecommunications, common control is a principle of switching telephone calls in an automatic telephone exchange that employs shared control equipment which is attached to the circuit of a call only for the duration of establishing or otherwise ...
elements, which were very reliable as relay sets. The design criteria specified only two hours of
downtime The term downtime is used to refer to periods when a system is unavailable. The unavailability is the proportion of a time-span that a system is unavailable or offline. This is usually a result of the system failing to function because of an un ...
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 Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics, electronic circuits which operate using digital signals ** Digital camera, which captures and stores 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 switch The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of it ...
es. 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. 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 to the other contacts on the same level by bare-strip wiring, often nicknamed ''banjo wiring''. 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 one hundred conversations in progress. The link principle was more efficient, but required a complex control system to find idle links through the
switching fabric Switched fabric or switching fabric is a network topology in which network nodes interconnect via one or more network switches (particularly crossbar switches). Because a switched fabric network spreads network traffic across multiple physica ...
. This meant
common control In telecommunications, common control is a principle of switching telephone calls in an automatic telephone exchange that employs shared control equipment which is attached to the circuit of a call only for the duration of establishing or otherwise ...
, 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
museum A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make th ...
s, such as the Museum of Communications in Seattle, Washington, and the
Science Museum A science museum is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in ...
in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
.


Semiconductor

Semiconductor implementations of crossbar switches typically consist of a set of input
amplifiers An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...
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 Multistage interconnection networks (MINs) are a class of high-speed computer networks usually composed of processing elements (PEs) on one end of the network and memory elements (MEs) on the other end, connected by switching elements (SEs). Th ...
that connect the various processing units in a
uniform memory access Uniform memory access (UMA) is a shared memory architecture used in parallel computers. All the processors in the UMA model share the physical memory uniformly. In an UMA architecture, access time to a memory location is independent of which proc ...
parallel processor to the array of memory elements.


Arbitration

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 Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by American National Standards Institute (ANSI) and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs of ...
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 A matrix mixer is an audio electronics device that routes multiple input audio signals to multiple outputs. It usually employs level controls such as potentiometers to determine how much of each input is going to each output, and it can incorp ...
*
Nonblocking minimal spanning switch A nonblocking minimal spanning switch is a device that can connect N inputs to N outputs in any combination. The most familiar use of switches of this type is in a telephone exchange. The term "non-blocking" means that if it is not defective, ...
- describes how to combine crossbar switches into larger switches. * RF switch matrix


References


Further reading

* *


External links


Images on an Ericsson ARF crossbar switch
{{Telecommunications Switches Telephone exchange equipment Electronic circuits