Private branch exchange

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(Redirected from PABX)
The PBX abbreviation is also used for polymer-bonded explosives.


A Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone switching center that is owned by a private business, as opposed to one owned by a common carrier or by a telephone company.

The term PABX (Private Automatic Branch eXchange) refers to an automatic version of a PBX, though the two terms are used interchangeably today since most PBX systems are now fully automatic.

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 be 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. 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 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.

There are many PBX hardware manufacturers. Some of the most well known include: Avaya (was Lucent was AT&T), Alcatel, Cisco Systems, Ericsson, Fujitsu, InterTel, Mitel, NEC, Nortel, Panasonic, Siemens AG (includes Rolm), ShoreTel, Toshiba, and Vodavi.

One of the latest trends in PBX development is VoIP PBXs, also known as IPBXs, which use the Internet Protocol to carry calls. Most modern PBXs support VoIP, examples of VoIP PBXs are: AIP, Shoretel, Cisco, Asterisk, EcoRouter, New EGW-804, PBX Gate, PBXpress (

Historically, the expense of PBX systems has put them out of reach of small businesses and individuals. However, recent open source projects combined with cheaper hardware are sharply reducing the cost of PBX ownership. In particular, the open source PBX application Asterisk combined with cheap PC hardware and interface cards is gaining popularity as an alternative to traditional PBX solutions.

PBX Functions

Functionally, the PBX performs three main duties:

  • Establishing connections (circuits) between the telephone sets of two users. (e.g. mapping a dialled number to a physical phone, ensuring the phone isn't already busy)
  • Maintaining such connections as long as the users require them. (e.g. channeling voice signals between the users)
  • Providing information for the Accounting Department (e.g. metering calls)

In addition to these basic functions, PBXs offer many other capabilities, with different manufacturers providing different features in an effort to differentiate their products. Here is a short list of common capabilities (note that each manufacturer may have a different name for each capability):

Interface standards

Interfaces for connecting extensions to a PBX include:

  • POTS (Plain Old Telephone System) - the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension.
  • proprietary - the manufacturer has defined a protocol. One can only connect the manufacturer's sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons.
  • DECT - a standard for connecting cordless phones.
  • Internet Protocol - For example, H.323 and SIP.

Interfaces for connecting PBXs to each other include:

  • proprietary protocols - if equipment from several manufacturers are on site, the use of a standard protocol is required.
  • QSIG - for connecting PBXs to each other, usually runs over E1 (E-carrier) physical circuits.
  • DPNSS - for connecting PBXs to trunk lines. Standardised by BT, this usually runs over E1 (E-carrier) physical circuits.
  • Internet Protocol - H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.

Interfaces for connecting PBXs to trunk lines include:

  • standard POTS (Plain Old Telephone System) lines - the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call.
  • ISDN - the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on E1 physical connections.
  • Internet Protocol - H.323, SIP, MGCP, and IAX protocols operate over IP and are supported by some network providers.

See Also

de:Nebenstellenanlage fr:Private Automatic Branch eXchange pl:PBX ru:PBX


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