A Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone exchange that is owned by a private business, as opposed to one owned by a common carrier or by a telephone company.
Overview
Originally an organisation's manual switchboard (operated by a person plugging cables into sockets) was known as a PMBX (Private Manual Branch eXchange). These were gradually replaced by automated electromechanical and then electronic switching systems, called PABXs (Private Automatic Branch eXchange). As PMBXes are almost unheard of, the terms PABX and PBX have become synonymous.
Using a PBX saves connecting all of a business's telephone sets separately to the public telephone network (PSTN or ISDN). Such a set-up would require every set to have its own line (usually with a monthly recurring line charge), and "internal" calls would have to be routed out of the building to a central switch, only to come back in again.
As well as telephone sets, fax machines, modems and many other communication devices can be connected to a PBX (although the PBX may degrade line quality for modems). For this reason, all such devices are generally referred to as extensions.
The PBX equipment is typically installed at a business's premises, and connects calls between the telephones installed there. In addition, a limited number of outside lines (called trunk lines) are usually available for making and receiving calls external to the site (i.e. to the public telephone network). Companies with multiple sites can connect their PBXs together with trunk lines. PBX-like services can also be provided by equipment located off site at a central provider, delivering services over the public telephone network. This is known as a [[hosted pbx|hosted PBX]]. For example, most local phone companies offer a Centrex service in which each extension has a trunk line connected to the telephone company's Central Office. Other companies offer a similar service.
PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated or selectable at an individual extension. From a user's point of view calls on a key system are made by selecting a specific outgoing line and dialing the external number; calls on a PBX are made by dialing 9 (or 0 in some systems) followed by the external number; an outgoing trunk line is automatically selected upon which to complete the call.
Current trends
One of the latest trends in PBX development is the VoiceOverIP PBX, also known as an IPBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP; examples of VoIP PBXs are: AIP, Pingtel, ShoreTel, Cisco, Asterisk, EcoRouter, New EGW-804, pbxnsip, PBX Gate, PBXpress,and Vertical TeleVantage.
Historically, the expense of PBX systems has put them out of reach of small businesses and individuals. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.
Examples of open source PBX applications are: *[[Asterisk PBX|Asterisk]], commercially supported by Digium *SIPfoundry's sipXpbx, a [[Session Initiation Protocol|SIP]]-focused solution commercially supported by Pingtel. *[[GNU Bayonne]] - A [[Free_software|free]] telephony application development platform.