PBX

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abbr.
private branch exchange

A telecommunications switching system physically located at a customer's place of business. The private branch exchange (PBX) provides internal station-to-station communications for a well-defined set of users. It also provides access to outside telephone lines, called trunks, which connect the private branch exchange to the telephone company's central office. A typical private branch exchange has more stations than trunks (often as many as 10 stations for each trunk) because the stations share the use of the trunks.

CENTREX service, a leased business telephone service provided by the local telephone operating company, provides many of the same station features and functions as a private branch exchange. However, the main switching vehicle for CENTREX is located at the telephone company central office rather than the customer's premises. Conceptually, however, it is similar to a private branch exchange because station users must access the public switched network (PSN) on a contention basis, whereby stations share trunks.

Three distinct generations of private branch exchanges have appeared. In the first generation (1900–1930), a human operator manually set up calls. Second-generation private branch exchanges (mid-1930s to mid-1970s) used mechanical relays to establish the call path. The third generation of private branch exchanges is the stored-program microprocessor-controlled system. Introduced in the mid-1970s, these systems use computer instructions to perform the call set-up and tear-down. Because the private branch exchange is software-based, features and upgrades can often be added without disturbing its operation and the system can be expanded modularly as necessary. The third-generation private branch exchange is physically much smaller than electromechanical models, uses less power, and generates less heat. See also Digital computer; Microprocessor.

One significant development of this generation is the rise of digital transmission as the predominant method of signal transmission within the private branch exchange. The analog voice signal can be encoded into a string of digital pulses by a coder/decoder (codec). After it is switched through the private branch exchange, the signal is reconverted to analog format. See also Analog-to-digital converter; Digital-to-analog converter; Electrical communications; Pulse modulation.

Telephone instruments have also evolved, becoming more intelligent and versatile. The trend is toward proprietary electronic digital multibutton telephone sets with their own microprocessors, which support enhanced features and functions. Many private branch exchanges permit transmission of data as well as voice conversations. Integrated voice-data terminals, data adapters mounted within the telephone, and stand-alone data units allow the user to conduct voice and data calls simultaneously.

Data transmissions switched through the private branch exchange can communicate with another data device or computer connected to the system or, via the public switched network, with a wide variety of remote data devices and computers. Communications between two data devices attached to the same private branch exchange is usually end-to-end digital. Completely digital transmission between two private branch exchanges is also possible by using public switched network digital trunk facilities, called T1 or DS1 trunks. However, in most cases when transmission through the public switched network is required, the originating location must convert the digital signal to analog format by using a modem. See also Data communications; Modem.

Modern private branch exchanges have extensive and virtually identical feature complements, which include several types of call forwarding, least-cost routing, station message detail recording (SMDR), conferencing, hunting, and calling restrictions. In addition to these features, many private branch exchanges interface with outboard applications processors, which enhance the call-processing function of the private branch exchange.

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(Private Branch eXchange) An inhouse telephone switching system that interconnects telephone extensions to each other as well as to the outside telephone network (PSTN). A PBX enables a single-line telephone set to gain access to one of a group of pooled (shared) trunks by dialing an 8 or 9 prefix. PBXs also include functions such as least cost routing for outside calls, call forwarding, conference calling and call accounting. Modern PBXs use all-digital methods for switching, but may support both analog and digital telephones and telephone lines. See IP PBX and WPBX.

An Early PBX

This PBX began operation in Bangor, Maine in 1883. (Image courtesy of AT&T.)

A Field Switchboard

This portable commutator was a one-wire telegraphic switchboard for military field service in 1914. Made by Northern Electric (now Nortel Networks), it was one of the first communications products built for the battlefields of World War I. (Image courtesy of Nortel Networks.)

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A private telephone switching system located on the customer’s premises, usually serving an organization (such as a business or government agency). It switches telephone calls within a building and also to an outside telephone network.

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is short for:

Meaning	Category
Peanut Butter Xtra	Miscellaneous->Food
Private Branch Exchange	Computing->General Governmental->Military Computing->Networking Computing->Telecom
Punctuation	Community->Media

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For a list of words related to PBX, see:

• Voice Transmission and Telephony - PBX: private branch exchange; telephone system that functions within one business or building

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