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.