![1970's Czechoslovka TESLA automatic telephone exchange](https://upload.wikimedia.org/wikipedia/commons/d/dd/1970%27s_Czechoslovka_TESLA_automatic_telephone_exchange.jpg)
A business telephone system is a multiline
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX).
A business telephone system differs from an installation of several telephones with multiple central office (CO) lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into key telephone systems, and private branch exchanges, but many hybrid systems exist.
A key telephone system was originally distinguished from a private branch exchange in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office system in capacity and features. With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps.
Key telephone system
![Bell telephone magazine (1922) (14756481265)](https://upload.wikimedia.org/wikipedia/commons/3/3f/Bell_telephone_magazine_%281922%29_%2814756481265%29.jpg)
Key telephone systems are primarily defined by arrangements with individual line selection buttons for each available telephone line. The earliest systems were known as ''wiring plans'' and simply consisted of telephone sets, keys, lamps, and wiring.
''Key'' was a
Bell System term of art
Jargon is the specialized terminology associated with a particular field or area of activity. Jargon is normally employed in a particular communicative context and may not be well understood outside that context. The context is usually a particu ...
for a customer-controlled switching system
[W.F.B. Wood, ''Key Telephone Systems: The Latest Chapter'', Bell Laboratories Record, March 1966, pp.85] such as the line buttons on the phones associated with such systems.
The wiring plans evolved into modular hardware building blocks with a variety of functionality and services in the 1A key telephone system developed in the
Bell System in the 1930s.
Key systems can be built using three principal architectures:
electromechanical
In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
shared-control, electronic shared-control, or independent key sets.
New installations of key telephone systems have become less common, as hybrid systems and private branch exchanges of comparable size have similar cost and greater functionality.
Electromechanical shared-control key system
Before the advent of
large-scale integrated circuits, key systems were typically composed of electromechanical components (
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
s) as were larger
telephone switching
telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
systems.
The systems marketed in
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
as the 1A, 6A, 1A1 and the
1A2 Key System are typical examples and sold for many decades. The 1A family of
Western Electric Company
The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment m ...
(WECo) key telephone units (KTUs) were introduced in the late 1930s and remained in use to the 1950s. 1A equipment was primitive and required at least two KTUs per line; one for line termination and one for station (telephone instrument) termination. The telephone instrument commonly used by 1A systems was the WECo
300-series telephone. Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased the features available. As the 1A1 systems became commonplace, requirements for intercom features grew. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time. The WECo 6A dial intercom system provided two talk links and was often installed as the dial intercom in a 1A1 or 1A2 key system. The 6A systems were complex, troublesome and expensive, and never became popular. The advent of 1A2 technology in the 1964 simplified key system set up and maintenance. These continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems.
Two lesser-known key systems were used at airports for air traffic control communications, the 102 and 302 key systems. These were uniquely designed for communications between the air traffic control tower and radar approach control (
RAPCON
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
) or ground control approach (GCA), and included radio line connections.
Automatic Electric Company
Automatic Electric Company (A.E. Co.) was an American telephone equipment supplier primarily for independent telephone companies in North America, but also had a worldwide presence. With its line of automatic telephone exchanges, it was also a lo ...
also produced a family of key telephone equipment, some of it compatible with Western Electric equipment, but it did not gain the widespread use enjoyed by Western Electric equipment.
Electronic shared-control system
With the advent of
LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to (or even identical to) that used by non-key systems. Electronic shared-control systems led quickly to the modern hybrid telephone system, as the features of PBX and key system quickly merged. One of the most recognized such systems is the
AT&T Merlin
AT&T Merlin is a corporate telephone system by American Telephone and Telegraph (AT&T) that was introduced in late 1983, when it was branded American Bell Merlin. After the breakup of AT&T in 1984, it was rebranded and later also supplied by Luce ...
.
Additionally, these more modern systems allowed a diverse set of features including:
*
Answering machine
An answering machine, answerphone or message machine, also known as telephone messaging machine (or TAM) in the United Kingdom, UK and some Commonwealth countries, ansaphone or ansafone (from a trade name), or telephone answering device (TAD), ...
functions
* Automatic call accounting
*
Caller ID
Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call i ...
* Remote supervision of the entire system
* Selection of signaling sounds
* Speed dialing
* Station-specific limitations (such as no long distance access or no paging)
Features could be added or modified simply using software, allowing easy customization of these systems. The stations were easier to maintain than the previous electromechanical key systems, as they used efficient
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s instead of
incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
s for line status indication.
LSI also allowed smaller systems to distribute the control (and features) into individual telephone sets that don't require any single shared control unit. Generally, these systems are used with a relatively few telephone sets and it is often more difficult to keep the feature set (such as speed-dialing numbers) in synchrony between the various sets.
Hybrid key telephone system
Into the 21st century, the distinction between key systems and PBX systems has become increasingly blurred. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected
PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
lines and stations.
The modern key system now supports
SIP,
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
, analog handsets (in addition to its own proprietary handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signaling, and of some PBX functionality gives rise to the ''hybrid'' designation.
A hybrid system typically has some call appearance buttons that directly correspond to individual lines and/or stations, but may also support direct dialing to extensions or outside lines without selecting a line appearance.
The modern key system is usually fully digital, although analog variants persist and some systems implement
VOIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
services. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope and complexity of the features and facilities offered.
Private branch exchange
![Alcatel_-_PABX_-_Omni_PCX_Enterprise_v1](https://upload.wikimedia.org/wikipedia/commons/5/52/Alcatel_-_PABX_-_Omni_PCX_Enterprise_v1.jpg)
A PBX is a
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
or switching system that serves a private organization and permits sharing of central office trunks between internally installed telephones, and provides intercommunication between those internal telephones within the organization without the use of external lines. The central office lines provide connections to the
public switched telephone network (PSTN) and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization. Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network. This method reduces the number of lines needed from the organization to the public switched telephone network.
Each device connected to the PBX, such as a telephone, a
fax machine
Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...
, or a computer modem, is referred to as an
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
and has a designated extension telephone number that may or may not be mapped automatically to the
numbering plan
A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reac ...
of the central office and the telephone number block allocated to the PBX.
Initially, PBX systems offered the primary advantage of cost savings for internal phone calls: handling the circuit switching locally reduced charges for telephone service via central-office lines. As PBX systems gained popularity, they began to feature services not available in the public network, such as
hunt groups, call forwarding, and extension dialing. From the 1960s, a simulated PBX, known as
Centrex
Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.
Centrex service was first installed ...
, provided similar features from the central
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
.
A PBX differs from a
key telephone system
A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX).
A business telephone syst ...
(KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select the outgoing line automatically. The telephone sets connected to a PBX do not normally have special keys for central-office line control, but it is not uncommon for key systems to be connected to a PBX to extend its services.
A PBX, in contrast to a key system, employs an organizational numbering plan for its stations. In addition, a
dial plan In telecommunication, a dial plan (or dialing plan) establishes the permitted sequences of digits dialed by telephone subscriber and the manner in which a telephone switch interprets these digits within the definitions of the prevailing telephone nu ...
determines whether additional digit sequences must be prefixed when dialing to obtain access to a central office trunk. Modern number-analysis systems permit users to dial internal and external telephone numbers without special codes to distinguish the intended destination.
History
The term PBX originated when
switchboard operators managed company switchboards manually using cord circuits. As automated electromechanical switches and later electronic switching systems gradually replaced the manual systems, the terms ''private automatic branch exchange'' (PABX) and ''private manual branch exchange'' (PMBX) differentiated them. Solid-state digital systems were sometimes referred to as ''electronic private automatic branch exchanges'' (EPABX). , the term PBX is by far the most widely recognized. The abbreviation now applies to all types of complex, in-house telephony switching systems.
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
In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...
. Companies needed packet-switched networks for data, so using them for telephone calls proved 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
voice over IP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...
PBX, or IP-PBX.
The other trend involved the idea of focusing on
core competence
A core competency is a concept in management theory introduced by C. K. Prahalad and Gary Hamel.Prahalad, C.K. and Hamel, G. (1990)The core competence of the corporation", Harvard Business Review (v. 68, no. 3) pp. 79–91. It can be define ...
. 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 the hosted PBX. In wireline telephony, the original hosted PBX was the
Centrex
Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.
Centrex service was first installed ...
service provided by telcos since the 1960s; later competitive offerings evolved into the modern
competitive local exchange carrier. In voice over IP, hosted solutions are easier to implement as the PBX may be located at and managed by any telephone service provider, connecting to the individual extensions via the Internet. The upstream provider no longer needs to run direct, local leased lines to the served premises.
Manual PBX
Many manufacturers provided manually operated private branch exchange systems in various sizes and features; examples are pictured here:
File:Jersey Telecom switchboard and operator.jpg, Cord PBX switchboard (c.1975)
File:1940s N935 telephone exchange.jpg, Early 1945 telephone exchange model N935 system featuring cordless operation.
File:Telephone exchange machine in Ainokura Traditional Industry Museum.jpg, Manual cord telephone exchange exhibited in Ainokura Traditional Industry Museum in Japan.
System components
A PBX often includes:
* Cabinets, closets, vaults and other housings.
* Console or switchboard allows the operator to control incoming calls.
* Interconnecting wires and cables.
* Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
*
Microcontroller or
microcomputer for arbitrary data processing, control and logic.
* Outside telco trunks that deliver signals to (and carry them from) the PBX.
* Stations or telephone sets, sometimes called lines.
* The PBX's internal switching network.
*
Uninterruptible power supply
An uninterruptible power supply or uninterruptible power source (UPS) is an electrical apparatus that provides emergency power to a load when the input power source or mains power fails. A UPS differs from an auxiliary or emergency power syste ...
(UPS) consisting of sensors, power switches and
batteries.
Current trends
Since the advent of Internet telephony (
Voice over IP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...
) technologies, PBX development has tended toward the
IP PBX
An IP PBX ("Internet Protocol private branch exchange") is a system that connects telephone extensions to the public switched telephone network (PSTN) and provides internal communication for a business. An IP PBX is a PBX system with IP connectivi ...
, which uses the
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
to carry calls. Most modern PBXs support VoIP.
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as
conference calling
A conference call is a telephone call in which someone talks to several people at the same time. The conference call may be designed to allow the called party to participate during the call or set up so that the called party merely listens into ...
, call forwarding, and programmable caller ID. As of 2015 ISDN is being phased out by most major telecommunication carriers throughout Europe in favour of all-IP networks, with some expecting complete migration by 2025.
Originally having started as an organization's manual
switchboard or
attendant console
Attendant or attendance may refer to:
* Car attendant, a railroad employee
* Flight attendant, flight crew employed to ensure the safety and comfort of the passengers
* Museum attendant
* Toilet attendant, maintains standards in a toilet and coll ...
operated by a
telephone operator or just simply the ''operator'', PBXs have evolved into
VoIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
centers that are hosted by the operators or even manufacturers.
Even though VoIP is considered by many people as the future of telephony, the
circuit switched
Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full ...
network remains the core of communications, and the existing PBX systems are competitive in services with modern IP systems. Five distinct scenarios exist:
* Hosted/virtual PBX (hosted and circuit-switched) or traditional Centrex
* IP Centrex or hosted/virtual IP (hosted and packet-switched)
* IP PBX (private and packet-switched)
* Mobile PBX solution (mobile phones replacing or used in combination with fixed phones)
* PBX (private and circuit-switched)
For the option to call from IP network to the circuit-switched
PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
(
SS7/
ISUP), the hosted solutions include interconnecting media gateways.
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 many small, consumer-grade and consumer-size PBXs have become available. These systems are not comparable in size, robustness or flexibility to commercial-grade PBXs, but still provide many features.
The first consumer PBX systems used analog (
POTS) telephone lines, typically supporting four private analog and one public analog line. They were the size of a small
cigar box
A cigar box is a box container for cigar packaging. Traditionally cigar boxes have been made of wood, cardboard or paper. Spanish cedar has been described as the "best" kind of wood for cigar boxes because of its beautiful grain, fine textu ...
. In Europe these systems for analog phones were followed by consumer-grade PBXs for
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
. Using small PBXs for ISDN is a logical step, since the ISDN
basic rate interface
Basic Rate Interface (BRI, 2B+D, 2B1D) or Basic Rate Access is an Integrated Services Digital Network (ISDN) configuration intended primarily for use in subscriber lines similar to those that have long been used for voice-grade telephone servi ...
provides two logical phone lines (via two ISDN B channels) which can be used in parallel. 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. Additionally, many telecommunications providers now offer hosted PBX systems where the provider actually hosts the PBX and the phone handsets are connected to it through an internet connection.
Open source projects have provided PBX-style features since the 1990s. These projects provide flexibility, features, and programmability.
Launched in 1997, IP-PBX took
business communication
Business communication is communication that is intended to help a business achieve a fundamental goal, through information sharing between employees as well as people outside the company. It includes the process of creating, sharing, listening, ...
to the next level. IP-PBX is a PBX phone system that uses
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
(IP) data networks to manage the routing and switching of calls as well as to handle messaging.
Virtual hosting
Virtual hosting is a method for hosting multiple domain names (with separate handling of each name) on a single server (or pool of servers). This allows one server to share its resources, such as memory and processor cycles, without requiring al ...
resulted in significant cost efficiencies. Apart from offering advanced features (like
voicemail
A voicemail system (also known as voice message or voice bank) is a computer-based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to ind ...
), the system could also use a
VoIP gateway to connect to traditional PSTN lines. This gave the user the option of continuing with the same carrier. Over time, improvements in the user interface and call quality, along with cost benefits, added to the popularity of IP-PBX among small businesses.
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 dialed 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 feature
A vertical service code (VSC) is a sequence of digits and the signals star (*) and number sign (#) dialed on a telephone keypad or rotary dial to enable or disable certain telephone service features. Some vertical service codes require dialing of ...
s 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
Auto may refer to:
* An automaton
* An automobile
* An autonomous car
* An automatic transmission
* An auto rickshaw
* Short for automatic
* Auto (art), a form of Portuguese dramatic play
* ''Auto'' (film), 2007 Tamil comedy film
* Auto (play), ...
* Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
*
Automatic call distributor
An automated call distribution system, commonly known as automatic call distributor (ACD), is a telephony device that answers and distributes incoming calls to a specific group of terminals or agents within an organization. ACDs direct calls based ...
*
Automatic ring back
* Busy override
*
Call blocking
*
Call forwarding
Call forwarding, or call diversion, is a telephony feature of all telephone switching systems which redirects a telephone call to another destination, which may be, for example, a mobile or another telephone number where the desired called party ...
on busy or absence
*
Call logging
*
Call park
*
Call pick-up
*
Call transfer
A call transfer is a telecommunications mechanism that enables a user to relocate an existing telephone call to another phone or attendant console, using a transfer button or a switchhook flash and dialing the required location. The transferred ...
*
Call waiting
Call waiting is a telephone service where a subscriber can accept a second incoming telephone call by placing an in-progress call on hold—and may also switch between calls. With some providers it can be combined with additional features such a ...
*
Camp-on
In telecommunication, the term camp-on busy signal has the following meanings:
*A signal that informs a busy telephone user that another call originator is waiting for a connection. Synonym: ''call waiting''
*A teleprinter exchange
Exchange m ...
*
Conference call
A conference call is a telephone call in which someone talks to several people at the same time. The conference call may be designed to allow the called party to participate during the call or set up so that the called party merely listens into ...
* Custom greetings
* Customized abbreviated dialing (
speed dialing
Speed dial is a function available on many telephone systems allowing the user to place a call by pressing a reduced number of keys. This function is particularly useful for phone users who dial certain numbers on a regular basis.
In most case ...
)
*
Direct inward dialing
Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a telecommunication service offered by telephone companies to subscribers who operate a private branch exchange (PBX) system. The feature provides service for ...
(DID)
* Direct inward system access (DISA) (the ability to access internal features from an outside telephone line)
*
Do not disturb (DND)
* Follow-me, also known as find-me: Determines the routing of incoming calls. The exchange is configured with a list of numbers for a person. When a call is received for that person, the exchange routes it to each number on the list in turn until either the call is answered or the list is exhausted (at which point the call may be routed to a
voice mail
A voicemail system (also known as voice message or voice bank) is a computer-based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to ind ...
system).
*
Interactive voice response
Interactive voice response (IVR) is a technology that allows telephone users to interact with a computer-operated telephone system through the use of voice and DTMF tones input with a keypad. In telecommunications, IVR allows customers to interac ...
* Local connection: Another useful attribute of a hosted PBX is the ability to have a local number in cities in which you are not physically present. This service essentially lets you create a virtual office presence anywhere in the world.
*
Music on hold
Music on hold (MOH) is the business practice of playing recorded music to fill the silence that would be heard by telephone callers who have been placed on hold. It is especially common in situations involving customer service.
Music on hold i ...
*
Night service
*
Public address voice paging
* Shared message boxes (where a department can have a shared voicemail box)
*
Voice mail
A voicemail system (also known as voice message or voice bank) is a computer-based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to ind ...
* Voice message broadcasting
* Welcome message
Interface standards
Interfaces for connecting extensions to a PBX include:
*
DECT
Digital enhanced cordless telecommunications (Digital European cordless telecommunications), usually known by the acronym DECT, is a standard primarily used for creating cordless telephone systems. It originated in Europe, where it is the common ...
– a standard for connecting cordless phones.
*
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
– For example,
H.323 and
SIP.
* POTS (
plain old telephone service
Plain old telephone service (POTS), or plain ordinary telephone system, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1 ...
) – 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.
Interfaces for connecting PBXs to each other include:
*
DPNSS
The Digital Private Network Signalling System (DPNSS) is a network protocol used on digital trunk lines for connecting to PABX. It supports a defined set of inter-networking facilities.
DPNSS was originally defined by British Telecom. The specif ...
– for connecting PBXs to trunk lines. Standardized by
British Telecom, this usually runs over E1 (E-carrier) physical circuits.
*
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
–
H.323 and the
Session Initiation Protocol (SIP) are IP-based solutions for multimedia sessions.
*
Primary rate interface (ISDN) – Provided over T1 (23 bearer channels and 1 signaling channel) or E1 carriers.
* Proprietary protocols – if equipment from several manufacturers is on site, the use of a standard protocol is required.
*
QSIG QSIG is an ISDN based signaling protocol for signaling between private branch exchanges (PBXs) in a private integrated services network (PISN). It makes use of the connection-level Q.931 protocol and the application-level ROSE protocol. ISDN "prop ...
– for connecting PBXs to each other, usually runs over T1 (
T-carrier
The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.
The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell Syste ...
) or E1 (
E-carrier
The E-carrier is a member of the series of carrier systems developed for digital transmission of many simultaneous telephone calls by time-division multiplexing. The European Conference of Postal and Telecommunications Administrations (CEPT) ori ...
) physical circuits.
Interfaces for connecting PBXs to trunk lines include:
*
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
–
H.323,
SIP,
MGCP, and
Inter-Asterisk eXchange
Inter-Asterisk eXchange (IAX) is a communications protocol native to the Asterisk private branch exchange (PBX) software, and is supported by a few other softswitches, PBX systems, and softphones. It is used for transporting VoIP telephony session ...
protocols operate over IP and are supported by some network providers.
*
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
– 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 In communications systems, robbed-bit signaling (RBS) is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using channel-associated signaling (CAS). The T1 carrier circuit is a type of dedicated circuit ...
) – delivers 24 digital circuits over a four-wire (T1) interface
* standard POTS (
plain old telephone service
Plain old telephone service (POTS), or plain ordinary telephone system, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1 ...
) 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 (commonly called
glare).
Interfaces for collecting data from the PBX:
* File – the PBX generates a file containing the call records from the PBX.
* Network
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
(listen mode) – an external application connects to the
TCP or
UDP port. The PBX streams information to the application.
* Network port (server mode) – the PBX connects to another application or buffer.
*
Serial interface – historically used to print every call record to a serial printer. In modern systems a software application connects via serial cable to this port.
A data record from a PBX or other telecommunication system that provides the statistics for a telephone call is usually termed a ''
call detail record
A call detail record (CDR) is a data record produced by a telephone exchange or other telecommunications equipment that documents the details of a telephone call or other telecommunications transactions (e.g., text message) that passes through that ...
'' (CDR) or a
Station Messaging Detail Record (SMDR).
Hosted PBX systems
Virtual PBX systems or hosted PBX systems deliver PBX functionality as a service, available over the public switched telephone network (PSTN) or the Internet. Hosted PBXs are typically provided by a telephone company or service provider, using equipment located in the premises of a
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
or the provider's data center.
This means the customer does not need to buy or install PBX equipment. Generally the service is provided by a lease agreement and the provider can, in some configurations, use the same switching equipment to service multiple hosted PBX customers.
The first hosted PBX services were
feature-rich
In software, the term feature has several definitions. The Institute of Electrical and Electronics Engineers defines the term ''feature'' in IEEE 829 as " distinguishing characteristic of a software item (e.g., performance, portability, or functio ...
compared to most premises-based systems of the time. Some PBX functions, such as follow-me calling, appeared in a hosted service before they became available in hardware PBX equipment. Since introduction, updates and new offerings have moved feature sets in both directions. It is possible to get hosted PBX services that include feature sets from minimal functionality to advanced feature combinations.
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).
* allows scalability so that a larger system is not needed if new employees are hired, and so that resources are not wasted if the number of employees is reduced.
* eliminates the need for companies to manage or pay for on-site hardware maintenance.
* 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).
Hosted PBX providers
The ongoing migration of most major telecommunication carriers to IP-based networks, coupled with the rise in
Cloud Communications
Cloud communications are Internet-based voice and data communications where telecommunications applications, switching and storage are hosted by a third-party outside of the organization using them, and they are accessed over the public Internet. C ...
has resulted in a significant rise in the uptake of hosted PBX solutions.
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.
A mobile PBX may exploit the functionality available in smartphones to run custom applications to implement the PBX specific functionality.
In addition, a mobile PBX may create extension identifiers for each handset that allow to dial other cell phones in the PBX via their extension shortcut, instead of a PSTN number.
IP-PBX
An
IP PBX
An IP PBX ("Internet Protocol private branch exchange") is a system that connects telephone extensions to the public switched telephone network (PSTN) and provides internal communication for a business. An IP PBX is a PBX system with IP connectivi ...
handles voice signals over Internet protocol, bringing benefits for
computer telephony integration
Computer telephony integration, also called computer–telephone integration or CTI, is a common name for any technology that allows interactions on a telephone and a computer to be coordinated. The term is predominantly used to describe desktop- ...
(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
*
Centrex
Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.
Centrex service was first installed ...
*
Circuit ID
A circuit ID is a company-specific identifier assigned to a data or voice network connection between two locations. This connection, often called a circuit, may then be leased to a customer referring to that ID. In this way, the circuit ID is simi ...
*
Cloud communications
Cloud communications are Internet-based voice and data communications where telecommunications applications, switching and storage are hosted by a third-party outside of the organization using them, and they are accessed over the public Internet. C ...
*
Ground start trunk
*
List of SIP software
This list of SIP software documents notable software applications which use Session Initiation Protocol (SIP) as a voice over IP (VoIP) protocol.
Servers
Free and open-source license
A SIP server, also known as a SIP proxy, mana ...
*
RJ21
A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in th ...
*
Switchboard operator
*
Telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
*
Telephone switchboard
A telephone switchboard was a device used to connect circuits of telephones to establish telephone calls between users or other switchboards, throughout the 20th century. The switchboard was an essential component of a manual telephone exchange, ...
s
References
External links
{{DEFAULTSORT:Business Telephone System
Telephone exchange equipment
Computer telephony integration
Telephony equipment