Number One Crossbar Switching System

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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.[1][2]


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.

Subscriber sender[edit]

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.

An outgoing marker, being a complex control instrument with a short holding time, had the additional task of decoding the first three digits of the seven-digit telephone number to determine the routing to a distant exchange. A cross connect field had a terminal for each office code, which was cross connected to the coil of a route relay. When the office code point was grounded, it operated the route relay, whose contacts were wired in another cross connect or data field. Thus, the outgoing marker could select two office frames to search for idle trunks to the destination, and had pulsing information for the sender. The sender sent the telephone number to the distant terminating office. When one office was constructed, retired, or changed, staff in other offices received a Routing Letter, ordering the cross connect fields to be changed at a particular date and time, usually after midnight, to accommodate the change in the network. Translation cross connect fields such as these were among the first to be converted from soldered terminals to wire wrap.

If all trunks were busy, the marker did a route advance to operate a different route relay to select an alternate route via a tandem. This feature kept trunk groups small, and more heavily loaded with traffic, thus saving cost in outside plant.

Terminating sender[edit]

At the terminating office, a Terminating Sender Link circuit connected a terminating sender to the trunk, in order to receive the phone number, typically using Revertive Pulse as in the panel switch to receive only the last four digits. Multi-frequency Terminating Senders were introduced in the 1950s as part of Direct Distance Dialing, and also used for incoming traffic from some local crossbar exchanges. The Terminating Sender activated an Incoming Marker, which then used a Number Group Circuit to find the line, marked an idle path, and operated the crossbar switches to use the links to connect the incoming trunk to the line.

The Revertive Pulse system as used in 1XB had a "High Five" feature by which the Incoming Brush parameter could be incremented by five. Thus, the new IB numbers 6 through 10 designated a second ten thousand telephone numbers, known as a "B" office. This allowed each 1XB incoming section to handle twenty thousand lines.

Multifrequency Terminating Senders accepted a fifth digit to discriminate among office codes served by the same office. The originating marker told the sender to delete the first two of seven digits in these cases. Sometimes the two office codes had the same third digit, in which case the first three digits for the "B" office were deleted and replaced with a single digit, indicated as AR for Arbitrary, usually a zero. The potential for five incoming digits to address a 100,000 line office was not exploited.


A tandem version of the 1XB omitted the incoming section and Line Link Frames and replaced the junctor circuits with incoming trunks, leaving only the ability to connect those incoming trunks to outgoing trunks. In many cases the resulting Crossbar tandem switch (XBT) replaced a Panel Sender Tandem, because its multi-frequency senders were able to receive seven digits and some were modified to accept ten. Sometimes it replaced an Office Select Tandem as well, since its Revertive Pulse senders were able to accept Office Brush and Group parameters. In big cities, some XBT were strictly incoming Class 4 telephone switches, some outgoing, a few both-way, and some only for tandem traffic within the metropolitan area. Specialization was less marked in less dense areas. XBT served local telephone companies until the late 20th Century when they were replaced by 4ESS switches or other digital switches.


1XB inspired the later 5XB switch which intensified the trend towards greater efficiency and complexity. 5XB installations seldom replaced 1XB but were operated side by side with them, or in towns too small to be economical for the 1XB. Most 1XB switches were replaced in the 1970s by the 1ESS switch generation with stored program control.

See also[edit]


  1. ^ W.J. Lacerte, Field Tests of the Crossbar System, Bell Laboratory Record 18(4), December 1939
  2. ^ Telecommunications Heritage Group (UK) Peter Walker on history of Crossbar

External links[edit]