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.[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 frames as in the panel switch. The LLF, uniting the telephone line circuit at its "column and switch", simplified administration.

In addition to the Line Link Frame, the 1XB consisted of a series of additional crossbar frames and junctors that were used in call completion. The frames actually used to establish paths through the switch were known as link frames, and included the District Link, Office Link, Incoming Link, and Line Link. Other frames were attached to the link frames and provided other functions including supervision, signaling, and control. Examples of these frames included District Junctors, Subscriber Senders, Originating Markers, and the like.

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, and sender link, which corresponded to the cord circuit of the old cord telephone switchboard. 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 the dialed digits from the subscriber, and, when enough were accumulated, passed digits to a Marker for translation and switching element control. After the marker had established the path to the terminating office, the sender was instructed to outpulse the required information to the terminating office to complete the call. In many cases, No. 1 Crossbar central offices could have in excess of 100 senders, as each sender only served one call at a time. When the call setup stage was complete, the sender returned to normal and awaited seizure by another caller.


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. The No. 1 Crossbar pioneered the link principle, with each individual 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 originating 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

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.


The No. 1 Crossbar inspired the later No. 5 Crossbar switch which intensified the trend towards greater efficiency and complexity. No. 5. Crossbar installations seldom replaced No. 1 machines, but were operated side by side with them, or in towns not large enough to warrant a No. 1 Crossbar. Most No. 1 Crossbar switches were replaced in the 1970s by the 1ESS switch generation with stored program control.

See also


  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
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