The PBX abbreviation is also used for polymer-bonded explosives.

The Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone switching center that is owned by a private business, compared to one that is owned by the common carrier or telephone company. Users of the PBX share a certain number of outside lines for making telephone calls external to the PBX. There are two ways to realize a PBX: with hardware or virtually. A hardware PBX has the equipment located on the business premises and a virtual PBX has the equipment at a central provider and delivers the PBX as a service over the commercial telephone network.

Historically, the expense of PBX systems has put them out of reach of small businesses and individuals. Recent open source projects and cheaper hardware are sharply reducing the cost though.

The term PABX (Private Automatic Branch eXchange) is simply an automatic version of the PBX. The PABX is also faster with data communication, and can handle more telephone calls at the same time. A PBX could be considered an older version of the PABX. Most companies now use the PABX.

In industrial countries, many companies need their employees to be able to call each other, call outside phone numbers (the public telephone network or PSTN/ISDN), and receive calls from outside.

For companies with multiple physical locations, PBXs are sometimes interconnected by so-called trunk lines.

PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated and selectable from 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) then the external number.

Another alternative is to connect all the telephone sets to the PSTN/ISDN, but the major disadvantage is that every extension requires its own line (usually with a monthly recurring line charge); also, to-"internal" calls would be dialed externally, and charged for.

Finally, most local phone companies offer Centrex service in which each extension has a trunk line connected to the telephone company's Central Office, where software on the CO switch enables PBX-like functionality.

Functionally, the PBX performs three main duties:

• Establishing connections (circuits) between the telephone sets of two users. Note that fax, modems and many communication devices can often be connected to the PBX (although the PBX may degrade line quality for modems). Therefore telephone sets are referred to as extensions.
• Maintaining such connections as long as the users require them.
• Providing information for the Accounting Department (e.g. metering calls)

There are many PBX manufacturers. Some of the most common include: Avaya (was Lucent was AT&T), Siemens AG (includes Rolm), NEC, Nortel, Toshiba, Fujitsu, Vodavi, Mitel, Ericsson, Panasonic, InterTel. There is also an open source PBX application, Asterisk.

PBXs offer many capabilities, although each manufacturer may have a different name for each capability. Here is a short list of common capabilities:

• calling
• you can attach other devices
• call transfer

The newer version of the PABX can do more applications. Here is a short list of common capabilities:

• Direct Dialing (DDD or DDI), also called Direct Inward Dialing (DID)
• Customised Abbreviated dialing (Speed Dialing)
• voice mail
• Follow-me
• Call forwarding on absence
• Call forwarding on busy
• Call transfer
• Music on hold
• Automatic ring back
• Night service
• Call distribution (ACD, fixed sequences, ...)

The extension interface can be:

• proprietary: the manufacturer has defined a protocol. One can only connect the manufacturer's sets on the PBX
• standard interfaces: any device supporting the standard can be connected

The most common digital standard for fixed devices is ISDN.

Cordless phones can also be used on sophisticated PBX: DECT devices have become a standard.

Potential links between PBX (trunk lines) can also use proprietary protocols, but if equipment from several manufacturers are on site, the use of a standard protocol is required. Most used standard protocols are QSIG and DPNSS.

One of the latest trends in PBX development are VoIP PBXes. There are many VoIP PBX models and manufacturers. Some of them include: AIP, Asterisk, EcoRouter, New EGW-804, PBX Gate, PBXpress, and more.

See Also

• Asterisk is an open source software implementation of a PBX.
• VoIP PBX Servers