(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.)
