page_title: Number One Crossbar Switching System
section_title: Markers
hierarchical_section_title: Number One Crossbar Switching System / Architecture / Markers
caption_attribution_description: English: This photograph shows an Originating Marker from a No. 1 Crossbar telephone switch. The cabinets in the center contain the relay logic, and the field of terminals on the right side encode the translation information for each office code dialed.
context_page_description: The Number One Crossbar Switching System, 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.
context_section_description: 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 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.  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 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 it to the call waiting on an incoming trunk. 
  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.
caption: An Originating Marker with cross-connect field at the Connections Museum, Seattle WA.
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