In 1938 the first crossbar switch (No. 1) was installed. Crossbar advanced the concept of common control of the switch beyond what panel switches had achieved.
Also in that era Bell Labs developed improvements in step-by-step allowing for very small "community dial" offices (see below).
Lastly, in 1938 the model 300 'combined' telephone set was introduced, along with the 1A key system.
Excerpts from the Bell Labs history:
The basic idea of a crossbar switch-to provide a matrix of n by m sets of contacts with only n + m actuators to select one of the n x m sets of contacts-was proposed early in the automation of switching. In 1913 a patent application was filed by Western Electric for J. N. Reynolds for a crossbar switch, and the basic patent was issued in May1915. The device was based on contact elements multipled in one direction, as in a panel bank.
Although the panel system had been providing excellent service and meeting the needs of heavy traffic in large metropolitan offices, a number of advantages that the marker-type crossbar system could include were felt to be important to the future of providing service economy and flexibility in large cities. (This was the start of the services evolution made possible with common-control switching.) By selecting trunks in the marker, it was not necessary to assign trunks in fixed group patterns. They could be identified nonconsecutively and spread over several switches or terminal groups. This feature was particularly attractive for large trunk groups serving PBXs where terminations not identified by directory numbers could be reached by "jump hunting" to other terminal groups . It meant that traffic could be distributed evenly over the switches. For the first time alternate routing was possible.
The use of precious metal contacts would reduce or eliminate the electrical noise in circuits caused by vibration created in the contact brushes or wipers of panel and step-by-step selectors. By using common control with crossbar switches in the network, one was no longer tied to the characteristics of the switch in determining the form of signaling to be employed; therefore, it was envisioned that higher-speed signaling between new system offices and shorter call completion times could be realized. An objective in designing the network was to minimize the relay equipment required to be associated with a call throughout its duration and preferably to concentrate these relays in a so-called junctor relay circuit in a link or trunk circuit. These capabilities were achieved from the very beginning in the No. 1 crossbar system.
The first No. 1 crossbar office was placed in service in Troy Avenue, Brooklyn in February 1938. It initially served 1400 telephones. Prior to the cutover, more than 350,000 test calls were placed through the system starting in September 1937. This installation was unique since it was the only No. 1 crossbar system to be installed without a line choice connector between the terminating markers and the line link switches.
The second No. 1 crossbar office, and the first large installation, was placed in service in July 1938, serving 16,000 telephones from the East 30th Street telephone building in New York City . Here for the first time the elaborate PBX features were introduced into the system, for example those that permitted jump hunting and busy testing of nonconsecutively numbered PBX trunks as part of the same group. All of these features, plus reduced maintenance costs together with the advantages enumerated earlier, proved to be quite successful and demonstrated not only the increased flexibility of crossbar switching over the panel machine-switching system, but also its economic viability for use in large metropolitan offices. Many features were later added to the No. 1 crossbar system, including automatic message accounting (AMA), automatic number identification (ANI), auxiliary senders, and even a new wire spring version of the originating senders.
At the peak in April 1970 of No. 1 crossbar service, there were 325 terminating entities serving 7.25 million lines. The last new terminating marker group was installed in Philadelphia in 1969, and the last originating marker group in Chicago in 1970.
Crossbar tandem: With crossbar switching it was possible to provide a crossbar switch link between the incoming trunks and senders (registers) so that different senders could be selected from different groups, depending upon the class of service of the incoming trunk. This meant that the trunks could serve panel, crossbar, and step-by-step offices each with their own type of pulsing (revertive, multifrequency, and dial pulse). The remainder of the crossbar tandem system was almost a copy of the originating switchtrain of the No. 1 crossbar system, except that the incoming trunks took the place of the district junctors, and of course there were no line link frames (see Fig. 4-12). For this reason it was considered a part of the No. 1 crossbar development and not given a separate code identity. The first crossbar tandem systems were placed in service in late 1941, in Detroit, Manhattan, and San Francisco.
In some metropolitan areas, particularly New York City, individual message registers per line were provided for charging purposes. Generally the register was scored once for each call. As the dialing distances increased, complex district selector and junctor circuits were developed for the panel and No. 1 crossbar systems that permitted the multiple scoring of registers on the answering of the call, as well as additional scorings at the beginning of each overtime period. [also known as message units, bulk billing or measured service].
Community dial offices (step by step):
In small offices, operator functions were located remote from the switching office. The maintenance needs were so infrequent that resident craftspeople were not necessary.
The success of and demand for small offices after the depression stimulated a new equipment design with uniform-size frames and switch types that were packaged for easy engineering and installation. This system, developed by Bell Laboratories, was known as the "355A". Among the features was a new combined line and cutoff relay as well as a separate optional "line lockout" relay for use of lines with a high incidence of line faults. The first installation was cut over in Batavia, Ohio in 1939. A total of about 3500 offices serving about 4 million lines demonstrate the success of this development. There is more of this code of switching system manufactured and placed in service in the Bell System than any other code at any time.
Many individual innovations continued to be made in the stepby- step system. For example, an arrangement was added to provide for early detection and operator interception of permanent signals in 1963, and also the timed disconnect of connectors when only the called party goes on-hook so that he or she may originate a new call. Other improvements to maintain the step-by-step system as a viable member of the Bell System network are described elsewhere in this chapter. The step-by-step system provided automatic service for more Bell System lines than any other switching system when it reached its peak of24,440,000 lines in 1973.