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.
In Europe, the term PABX (Private Automatic Branch eXchange) is often used.
In industrial countries, most companies (with more than around 10 employes) need their employes to be able call each other, call outside phone numbers (the public telephone network or PSTN), 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 then the external number.
Another alternative is to connect all the telephone sets to the PSTN, but the major disandvantage is that every extension requires its own line (usually with a monthly recurring line charge); also, "internal" calls would be dialed externally, and charged for.
Finally, most local phone companies offer Centrex or "Virtual PBX" service in which each extension has a trunk line are 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 Accouting Department (e.g. metering calls)
The are many PBX manufacturers. Some of the most common include: Agilent (was Lucent was AT&T), Siemens (includes Rolm), NEC, Toshiba, Fujitsu, Vodavi, Mitel, Ericson.
PBXs offer many capabilities, although each manufacturer may have a different name for each capability. Here is a short list of common capabilities:
- Direct Dialing (DDD or DDI), also called Direct Inward Dialing (DID) - customised Abreviated dialing (Speed Dialing) - follow-me - call forwarding on absence - call forwarding on busy - call transfer - music on hold - automatic ring back - night service - call disribution (ACD,fixed sequences,...)
The extension interface can be:
- proprietary: the manufacter 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 sofisticated PBX: DECT devices become a standard.
Potential links between PBX (trunk lines) can also use proprietary protocols, but if severals manufacters are on site, the use of a standard protocol is required. Most used standard protocols are QSIG and DPNSS.