New Time Division Switch Units for No. 101 ESS

The initial development of the No. 101 Electronic Switching System to provide commercial private branch exchange (PBX) service was completed in November 1963.1 This system utilizes a stored program common control located in the central office and time-division switching units located on customers premises, connected to the common control by data links and trunks. This initial system design included a time-division switch unit, which served customers having a maximum of 200 extensions and 40 trunks. 2 Although this line capacity meets the requirements of a large percentage of the P B X customers of the Bell System, there are a significant number of customers who require service for more than 200 lines. These customers may now be served by No. 101 ESS through the use of the 2A (364 lines), 3A (820 lines), and 4A3 (800 to 4000" lines) switch units in concert with the control unit. This paper describes the 2A and 3A switch units; the 4A is described in a separate article in this issue. * T h e 4A switch unit capacity is 2000 lines. Expansion to a 4000 line capacity is under development. 443