Private Branch Exchange (PBX) is a telephone switch used by enterprises and located at the premises of a company. PBX switches calls among enterprise users on local lines while allowing all users to share a certain number of lines for external calls to PSTN. For companies with multiple physical locations, PBXs are sometimes interconnected by so called trunk lines.
The PBX performs three main functions: a) Establishing connections (circuits) between the telephone sets of two users. Fax, modems and many communication devices can also be connected to the PBX. b) Maintaining phone connections as long as the users require them. c) Providing management information for the accounting purpose. Here is a short list of common capabilities of a PBX:
- Direct Dialing (DDD or DDI), also called Direct Inward Dialing (DID)
- Customised Abbreviated dialing (Speed Dialing)
- 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,...)
A PBX typically includes the following components:
- Telephone trunk lines that terminate at the PBX
- A computer that manages the switching of the calls within the PBX and the access to PSTN
- The network of local lines within the PBX
- Usually a console or switchboard for a human operator
PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated and selectable from an individual extension. In addition, telephone service providers offer Centrex or "Virtual PBX" 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.
The traditional PBX based on the TDM technology is reaching the end of its lifecycle due to the emergence of IP-PBX. The IP-PBX offers easier user administration and advanced applications.