The Number One Crossbar Switching System (1XB) was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first No. 1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.
Evolution The predecessor to the No. 1 Crossbar was the panel system, which was used in many large metropolitan areas beginning in the early 1920s. By the 1930s, there was a growing need for a new type of switching machine that did not have the drawbacks inherent in the panel system. Desirable features of the crossbar system included:
Elimination of the motor-driven equipment Reduction in the number of unique parts Reduction in the cost of maintenance Bifurcated, precious metal relay contacts, resulting in improved contact performance and reduced noise More efficient manufacturing methods The development of the multi-contact relay, for opening and closing a large number of contacts (usually greater than 30 at once)
Architecture
The layout of the Number One Crossbar separated incoming and outgoing traffic into distinct sections. Each section had its own central control elements known as markers. The originating marker handled call routing up to the outgoing trunk, and the terminating marker routed calls from the incoming trunk all the way up to the termination point at the line link frame (LLF). Notably, this design meant that interoffice calls were handled in the same basic way as intraoffice calls. No distinction was made between calls terminating in the same office, or a different crossbar office. Any originating call that was to terminate on the same machine would pass through the entire originating section, then a trunk would be selected to the terminating section, where the last 4 or 5 digits of the telephone number were used to determine the location of the called line. Despite the division of the system into two logical halves, the No. 1 Crossbar placed all subscriber lines on one of several line link frames, which were used for both origination and termination. This simplified administration and reduced the number of frames required, compared to the earlier panel system, where subscriber lines were split between distinct frames. 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. "Link" frames, including the district link, office link, and incoming link provided the actual switching fabric that connected calls through the machine. Other frames were attached to the link frames as necessary, and provided functions including supervision, signaling, and control. Examples of these frames included district junctors, subscriber senders, and originating and terminating markers.
Markers
Unlike the motor driven, clutch controlled panel switch selectors, crossbar switches using the link principle required originating and terminating markers to find an idle path and set up the switch train for each call. A marker, being a complex control instrument with a short holding time, had the task of decoding the digits of the seven-digit telephone number to determine the routing required to set up the switching fabric for call completion. 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. In an originating marker, a cross connect field had a terminal for each two- or three-digit office code. A particular office code terminal 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. These cross connects were in turn used to activate relays in the marker that controlled the treatment, or handling for the office code dialed. Using the output provided by its decoding stage, the originating marker could select two office link frames to search for idle trunks to the destination. Once the originating marker established a path to the called office, it returned pulsing information to the subscriber sender. The sender then sent the remaining digits of the called telephone number to the distant terminating office. In a terminating marker, there was also a cross-connect field which was used to declare which frames the marker had to access in order to complete the call to the desired line. Subscriber lines were terminated in arbitrary locations on the line link frame, and it was the task of the terminating marker to locate the line, and close the required crosspoints to connect the call to its destination. 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. An outstanding feature of the originating marker was route advance, where if all trunks were busy, the marker would 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.
Senders
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