Panel switch
In c. 1906, AT&T organized two research groups for solving the unique challenges in switching telephone traffic in the large urban centers in the Bell System.
The two groups at the Western Electric Laboratories focused on different technologies, using competitive development to stimulate invention and increase product quality, a concept that had been successful at AT&T previously in transmitter design.
By 1910, the design of the Rotary system had progressed farther and internal trials employed it at Western Electric as a private branch exchange (PBX).
[3] After a trial installation as a PBX within Western Electric in 1913, Panel system planning commenced with design and construction of field trial central offices using a semi-mechanical method of switching, in which subscribers still used telephones without a dial, and operators answered calls and keyed the destination telephone number into the panel switch, which then completed the call automatically.
The first fully machine-switching Panel systems using common control principles were the Douglas and Tyler exchanges in Omaha, Nebraska, completed in December 1921.
[8] When a subscriber removes the receiver (earpiece) from the hookswitch of a telephone, the local loop circuit to the central office is closed.
The line finder then operates a cutoff relay, which prevents that telephone from being called, should another subscriber happen to dial the number.
As soon as the two, or three digits of the office code were dialed and stored, the sender performed a lookup against a translator (early-type) or decoder (later-type).
The translator or decoder took the two or three digits as input, and returned data to the sender that contained the parameters for connecting to the called central office.
The sender also stored and utilized other information pertaining to the electrical requirements for signaling over the newly established connection, and the rate at which the subscriber should be billed, should the call successfully complete.
As in the Strowger system, each central office could address up to 10,000 numbered lines (0000 to 9999), requiring four digits for each subscriber station.
The cutoff relay was controlled by a sleeve lead that, as with the multiple switchboard, could be activated by either the originating section or the terminating.
Thus, when a call was completed to a subscriber, the final selector circuit connected to the desired line, and then performed a sleeve (busy) test.
If the line was not busy, the final selector operated the cut-off relay via the sleeve lead, and proceeded to ring the called subscriber.
[11] Some District frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals for handling calls from payphones.
The use of senders provided advantages over the previous direct control systems, because they allowed the office code of the telephone number to be decoupled from the actual location on the switching fabric.
As it was responsible for driving the selectors to their destinations, it was able to detect errors (known as trouble) and alert central office staff of the problem by lighting a lamp at the appropriate panel.
When the sender's job was complete, it connected the talk path from the originating to the terminating side, and dropped out of the call.
In this way, a comparatively small number of senders could handle a large amount of traffic, as each was only used for a short duration during call setup.
In the case of the final frame, the last selection would result in connection to an individual's phone line and would begin ringing.
When the sender counted the appropriate number of pulses, it cut the power to the solenoid in the terminating office, and caused the brush to stop at its current position.
[12] This is in contrast to more modern forms of forward pulsing, where the originating equipment will directly outpulse to the terminating side the information it needs to connect the call.
In areas with mostly manual switches, the Call Annunciator signaling system was used to avoid installing lamp panels at every operator station.
In the 1950s, auxiliary senders were added for storing more than eight digits, and sending by multi-frequency (MF) signaling for direct distance dialing (DDD).
At the time of its design, it was decided that maintenance should be done on a preventative basis, and regular testing of the equipment would be used to identify faults before they became severe enough to affect subscribers.
Early two- and three-digit type senders stored dialed digits on rotary selector switches.
The senders employed translators to convert the dialed digits into the appropriate brush and group selections needed to complete the call.
These were more reliable, and in addition, replaced the translator equipment with decoders, which also operated entirely with relays, rather than with motor-driven apparatus, which yielded faster call completion, and required less maintenance.
A busy line condition was indicated by -48 volt battery applied to the other side of the cut-off relay winding, and thus at the sleeve lead.
As these line finders went into service, however, it became evident that 15 brushes on each vertical selector rod were quite heavy, and needed springs and pulleys at the top of the frame to compensate for their mass.
