PBX
From Skip2PBX-WIKI
A Private Branch eXchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. PBXs are also referred to as:
- PABX - Private Automatic Branch eXchange
- EPABX - Electronic Private Automatic Branch eXchange
Contents |
Overview
PBXs make connections among the internal telephones of a private organization — usually a business — and also connect them to the public switched telephone network (PSTN) via trunk lines. Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch.
PBXs are differentiated from "Key telephone system" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Hybrid telephone system combine features of both.
Initially, the primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone service. As PBXs gained popularity, they started offering services that were not available in the operator network, such as hunt groups, call forwarding, and extension dialing. In the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.
Two significant developments during the 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the VoIP PBX. (Technically, nothing was being "exchanged" any more, but the abbreviation PBX was so widely understood that it remained in use.)
The other trend was the idea of focusing on core competence. PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of hosted PBX. In a hosted setup, the PBX is located at and managed by the telephone service provider, and features and calls are delivered via the Internet. The customer just signs up for a service, rather than buying and maintaining expensive hardware. This essentially removes the branch from the private premises, moving it to a central location.
PBX functions
Functionally, the PBX performs four main call processing duties:
- Establishing connections (circuits) between the telephone sets of two users. (e.g. mapping a dialled number to a physical phone, ensuring the phone isn't already busy)
- Maintaining such connections as long as the users require them (i.e. channelling voice signals between the users)
- disconnecting those connections as per the users requirement
- Providing information for accounting purposes (e.g. metering calls)
In addition to these basic functions, PBXs offer many other calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include (manufacturers may have a different name for each capability):
- Auto attendant
- Auto dialing
- Automatic call distributor
- Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
- Automatic ring back
- Call accounting
- Call forwarding on busy or absence
- Call park
- Call pick-up
- Call transfer
- Call waiting
- Camp-on
- Conference call
- Custom greetings
- Customised Abbreviated dialing (Speed Dialing)
- Busy Override
- Direct Inward Dialing
- [Direct Inward System Access] (DISA) (the ability to access internal features from an outside telephone line)
- Do not disturb(DND)
- Follow-me
- Interactive voice response
- Music on hold
- Night service
- Shared message boxes (where a department can have a shared voicemail box)
- Voice mail
- Voice message broadcasting
- Voice paging (PA system)
- Welcome Message
IP-PBX
An IP PBX handles voice signals under Internet protocol, bringing benefits for Computer telephony integration (CTI). An IP-PBX can exist as physical hardware, or can carry out it functions virtually, performing the call-routing activities of the traditional PBX or key system as a software system. The virtual version is also called a "Soft PBX".
Interface standards
Interfaces for connecting extensions to a PBX include:
- POTS (Plain Old Telephone System) - the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension.
- proprietary - the manufacturer has defined a protocol. One can only connect the manufacturer's sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons.
- DECT - a standard for connecting cordless phones.
- Internet Protocol - For example, H.323 and SIP.
Interfaces for connecting PBXs to each other include:
- proprietary protocols - if equipment from several manufacturers is on site, the use of a standard protocol is required.
- QSIG - for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits.
- DPNSS - for connecting PBXs to trunk lines. Standardized by British Telecom, this usually runs over E1 (E-carrier) physical circuits.
- Internet Protocol - H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.
Interfaces for connecting PBXs to trunk lines include:
- standard POTS (Plain Old Telephone System) lines - the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call.
- ISDN - the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections.
- RBS - (Robbed bit signaling) - delivers 24 digital circuits over a four-wire (T1) interface.
- Internet Protocol - H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers.
Interfaces for collecting data from the PBX:
- Serial interface - historically used to print every call record to a serial printer. Now an application connects via serial cable to this port.
- Network Port (Listen mode) - where an external application connects to the TCP or UDP port. The PBX then starts streaming information down to the application.
- Network Port (Server mode) - The PBX connects to another application or buffer.
- File - The PBX generates a file containing the call records from the PBX.
The call records from the PBX are called SMDR, CDR, or CIL.
It is possible to use a Voice modem as FXO card.
