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INTERNET WORKING DEVICES SWITCH
In a telecommunications network, a switch is a device that channels incoming data from any of multiple input ports to the specific output port that will take the data toward its intended destination.
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.
Basically switches are three types –
1-Manual Switches
2-Eletcromechanically switches
3-Electronics Switches
4-Digital Switches
MANUAL SWITCHES
With manual service, the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. Provided that the number is in the same central office, the operator connects the call by plugging into the jack on the switchboard corresponding to the called customer's line. If the call is to another central office, the operator plugs into the trunk for the other office and asks the operator answering (knownas the "inward" operator) to connect the call.
Figure 1
Electromechanically Switches ( Automatic switching)
The Invention of Automatic Switchin
Almon B. Strowger was an undertaker in Kansas City, USA. The story goes that there was a competing undertaker locally whose wife was an operator at the local (manual) telephone exchange. Whenever a caller asked to be put through to Strowger, calls were deliberately put through to his competitor. This obviously frustrated Strowger greatly and he set about devising a system for doing away with the human part of the equation Strowger developed a system of automatic switching using an electromechanical switch based around around electromagnets and pawls. With the help of his nephew (Walter S. Strowger) he produced a working model in 1888 (US Patent No. 447918 10/6/1891). In this selector, a moving wiper (with contacts on the end) moved up to and around a bank of many other contacts, making a connection with any one of them.
Strowger did not invent the idea of automatic switching; it was first invented in 1879 by Connolly & McTigthe but Strowger was the first to put it to effective use. Together with Joseph B. Harris and Moses A. Meyer, Strowger formed his company 'Strowger Automatic Telephone Exchange' in October 1891.
In the late 1890's Almon B. Strowger retired and eventually died in 1902. In 1901, Joseph Harris licensed the Strowger selectors to the Automatic Electric Co. (AE); the two companies merged in 1908. The company still exists today as AG Communications Systems, having undergone various corporate changes and buyouts along the way.
Selector Theory
A selector starts in the 'home' position and with each 'impulse' the wiper contacts would progress round the output bank to the next position. Each output would be connected to a different subscriber, thus the caller could connect to any other subscriber who was connected to that bank, without any manual assistance from an operator.