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Crossbar switch

A crossbar switch (also known as cross-point switch, crosspoint switch, or matrix switch) is a switch connecting multiple inputs to multiple outputs in a matrix manner. Originally the term was used literally, for a matrix switch controlled by a grid of crossing metal bars, and later was broadened to matrix switches in general. It's one of the principal switch architectures, together with a memory switch and a crossover switch.

General properties

Crossbar switch has a characteristic matrix of switches between the inputs and the outputs. If the switch has M inputs and N outputs, then a crossbar has a matrix with M x N cross-points or places where 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 inputs.

The crossbar "format" is also being used in some experimental high-density memory devices (see nanotechnology). 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. Depending on the design, the devices may be Programmable read-only memory or reprogrammable.

At the 2008 NSTI Nanotechnology Conference a paper was presented discussing a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation.

Implementations

Historically, a crossbar switch consisted of metal bars associated with each input and output, 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 are being implemented with MEMS technology.

Mechanical

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.

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. It was little used in America, but the LM Ericsson company used an improved version for rural exchanges in Sweden. To save money on control systems, this system was organized on the stepping switch or selector principle rather than the link principle. The system design used in AT&T's 1XB crossbar exchanges, which entered revenue service from 1938, was developed by Bell Telephone Labs, based on the rediscovered link principle. Delayed by the Second World War, several millions of urban 1XB lines were installed from the 1950s in the United States. Crossbar switching quickly spread to the rest of the world, replacing most earlier designs like the Strowger and Panel 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 features including short-code and speed dialling. In the UK the Plessey Company produced a range of 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 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 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 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 fingers 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 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 switch (Telephone eXchange Crossbar) - in the UK.

The Bell System Type B crossbar switch of the 1960s was the most produced type. The majority were 200 point switches, with twenty verticals and ten levels of three wires, but this example is a 100 point six wire switch. Each select bar carries ten fingers so 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. Note the Vertical Off-Normal contacts next to the hold magnets, 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; many six. 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.

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 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 VAC 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 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.

Two principles of using crossbar switches were used. 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 dialled. 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 10x10 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 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 10x10 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 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.

In some countries, no crossbar exchanges remain 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 Museum of Communications in Seattle, Washington, and the Science Museum in London.

Changing nomenclature can confuse: in current American terminology a 'switch' now frequently refers to a system which is also called a 'telephone exchange' (the usual term in English) - that is, a large collection of selectors of some sort within a building. For most of the twentieth century a 'Strowger switch' or a '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

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.

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.