Key telephone system - AbsoluteAstronomy.com

A business telephone system is any of a range of a multiline telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch.
A business telephone system differs from simply using a telephone with multiple lines in that the lines used are accessible from multiple telephones, or "stations" in the system, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into "key systems", "hybrid systems", and "private branch exchanges".

A key system was originally distinguished from a private branch exchange (PBX) in that it allowed the station user to see and control the calls directly, manually, using lighted line buttons, while a private branch exchange operated in a manner similar to the public telephone system, in that the calls were routed to the correct destination by being dialed directly. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office in capacity and features.

Key systems

Key was a Bell System

Bell System

The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...

term of art

Technical terminology

Technical terminology is the specialized vocabulary of any field, not just technical fields. The same is true of the synonyms technical terms, terms of art, shop talk and words of art, which do not necessarily refer to technology or art...

for a manually operated switch, such as the line-buttons on the phones associated with such systems.

Key systems are primarily defined by their individual line selection buttons for each connected phone line, a feature shared with hybrid systems. New installations of true "key" systems have become less common, as hybrid systems and private branch exchanges of comparable size now have similar costs and greater functionality.

Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent keysets.

Electromechanical shared-control key systems

Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relay

Relay

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

s) as were larger telephone switching systems.

The systems marketed in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

as the 1A, 6A, 1A1 and the 1A2 Key System

1A2 Key System

The 1A2 Key System is an analog multiline business key telephone system. Unlike more modern multiline systems, every telephone line serving a particular phone is wired into that phone, and electromechanical switches switch the lines in the phone itself...

were typical and sold for many decades. The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were in use in 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

Model 302 telephone

The Model 302 telephone subscriber set was manufactured by Western Electric from 1937; manufacture of entirely new units was ceased after the introduction of the Model 500, but Model 302 units were continually remanufactured as such at least until 1958, and as the Model 5302 telephone which was...

. In the 1960s, 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 increased. 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. Unfortunately, the 6A systems were complex, troublesome and expensive, and never became popular. The advent of 1A2 technology in the 1970s 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 obscure 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

Terminal Control Center

A terminal radar approach control is an air traffic control facility usually located within the vicinity of a large airport. Typically, the TRACON controls aircraft within a 20-50 nautical mile radius of the major airport and a number of "satellite airports" between surface and up to between and...

) or ground control approach (GCA), and included radio line connections.

Automatic Electric Company

Automatic Electric Company

Automatic Electric Company was a telephone equipment supplier for independent telephone companies, comparable to the Bell System's Western Electric. It was located in Northlake, Illinois, with research and development labs in Melrose Park and Elmhurst, Illinois...

also sold a family of key telephone equipment, but it never gained the widespread use enjoyed by Western Electric equipment.

Electronic shared-control systems

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

The AT&T Merlin telephone system was introduced in late 1983, branded American Bell Merlin. After the breakup of AT&T in 1984, it was rebranded AT&T Merlin. It was designed at the beginning of the 1980s prior to the Bell System Divestiture as a modern electronic replacement for the dated...

.

Additionally, these more modern systems allowed a vast set of features including:

Answering machine
Answering machine
The answering machine or message machine, also known as the telephone answering machine in the UK and some Commonwealth countries) and previously known as an ansaphone, ansafone, or telephone answering device is a device for answering telephones and recording callers' messages.Unlike voicemail,...

functions
Remote supervision of the entire system
Automatic call accounting
Speed dialing
Caller ID
Caller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
Station-specific limitations (such as no long distance access or no paging)
Selection of signaling sounds

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

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

s instead of incandescent light bulb

Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

s for line status indication.

Independent keysets

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 keyphone systems

Into the 21st century, the distinction between key systems and PBX has become increasingly confusing. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations.

The modern key system now supports SIP

Session Initiation Protocol

The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

, ISDN, 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 signalling, 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 directly 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 embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope and complexity of the features and facilities offered.

Hybrid systems are a common tool in the financial services industry used on trading floors. These advanced hybrid key systems generally only require attached PBXs for interaction with backroom staff and voicemail. These systems commonly have their front end units referred to as Turrets

Trading turret

A trading turret is a specialised telephony key system, also known as a Dealer Board, that is generally used by financial traders in conjunction with other tools that make up their Electronic trading platform.-Voice Trading Turrets:...

and are notable for their presentation of hoot-n-holler

Hoot-n-holler

Hoot-n-holler is a type of telecommunications system where you have a permanent open circuit between two or more parties. Anyone can speak at any time over a distance without having to pick up a phone or press a button...

circuits. Multiple Hoots are presented to multiple users over multiplexed speakers to multiple locations.

Private branch exchange

A private branch exchange (PBX) is a telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

that serves a particular business or office, as opposed to one that a common carrier

Common carrier

A common carrier in common-law countries is a person or company that transports goods or people for any person or company and that is responsible for any possible loss of the goods during transport...

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

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

Fax

Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...

, modem

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

s, and more, the general term "extension

Extension (telephone)

An extension telephone is an additional telephone wired to the same telephone line as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions". Such extension phones allow making or receiving calls in different rooms,...

" is used to refer to any end point on the branch.

PBXs are differentiated from "key systems

Key telephone system

A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch....

" 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

Hunting (telephony)

Hunting, or a hunt group is a telephony concept that refers to the methodology of distributing phone calls from a single telephone number to a group of several phone lines. Specifically, it refers to the process or algorithm used to select which line will receive the call.Hunt groups are supported...

, call forwarding, and extension dialing. In the 1960s a simulated PBX known as Centrex

Centrex

Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of Centrex:...

provided similar features from the central telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

.

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

Packet switching

Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...

. 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 operator

Switchboard operator

In the early days of telephony, through roughly the 1960s, companies used manual telephone switchboards and switchboard operators connected each call by inserting a pair of phone plugs into the appropriate jacks. Each pair of plugs was part of a cord circuit with a switch associated that let the...

s 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

Key telephone system

" 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

Emergency telephone number

Many countries' public telephone networks have a single emergency telephone number, sometimes known as the universal emergency telephone number or occasionally the emergency services number, that allows a caller to contact local emergency services for assistance. The emergency telephone number may...

) 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 often includes:

The PBX’s internal switching network.
Microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

or microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...

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
Uninterruptible power supply
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...

(UPS) consisting of sensors, power switches and batteries
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

.
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

IP PBX

An IP PBX is a business telephone system designed to deliver voice or video over a data network and interoperate with the normal Public Switched Telephone Network ....

or IPBX, which uses the Internet Protocol

Internet Protocol

The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

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

Open source

The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

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

Telephone switchboard

A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...

or attendant console

Attendant console

An attendant console is a telephone station that is generally part of a private branch exchange or Centrex or other private telephone system...

operated by a telephone operator

Telephone operator

A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...

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

Centrex

Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of 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

SS7

SS-7 can stand for:* Signaling System #7, a set of telephone signaling protocols.* The R-16 missile, with NATO reporting name SS-7 Saddler.* China Railways SS7, an electric locomotive model in China.* Super Socket 7, a chip socket introduced by AMD...

/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 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

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

) telephone lines, typically supporting four private analog and one public analog line. They are the size of a small cigar box

Cigar box

Cigar boxes are popular juggling props. Their rectangular shape resembles that of its namesake; they are sometimes padded on the ends and/or the sides with a felt-like material....

. 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

Basic rate interface

Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...

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

IP PBX

An IP PBX is a business telephone system designed to deliver voice or video over a data network and interoperate with the normal Public Switched Telephone Network ....

have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches.

Open source

Open source

projects have provided PBX-style features since the 1990s. These projects provide extreme flexibility and features, including the means to 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

Call processing

In telecommunication, the term call processing has the following meanings:# The sequence of operations performed by a switching system from the acceptance of an incoming call through the final disposition of the call...

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
Clearing (telecommunications)
Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....

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
Automated attendant
In telephony, an automated attendant allows callers to be automatically transferred to an extension without the intervention of an operator/receptionist). Many AAs will also offer a simple menu system...
Auto dialing
Auto dialer
An auto dialer, autodialer, or autodialler is an electronic device or software that automatically dials telephone numbers. Once the call has been answered, the autodialer either plays a recorded message or connects the call to a live person....
Automatic call distributor
Automatic call distributor
In telephony, an Automatic Call Distributor , also known as Automated Call Distribution, is a device or system that distributes incoming calls to a specific group of terminals that agents use. It is often part of a computer telephony integration system.Routing incoming calls is the task of the ACD...
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
Automatic ring back
Automatic ring back is a service offered by phone companies.Normally, when a person's line is busy, one has to call back every few minutes to check if their line is free yet. With automatic ring back, a code can be dialed into the telephone keypad to enable ringback...
Call accounting
Call accounting
A Call Accounting System is a telecommunications software or hardware application that captures, records, and costs telephone usage events. Internationally call accounting systems may be referred to as call logging systems...
Call blocking
Call Blocking
Call blocking is the ability to prevent incoming telephone calls from a given phone number. Various methods of call blocking include:*Sending caller to voicemail*Sending caller to busy signal*Sending caller to "Number No Longer in Service"...
Call forwarding
Call forwarding
Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...

on busy or absence
Call park
Call pick-up
Call pick-up
Call pick-up is a feature used in a telephone system that allows one to answer someone else’s telephone call. The “call pick-up” feature is accessed by pressing a preprogrammed button , or by pressing a special sequence of buttons on the telephone set.In places where “call pick-up” is used, the...
Call transfer
Call transfer
A call transfer is a telecommunications mechanism that enables a user to relocate an existing call to another phone or attendant console by using the transfer button and dialing the required location. The transferred call is either announced or unannounced....
Call waiting
Call waiting
Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
Camp-on
Conference call
Conference call
A conference call is a telephone call in which the calling party wishes to have more than one called party listen in to the audio portion of the call. The conference calls may be designed to allow the called party to participate during the call, or the call may be set up so that the called party...
Custom greetings
Customised Abbreviated dialing (Speed Dialing)
Busy Override
Direct Inward Dialing
Direct Inward Dialing
Direct inward dialing , also called direct dial-in in Europe and Oceania, is a feature offered by telephone companies for use with their customers' private branch exchange systems...
Direct Inward System Access (DISA) (the ability to access internal features from an outside telephone line)
Do not disturb
Do Not Disturb (telecommunications)
The Do Not Disturb or function on most PBX or PABX systems prevents calls from ringing on an extension for which DND is activated. Some Do Not Disturb attributes include directing the call to a preassigned extension , busy signal, DND signal, or recorded message generated by the telephone switch...

(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 system).
Interactive voice response
Interactive voice response
Interactive voice response is a technology that allows a computer to interact with humans through the use of voice and DTMF keypad inputs....
Music on hold
Music on hold
Music on hold 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...
Night service
Night service
In general, a night service is any service that operates at night, such as a night-time public transport service or a 24-hour telephone support service....
Public address
Public address
A public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...

voice paging
Shared message boxes (where a department can have a shared voicemail box)
Voice mail
Voice message broadcasting
Welcome Message

Interface standards

Interfaces for connecting extensions to a PBX include:

POTS (plain old telephone service
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

) - 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
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

- For example, H.323
H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

and SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

.

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
Primary rate interface
The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....

- Runs over T1, 23 bearer channels + 1 signalling channel
QSIG
QSIG
QSIG is an ISDN based signaling protocol for signaling between private branch exchanges in a Private Integrated Services Network . It makes use of the connection-level Q.931 protocol and the application-level ROSE protocol...

- for connecting PBXs to each other, usually runs over T1 (T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....

) or E1 (E-carrier
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

) physical circuits.
DPNSS - for connecting PBXs to trunk lines. Standardized by British Telecom, this usually runs over E1 (E-carrier) physical circuits.
Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

- H.323
H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

, SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

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
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

) 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
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

- 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
Robbed bit signaling
In communication systems robbed-bit signaling is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using Channel Associated Signaling ....

) - delivers 24 digital circuits over a four-wire (T1) interface.
Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

- H.323
H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

, SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers.

Interfaces for collecting data from the PBX:

Serial
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...

interface - historically used to print every call record to a serial printer. Now an application connects via serial cable to this port.
Network Port
TCP and UDP port
In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication...

(listen mode) - where an external application connects to the TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

or UDP
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...

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

Call detail record

A call detail record , also known as call data record, is a data record produced by a telephone exchange or other telecommunications equipment documenting the details of a phone call that passed through the facility or device...

, CDR

Call detail record

, 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

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

. 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

Application service provider

An application service provider is a business that provides computer-based services to customers over a network. Software offered using an ASP model is also sometimes called On-demand software or software as a service ....

(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.
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.

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

IP PBX

An IP PBX is a business telephone system designed to deliver voice or video over a data network and interoperate with the normal Public Switched Telephone Network ....

handles voice signals under Internet protocol, bringing benefits for computer telephony integration

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 integrated or coordinated...

(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".

External links

SIP Why isn't anyone drinking? - Article by Andrew Taub 2010

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Key systems

Electromechanical shared-control key systems

Electronic shared-control systems

Independent keysets

Hybrid keyphone systems

Private branch exchange

History

System components

Current trends

Home and small business usage

PBX functions

Interface standards

Hosted PBX systems

Mobile PBX

IP-PBX

See also

External links