A voice-optimized switching system physically located on the customer premises, serving the internal station-to-station communications requirements of one or more user organizations and with trunk circuits connecting to the public switched telephone network (PSTN) via one or more central office (CO) switches, and perhaps one or more other PBXs composing a private network.The term refers to the fact that the PBX originated as a switching system located on the subscriber's private premises and serving the subscriber's private communications requirements, while functioning as a branch (i.e., partition) of the public exchange. The first PBX was placed into service in the Old Soldiers' Home in Dayton, Ohio, in 1879. The first systems were non-standard modifications of CO switches. AT&T offered the first standard PBX, the No. 1 PBX, in 1902.The evolution of PBX technology can be organized along generational lines, as shown in Table P-1.

Table P-1: Evolution of PBX technology

GenerationDesignationNature of Technology 0 Cordboard Manual Switchboard 1 Electro-Mechanical Step-by-Step (SxS) 2 Electro-Magnetic Crossbar (XBar) or Crossreed 3 Electronic Common Control (ECC) Analog or Digital Stored Program Control (SPC) 4 Digital Stored Program Control (SPC) LAN-based Voice over Internet Protocol (VoIP) IP PBX or IPBX (Internet Protocol Private Branch eXchange or Intranet Private Branch eXchange) EPABX (Electronic Private Automatic Branch eXchange) PABX (Private Automatic Branch eXchange) PBX (Private Branch eXchange) PBX generations 0