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Number One Crossbar Switching System
The Number One Crossbar Switching System, or 1XB switch, was the primary urban local telephone exchange design used by the Bell System in the mid-20th century. Its switch fabric topology was based on the earlier urban panel switch system, which, in turn, was based on the turn of the century Divided Multiple Switchboard. Thus, it had separate incoming and outgoing 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. The first 1XB was the PR7 exchange at Troy Avenue in Brooklyn, New York in 1937.〔(Telecommunications Heritage Group (UK) ) Peter Walker on history of Crossbar〕
==Subscriber sender==
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 was able to accept Dial Pulses and (in later models) dual-tone multi-frequency signaling (DTMF). 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. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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