The crossbar switch – from concept to success

The crossbar switch was most popular from 1950 to 1980. Most new telephone switches around the world were based on some form of crossbar switching system. There were several different manufacturers, and from the 1940s and onward, Ericsson was one of the leaders in the development of this technology. In the early 1960s, the company's sales of crossbar switches exceeded those of the 500-swiching system, as measured in the number of lines.

Although the crossbar switch for all practical purposes was the successor to Ericsson's 500-switch, the two systems were conceived almost simultaneously. G A Betulander, a mechanical engineer on a leave of absence from Televerket, the Swedish PTT, worked together with Nils Palmgren in the 1910s to lay the foundation for the crossbar switching system. His small company, Nya Autotelefon Betulander, worked with the development of automatic telephone switches based on register-based and relay-controlled selectors. A major accomplishment was a patent application in 1912 for a system with links and markers that allowed large telephone stations to be built using relay-based selectors.

While operating a test station using this system that had been ordered by Televerket, Betulander and Palmgren began testing a new selector design in 1919 based on a crossbar switch that used fewer relays. This was possibly inspired by a previous design developed by the Bell company Western Electric, but the decisive factor was that their own system of links and markers was used.

Although the test station seems to have worked well, Betulander and Palmgren's crossbar switching system was not an immediate success. In Stockholm, their competition included two other systems offered by Ericsson that were both based on patents held by Televerket's chief engineer in Stockholm, Axel Hultman. As the representative for the purchaser Televerket, Hultman doubted that call completion rates could be determined for the crossbar system.

In late-1919, the small company Nay Autotelefon Betulander was acquired by Ericsson in exchange for a cash payment plus a royalty agreement on sales to Televerket, irregardless of whether the systems offered by Ericsson were based on Betulander and Palmgren's system. While Ericsson assumed responsibility for the marketing of this switch, the company in large part accepted Hultman's doubts regarding the calculation of call completion rates. It was therefore hardly surprising that Televerket one year later selected the 500-switch for automated equipment in Stockholm and Gothenburg.

After the sale of his company, G A Betulander resumed his employment at Televerket, where he was given the assignment of designing a station for Sundsvall based on his crossbar switch, although without the links that were considered to make calculations of the call completion rate more difficult, but which were required to make the system competitive for large stations. The station was taken into operation in 1926. More than one thousand such crossbar stations without links and markers were manufactured and installed by Televerket over the next two decades. The crossbar switching technology thus survived as a system for telephone stations in Sweden because it became the established standard for small and medium-size stations.

The crossbar switch's development into a system for large stations as Betulander and Palmgren had anticipated proceeded oddly enough via the US and Bell Laboratories. When the Bell companies attempted to develop a new system for large stations in the 1920s, their attention was drawn to the Swedish crossbar switches.

Following a study trip to Sweden in 1930, the American began development work spanning several years. In calculating call completion rates, they initially used rough estimates and relied on being able to empirically determine the system's traffic-handling characteristics, rather than calculating these characteristics in advance.

By 1938, this development effort had resulted in taking into operation two crossbar switching stations that used links and markers. When Televerket learned of the positive experiences from New York, where the installations had taken place, the operator began to develop a similar system, which was used for the first time in the automatic switching station in Sundsvall, when its capacity was expanded in 1945.

Ericsson also began to show interest in crossbar switching systems in the early 1940s. The company had some experience through orders from Televerket and began at this time to use crossbar switches in the registers for 500-switch stations. After several years of development work, the first Ericsson crossbar switching system using links was installed in Helsinki in 1950. Ericsson's sales of crossbar switches continued to grow over the next decade.

In its development work, Ericsson refined methods for calculating call completion rates in crossbar switching systems with links. These methods subsequently won widespread international approval.

Author: Claes-Fredrik Helgesson

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