Editing Crossbar switch (section)

===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 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]] 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 data|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]]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.{{citation needed|date=January 2022}}

[[File:Crossbar-banjo2-hy.jpg|thumb| Bare-strip 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 {{not a typo|multipled}} to the other contacts on the same level by bare-strip wiring, often nicknamed ''banjo wiring''.<ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |date=1961 |title=The Western Electric Engineer: Volumes 5-7 |url=https://books.google.com/books?id=bXMjAQAAMAAJ&q=%22banjo+wiring%22 |publisher=[[Western Electric]] |page=23}}</ref> 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]].

This meant [[common control]], as described above: all the digits were recorded, then passed to the common control equipment, the [[Marker (telecommunications)|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.

Crossbar exchanges remain in revenue service only in a few telephone networks. Preserved installations are maintained in [[museum]]s, such as the [[Museum of Communications]] in Seattle, Washington, and the [[Science Museum (London)|Science Museum]] in [[London]].