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
Overview
PBXs make connections among the internal telephones of a private organization — usually a business — and also connects 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 systems" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Hybrid systems 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.
History
The term PBX was first applied when switchboard operators ran company switchboards by hand. As automated electromechanical and then electronic switching systems gradually began to replace the manual systems, the terms PABX (private automatic branch exchange) and PMBX (private manual branch exchange) were used to differentiate them. Solid state digital systems were sometimes referred to as EPABX s (electronic private automatic branch exchange). Now, the term PBX is by far the most widely recognized. The acronym is now applied to all types of complex, in-house telephony switching systems, even if they are not p rivate, b ranches, or e x changing anything.
PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated or selectable at an individual extension. From a user's point of view, calls on a key system are made by selecting a specific outgoing line and dialing the external number. A PBX, in contrast, has a dial plan. Users dial an escape code (usually a single digit; often the same as the first digit of the local emergency telephone number) that connects them to an outside line (DDCO or Direct Dial Central Office in Bell System jargon), followed by the external number. Some modern number analysis systems allow users to dial internal and external numbers without escape codes by use of a dialplan which specifies how calls to numbers beginning with certain prefixes should be routed.
System components
A PBX will often include:
- The PBX’s internal switching network.
- Microcontroller or microcomputer for arbitrary data processing, control and logic.
- Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
- Stations or telephone sets, sometimes called lines.
- Outside Telco trunks that deliver signals to (and carry them from) the PBX.
- Console or switchboard allows the operator to control incoming calls.
- Uninterruptible Power Supply (UPS) consisting of sensors, power switches and batteries.
- Interconnecting wiring.
- Cabinets, closets, vaults and other housings.
Current trends
One of the latest trends in PBX development is the VoIP PBX, also known as an IP-PBX or IPBX , which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, and programmable caller ID. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.
For some users, the private branch exchange has gone full circle as a term. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator , they have evolved into VoIP centres that are hosted by the operators or even hardware manufacturers. These modern IP Centrex systems offer essentially the same service, but they have moved so far from the original concept of the PBX that the term hardly applies at all.
Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrexes. Currently, there are four distinct scenarios in use:
- PBX (Private and Circuit Switched)
- Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
- IP PBX (Private and Packet Switched)
- IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)
Since in reality people want to call from the IP side to the circuit switched PSTN (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.
Home and small business usage
Historically, the expense of full-fledged PBX systems has put them out of reach of small businesses and individuals. However, since the 1990s there has been a large set of small, consumer-grade and consumer-size PBXs available. These systems are not comparable in size, robustness or flexibility to commercial-grade PBXs, but still provide a surprising set of features.
The first consumer PBX systems were for the analog telephone systems, typically supporting four private analog and one public analog line. They are the size of a small cigar box.
Particularly in Europe these systems for analog phones were followed by consumer-grade PBXs for ISDN. Using small PBXs for ISDN is a logical step, since the basic rate interface of ISDN (which is the phone interface individuals and small businesses typically get) provides two logical phone lines (two B channels) which can be used in parallel. Small, entry-level systems are also extremely cheap (e.g. US$100).
With the pickup of VoIP by consumers, of course consumer VoIP PBXs have seen the light, and PBX functions have become simple additional features of consumer-grade routers and switches.
Open source projects have been available since the beginning of the 90s. These projects provide flexibility and features (often not needed or understood by average users), plus the means to actually inspect and change the inner working of a PBX. They have also opened business opportunities for newcomers to the market of mid-size PBXs, since they have lowered the entry barrier for new manufacturers.
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 user's 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 Blocking
- Call forwarding on busy or absence
- Call park
- Call pick-up
- Call transfer <
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