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Private branch exchange

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This redirect is about the type of telephone exchange. For other uses of "PBX", see PBX.
An Avaya G3si PBX with front cover removed (view from the top).

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 EPABXs (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 private, branches, or exchanging 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.

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 utilized analog ('POTS') telephone lines, typically supporting four private analog and one public analog line. They are the size of a small cigar box. 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 ISDN basic rate interface of provides two logical phone lines (via two ISDN B channels) which can be used in parallel. Small, entry-level systems are also extremely cheap (e.g. US$100). With the adoption of VoIP by consumers, consumer VoIP PBXs have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches.

Open source projects have provided PBX-style features since the 1990s. These projects provide extreme flexibility and features, including the means to actually inspect and change the inner working of a PBX. Lowered entry barriers for new manufacturers created business opportunities for newcomers.

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):

Interface standards

Interfaces for connecting extensions to a PBX include:

  • POTS (plain old telephone service) - 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.
  • ISDN PRI - Runs over T1, 23 bearer channels + 1 signalling channel
  • 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 service) 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.

Hosted PBX systems

A hosted PBX system delivers PBX functionality as a service, available over the Public Switched Telephone Network (PSTN) and/or the internet. Hosted PBXs are typically provided by the telephone company, using equipment located in the premises of the telephone company's exchange. This means the customer organization doesn't need to buy or install PBX equipment (generally the service is provided by a lease agreement) and the telephone company can (in some configurations) use the same switching equipment to service multiple PBX hosting accounts.

Instead of buying PBX equipment, users contract for PBX services from a hosted PBX service provider, a particular type of application service provider (ASP). The first hosted PBX service was very feature-rich compared to most premise-based systems of the time. In fact, some PBX functions, such as follow-me calling, appeared in a hosted service before they became available in hardware PBX equipment. Since that introduction, updates and new offerings from several companies have moved feature sets in both directions. Today, it is possible to get hosted PBX service that includes far more features than were available from the first systems of this class, or to contract with companies that provide less functionality for simple needs.

In addition to the features available from premises-based PBX systems, hosted-PBX:

  • Allows a single number to be presented for the entire company, despite its being geographically distributed. A company could even choose to have no premises, with workers connected from home using their domestic telephones but receiving the same features as any PBX user.
  • Allows multimodal access, where employees access the network via a variety of telecommunications systems, including POTS, ISDN, cellular phones, and VOIP. This allows one extension to ring in multiple locations (either concurrently or sequentially).
  • Supports integration with custom toll plans (that allow intra company calls, even from private premises, to be dialed at a cheaper rate) and integrated billing and accounting (where calls made on a private line but on the company's behalf are billed centrally to the company).
  • Eliminates the need for companies to manage or pay for on-site hardware maintenance.

Mobile PBX

A mobile PBX is a hosted PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other hosted PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys and other input devices, to control PBX phone functions and to manage communications without having to call into the system first.

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 its 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".

See also

Notes

  • The acronym WPBX, meaning Wireless PBX, is sometimes used. See also IP PBX

References

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