Common control for an electronic private branch exchange

IN THE BELL System´s No. 101 Electronic Switching System (ESS), also known as the electronic private branch exchange (PBX), a single centrally located unit has been designed to control the operation of a maximum of 32 remotely located switch units.^1,2 The control unit will be located at a central office, while the switch units each will be located in a different PBX customer´s office. A particular switch unit may serve up to 200 lines, while the control unit may accommodate up to 3,200 lines.