No. 101 ESS 4A Switch Unit

The No. 101 electronic switching system (ESS) has brought new conveniences and services to private branch exchange and Centrex customers with the 200-line 1A, 364-line 2A, 820-line 3A switch units. 1-4 To extend the No. 101 ESS features to still larger customers, the 4A 4000 line switch unit was designed. The No. 101 ESS 1A, 2A, and 3A switch units utilize time division for the switching networks. In the 1A switch unit, 240 lines and 40 trunks have access to two buses through a total of 50 time slots. With a partially duplicated bus system in the 2A switch unit 364 lines and 58 trunks are accessed by 60 time slots. The 3A switch unit accommodates 820 lines and 112 trunks with either 120, 180, and 240 time slots and a bus system similar to the 2A. In general, the traffic capacity of a time division network depends upon the number of time slots; the line capacity depends upon the number of line circuits. As the number of lines increases, the traffic also increases; thus large line capacities require more time slots. The maximum number of time slots per talking bus is determined by the sampling rate and pulse width. The sampling rate is set by the transmission requirements; the pulse width is largely determined by active component performance and stray capacitance. 4 Substantially larger time division switching networks require considerable advancement in components and fabrication techniques.