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 Number 1 Crossbar offices made use of complex and versatile originating and terminating senders to communicate within themselves, and to other switching offices. When a subscriber lifted their phone off the hook, they were connected to an originating (also known as a s''ubscriber's'') sender. This sender received and registered the dialed digits, communicated with an originating marker to establish the path for the call, and then pulsed the dialed digits forward to the terminating office in whatever format was required. In many cases, No. 1 Crossbar central offices could have in excess of 100 subscriber's 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. In the terminating half of the crossbar office, a
terminating sender link circuit connected a terminating sender to the trunk, in order to receive the information from the originating sender. This was typically done using Revertive Pulse, as in the
panel switch.
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. The revertive pulse system as used in 1XB had a "High Five" feature by which the "incoming brush" selection 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, whereas a panel office using the revertive pulse system could handle a maximum of 10,000 lines per office. Multifrequency terminating senders accomplished the same goal by accepting 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. == Maintenance facilities ==