Number One Crossbar Switching System

1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.

Examples of these frames included district junctors, subscriber senders, and originating and terminating markers.

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.

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.

In an originating marker, a cross connect field had a terminal for each two- or three-digit office code.

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.

This feature kept trunk groups small, and more heavily loaded with traffic, thus saving cost in outside plant.

When the call setup stage was complete, the sender returned to normal and awaited seizure by another caller.

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.

Once the terminating sender received and stored the digits of the called telephone number, it activated a terminating 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.

Whereas earlier systems, such as the panel switch had maintenance facilities spread throughout the office, it was found desirable to centralize these processes into a single space so that technicians could work more effectively.

[6] This allowed technicians to quickly identify and resolve issues with the equipment without taking it out of service to perform lengthy troubleshooting routines.

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.

The No. 1 Crossbar architecture consists of an originating and terminating half. An intra-office call (from and to the same office) will extend from the beginning of the originating side, to the end of the terminating side.
An Originating Marker with cross-connect field at the Connections Museum, Seattle WA.
A vertical panorama showing tall aisles of equipment. On the right is a framework containing five large metal enclosures with access doors for each on the front.
A terminating sender frame (right) contains five senders of either type, revertive pulse, or multifrequency. Several such frames would be present in a full size 1XB office.
Maintenance center for the VErmont No. 1 Crossbar office at Connections Museum, Seattle. This switch was installed in 1942 and served the neighborhoods of northeast Seattle.