The K24A syncroplex telephone carrier system

A most significant turning point in the history of telephone transmission occurred shortly before 1920 with the introduction of carrier equipment as a means of multiplying the capacity of open-wire line facilities. The 20-year period between 1918 and 1938 witnessed the firm establishment of single-side-band techniques in telephone transmission through the C carrier and the development of the 12-channel J and K systems for open wire and cable, respectively. This was a period of rapid expansion of carrier applications to long-haul toll circuits and, by and large, a consolidation of carrier design for this realm of service. If the L system is included, which is essentially an expansion of the K, the equipment which supports the majority of long-haul toll transmissions is still of the J, K, or L type.