No. 1 ESS ADF: Teletypewriter Stations and Transmission Facilities

l.i Facilities With the advent of the No. 1 Electronic Switching System, Arranged with D a t a Features (No. 1 ESS A D F ) , the art of store-and-forward switching has been significantly advanced in the areas of reliability, capacity and feature capability. Interconnection between the newly designed stations and the message switcher is provided by existing Bell System and Independent Company private-line telegraph facilities. Only minor modification was required to modernize these facilities for use with the higher data rates of No. 1 ESS ADF. The private-line telegraph system contains a large number of centrally located offices which provide both transmission and maintenance facilities. These locations are interconnected by a variety of carrier systems, utilizing channels derived from frequency division of voice channels shared with other subscribers. This sharing of facilities pro2941 2942 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1070 vides the subscriber with significant savings. At each location, the data signals of a channel are demodulated to baseband. Any number of individual baseband "legs" can be combined by an interconnecting circuit called a "hub." The basic function of a hub is to permit all outgoing legs to transmit what has been received on an incoming leg. As shown in Fig. 1, hubs can be interconnected to permit multistation arrangements or a hub may simply be used to interconnect two carrier channels, or to connect a final channel to the local loop of a station.