Private branch exchange
A private branch exchange (PBX) is a telephone switch that services telephones internal to an organization, permitting internal calls without going to the Public Switched Telephone Network (PSTN), but also allowing internal telephones to connect to the PSTN. Typically, using a PBX reduces the number of PSTN lines, and their cost, compared with providing a direct line to each instrument.
The original PBX were manual switchboards. Later, they used circuit switching technology. Starting in the 1990s, they began to use Voice over Internet Protocol and could be internally packet-switched. The abbreviation PABX, for private automatic branch exchange, is sometimes seen, to distinguish it from the manual switchboard; all PBX now use automatic switching.
More advanced PBX can provide conferencing, detailed call accounting, and other features. With convergence of communications, the voice PBX function may very well be a software function in a router, or general-purpose computer with routing and telephone switching software. Asterisk is a widely used open source PBX software package.[1]
Large organizations' PBXs can be as large as telephone central office switches. The Pentagon Building, for example, requires several large switches.