Number One Crossbar Switching System

The Number One Crossbar Switching System (1XB switch), was the primary technology for designing urban telephone exchanges in the Bell System in the mid-20th century. Its switch fabric used the new electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first 1XB system was installed in the PResident-2 office at Troy Avenue in Brooklyn, New York which became operational in February 1938.

The layout of the Number One Crossbar separated incoming and outgoing traffic into distinct sections. However, lines appeared only on the Line Link Frame (LLF), rather than requiring a multi-wire connection to two different sections as in a panel switch. The LLF uniting the telephone line circuit at its "column and switch" simplified administration.

For outgoing calls, the Line Link Frame acted like the line finder of the panel switch, autonomously connecting the line to a junctor, which corresponded to the cord circuit of the old cord telephone switchboard. As in the panel switch, the sender then found the chosen junctor and supplied dial tone. Like the panel switch, the 1XB common control was based on a complex, versatile sender circuit. The sender decoded dial pulses and was retrofitted for dual-tone multi-frequency signaling (DTMF) in the 1960s. A large number of senders used a common translator circuit to detect a call going to a nearby area code to be stored in abbreviated form. It called in an auxiliary sender when necessary to implement Direct Distance Dialing (DDD). Like the panel switch, two or more offices with separate incoming sections could share an outgoing section for more efficient trunking. Unlike the panel switch, it was rare to combine more than two this way.

Unlike the motor driven, clutch controlled panel switch selectors, crossbar switches using the link principle required incoming and outgoing markers to find an idle path and set up the switch train for each call. Earlier crossbar exchanges had used the crossbar switch according to the selector principle, with one input and typically 100 or 200 outputs, similar to a stepping switch. 1XB pioneered the link principle, with each switch able to handle as many phone calls as it had inputs or outputs, typically ten. This innovation diminished the cost of switches, at the expense of more complex controls. The complexity of the circuitry challenged the art of circuit drawings, leading to the development of detached contact drawings, which in turn led to the application of Boolean algebra and Karnaugh maps.

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