Local Central Office Switches
Short-Haul Tandem Switches
Long-Haul Tandem Switches
Crossbar telephone switching systems were developed by LM Ericsson of Sweden in the 1930s. Ericsson switches were very popular in Europe, especially in the United Kingdom.
Western Electric (aka the Bell System) obtained licenses from Ericsson to develop their own crossbar switches. The Crossbar switch had a significant advantage over either Panel or Step by Step since it was able to use a "store and forward" concept where it would take incoming digits, store them, and then process the call. It was also able to do call routing and determine where a call should be sent to by doing "translations" of the incoming digits and deciding how to send the call.
The concept of the crossbar is the crossbar mechanism operates on a matrix concept where dialed digits would change the position of the crossbar and connect to other parts of the matrix. It was still the same "level" concept of the step by step and panel but using more efficient and smaller equipment.
Local Central Office Crossbar Switches
The first Western Electric developed crossbar switch was the #1 Crossbar (#1XB for short). The #1 Crossbar was designed for use in large metropolitan areas such as New York City, Chicago, Pittsburgh, Boston and the like. Often times the #1 Crossbar was used in conjunction with existing Panel switches in the same building. The first installation of a #1 Crossbar was in 1938 at the Troy Avenue central office in Brooklyn, NY.
Technolgical advancements came fast and furious after World War II. Western Electric developed an advanced crossbar switch known as the #5 Crossbar (#5XB for short). The #5 Crossbar was similar to the #1 Crossbar but an advanced design. The first installation of a #5 Crossbar was on July 11, 1948 in Media, PA.The #5 Crossbar became the “bread and butter” telephone switch of a growing nation in the 1950s and 1960s. The last #5 Crossbar was built in 1969.
The #5XB was also significant since it was was the first switch to utilize (with modified
originating registers) to handle customer dialed Dual Tone Multi-Frequency (DTMF, aka “Touch Tone”) dialing in the early 1962.
Western Electric #5 Crossbar Master Test Frame
at the Museum of Communications (Seattle, WA)
Short-Haul Tandem Crossbar Switches
In the late 1930s, it became apparent that switches needed to be developed to handle “in route” short and long haul handling of regional and toll (long distance) calling. These specialized switches would be able to connect groups of switches in a region to groups of other switches in other regions more effectively. In large cities such as New York City, there were specialized switches just to handle the local calling for several million people. These specialized switches are known as “tandem” switches.
The first Western Electric tandem crossbar switch was known as the “Crossbar Tandem” or XBT for short. The XBT was developed in 1941 based upon the existing #1 Crossbar local central office switch. The XBT was meant for use as a regional or city-wide tandem switch, though in the early days XBT switches were often uses a long-haul tandem switches. There were many XBT switches used throughout the United States and Canada (developed under licesnse by Northern Electric).
Long Haul Tandem Crossbar Switches
By the early 1940s, Western Electric realized that the XBT was not enough to handle the many long distance calls that were placed daily. A much more robust switch needed to be developed.
The #4 Crossbar switch was developed to meet this need. The first installation of the #4 Crossbar was at Philadelphia in 1943. Further installations were put on hold until after World War II. Subsequent installations started up again in 1948 and 1949 in New York, Chicago, Cleveland, Oakland (CA) and Boston.
By 1950, Western Electic had anticipated the need for the #4 Crossbar to change “routing translations” dynamically when needed, instead of being hard wired. New #4 Crossbar switches were called #A4A for “Anticipated #4 Advanced”. These machines were installed between 1950 and 1953.
By 1953, a “card translator” system had been developed using punched metal cards and light detecting “photo cells” would detect patterns in these punched cards to determing routing transations and alternate routing translations (if the first routing was busy). The new #4 Crossbar switches were called #4A (#4 Advanced). The existing #4 Crossbar switches were modified for use with the card translator – their names became #4M (#4 Modified). The #4A switches were installed between 1953 and 1968.
By 1969, an electronic equivalent of the mechancical card translator system had been developed. Further installations of the #4 Crossbar switch utilized this new system. They were referred to as #4-ETS (Electronic Translator System). Some existing #4 Crossbar switches were modified to use the ETS, but not all. The ETS line of #4 Crossbar switches lasted from 1969 through the end of the #4 Crossbar line in 1976.
The #4 Crossbar series of switches were in service until the 1980s when they were replaced with the
#4ESS series of digital tandem switches.
Western Electric #4 Crossbar Sender Test Frame
List of Telephone Switches
Switching System Pages
by Step (Strowger) Switching Systems
Vintage Switching Systems of Note
Number 4 Crossbar (#4XB) List
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Page last modified December 20, 2008
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