Panel switch
The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983.
The Panel switch was named for its tall panels which consisted of layered strips of terminals. Between each strip was placed an insulating layer, which kept each metal strip electrically isolated from the ones above and below. These terminals were arranged in banks, five of which occupied an average selector frame. Each bank contained 100 sets of terminals, for a total of 500 sets of terminals per frame. At the bottom, the frame had two electric motors to drive sixty selectors up and down by electromagnetically controlled clutches. As calls were completed through the system, selectors moved vertically over the sets of terminals until they reached the desired location, at which point the selector stopped its upward travel, and selections progressed to the next frame, until finally, the called subscriber's line was reached.
History
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. Large cities had a complex infrastructure of manual switching that prevented complete ad hoc conversion to mechanical switching, but more favorable economics was anticipated from conversion to mechanical operation. No satisfactory methods existed for interconnecting manual systems with machines for switching. 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. One group continued existing work that yielded the Rotary system, while the second group developed a system that was based on linear movement of switch components, which became known as the panel bank. As work continued, many subassemblies were shared, and the two switches only distinguished themselves in the switching mechanisms.By 1910, the design of the Rotary system had progressed farther and internal trials employed it at Western Electric as a private branch exchange. However, by 1912, the company had decided that the panel system showed better promise to solve the large-city problem, and delegated the use of the Rotary system for use in Europe to satisfy the growing demand and competition from other vendors there, under the management and manufacture by the International Western Electric Company in Belgium.
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.
These first panel-type exchanges were placed in service in Newark, New Jersey, on January 16, 1915 at the Mulberry central office serving 3640 subscribers, and on June 12 in the Waverly central office, which had 6480 lines. Panel development continued throughout the rest of the 1910s and in the 1920s in the United States. A third system in Newark followed in April 1917 for testing automatic call distribution.
The first fully machine-switching Panel systems using common control principles were the Douglas and Tyler exchanges in Omaha, Nebraska, completed in December 1921. Subscribers were issued new telephones with dials, that permitted the subscriber to place local calls without operator assistance. This installation was followed by the first installations in the eastern region in the Sherwood and Syracuse-2 central offices in Paterson, New Jersey, in May and July 1922, respectively. The storied Pennsylvania exchange in New York City was cut-over to service in October 1922.
Most Panel installations were replaced by modern systems during the 1970s. The last Panel switch, located in the Bigelow central office in Newark, was decommissioned by 1983.
Operational overview
When a subscriber removes the receiver from the hookswitch of a telephone, the local loop circuit to the central office is closed. This causes the flow of current through the loop and a line relay, which causes the relay to operate, starting a selector in the line finder frame to hunt for the terminal of the subscriber's line. Simultaneously, a sender is selected, which provides dial tone to the caller once the line is found. The line finder then operates a cutoff relay, which prevents that telephone from being called, should another subscriber happen to dial the number.Dial tone confirms to the subscriber that the system is ready for dialing. Depending on the local numbering system, the sender required either six or seven digits in order to complete the call. As the subscriber dialed, relays in the sender counted and stored the digits for later usage. As soon as the two, or three digits of the office code were dialed and stored, the sender performed a lookup against a translator or decoder. 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. After the sender received the data provided by the translator or decoder, the sender used this information to guide the district selector and office selector to the location of the terminals that would connect the caller to the central office where the terminating line was located. 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.
On the district or office selectors themselves, idle outgoing trunks were picked by the "sleeve test" method. After being directed by the sender to the correct group of terminals corresponding to the outgoing trunks to the called office, the selector continued moving upward through a number of terminals, checking for one with an un-grounded sleeve lead, then selecting and grounding it. If all the trunks were busy, the selector hunted to the end of the group, and finally sent back an "all circuits busy" tone. There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones.
Once the connection to the terminating office was established, the sender used the last four digits of the telephone number to reach the called party. It did so by converting the digits into specific locations on the remaining incoming and final frames. After the connection was established all the way to the final frame, the called party's line was tested for busy. If the line was not busy, the incoming selector circuit sent ringing voltage forward to the called party's line and waited for the called party to answer their telephone. If the called party answered, supervision signals were sent backwards through the sender, and to the district frame, which established a talking path between both subscribers, and charged the calling party for the call. At this time, the sender was released, and could be used again in service of an entirely new call. If the called subscriber's line was busy, the final selector sent a busy signal back to the called party to alert them that the caller was on the phone and could not accept their call.
Telephone numbering
As in the Strowger system, each central office could address up to 10,000 numbered lines, requiring four digits for each subscriber station.The panel system was designed to connect calls in a local metropolitan calling area. Each office was assigned a two- or three-digit office code, called an office code, which indicated to the system the central office in which the desired party was located. Callers dialed the office code followed by the station number. In larger cities, such as New York City, dialing required a three-digit office code, and in less-populated cities, such as Seattle, WA and Omaha, NE, a two-digit code. The remaining digits of the telephone number corresponded to the station number, which pointed to the physical location of the subscriber's telephone on the final frame of the called office. For instance, a telephone number may be listed as PA2-5678, where PA2 is the office code and 5678 is the station number.
In areas that served party lines, the system accepted an additional digit for party identification. This allowed the sender to direct the final selector not only to the correct terminal, but to ring the correct subscriber's line on that terminal. The panel system supported individual, 2-party, and 4-party lines.
Circuit features
Similar to the divided-multiple telephone switchboard, the panel system was divided into an originating section and a terminating section. The subscriber's line had two appearances in a local office: one on the originating side, and one on the terminating side. The line circuit consisted of a line relay on the originating side to indicate that a customer had gone off-hook, and a cutoff relay to keep the line relay from interfering with an established connection. 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. On the terminating end, the line circuit was connected to a final selector, which was used in call completion. Thus, when a call was completed to a subscriber, the final selector circuit connected to the desired line, and then performed a sleeve 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.Supervision was supplied by a District circuit, similar to the cord circuit that plugged into a line jack on a switchboard. The District circuit supervised the calling party, and when the calling party went on-hook, it released the ground on the sleeve lead, thus releasing all selectors except the final, which returned down to their start position to make ready for further traffic. The final selector circuit was not supervised by the district circuit, and only returned to normal once the called party hung up. 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.
Many of the urban and commercial areas where Panel was first used had message rate service rather than flat rate calling. For this reason the line finder had a fourth wire known as the "M" lead. This enabled the District circuit to send metering pulses to control the subscriber's message register. The introduction of direct distance dialing in the 1950s required the addition of automatic number identification equipment for centralized automatic message accounting.
The terminating section of the office was fixed to the structure of the last four digits of the telephone number, had a limit of 10,000 phone numbers. In some of the urban areas where Panel was used, even a single square mile might have three or five times that many telephone subscribers. Thus the incoming selectors of several separate switching entities shared floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Selector Tandem was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common control equipment; just one stage of selectors and accepting only the Office Brush and Office Group parameters. Panel Sender Tandems were also used when their greater capabilities were worth their additional cost.