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Early model 464G Western Electric key telephone set

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. 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^[1] 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 [link]

Key was a Bell System term of 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 [link]

A typical rotary dial key telephone: the Western Electric eighteen button Call Director, manufactured from 1958 to the early 80s.

Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.

The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System^[2][3] 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. 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) or ground control approach (GCA), and included radio line connections.

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

Electronic shared-control systems [link]

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.

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

• Answering machine functions
• Remote supervision of the entire system
• Automatic call accounting
• Speed dialing
• Caller ID
• 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 LEDs instead of incandescent light bulbs for line status indication.

Independent keysets [link]

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 [link]

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, 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.^[4]

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 and are notable for their presentation of hoot-n-holler circuits. Multiple Hoots are presented to multiple users over multiplexed speakers to multiple locations.

Private branch exchange [link]

This unreferenced section requires citations to ensure verifiability.

An Avaya G3si PBX with front cover removed (view from the top).

A private branch exchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. PBXs are also referred to as:

• PABX – private automatic branch exchange
• EPABX – electronic private automatic branch exchange

PBXs make connections among the internal telephones of a private organization—usually a business—and also connect them to the public switched telephone network (PSTN) via trunk lines. Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch.

PBXs are differentiated from "key systems" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Hybrid systems combine features of both.

Initially, the primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone service. As PBXs gained popularity, they started offering services that were not available in the operator network, such as hunt groups, call forwarding, and extension dialing. In the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.

Two significant developments during the 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the VoIP PBX. (Technically, nothing was being "exchanged" any more, but the abbreviation PBX was so widely understood that it remained in use.)

The other trend was the idea of focusing on core competence. PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of hosted PBX. In a hosted setup, the PBX is located at and managed by the telephone service provider, and features and calls are delivered via the Internet. The customer just signs up for a service, rather than buying and maintaining expensive hardware. This essentially removes the branch from the private premises, moving it to a central location.

History [link]

The term PBX was first applied when switchboard operators ran company switchboards by hand. As automated electromechanical and then electronic switching systems gradually began to replace the manual systems, the terms PABX (private automatic branch exchange) and PMBX (private manual branch exchange) were used to differentiate them. Solid state digital systems were sometimes referred to as EPABXs (electronic private automatic branch exchange). Now, the term PBX is by far the most widely recognized. The acronym is now applied to all types of complex, in-house telephony switching systems, even if they are not private, branches, or exchanging anything.

PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated or selectable at an individual extension. From a user's point of view, calls on a key system are made by selecting a specific outgoing line and dialing the external number. A PBX, in contrast, has a dial plan. Users dial an escape code (usually a single digit; often the same as the first digit of the local emergency telephone number) that connects them to an outside line (DDCO or Direct Dial Central Office in Bell System jargon), followed by the external number. Some modern number analysis systems allow users to dial internal and external numbers without escape codes by use of a dialplan which specifies how calls to numbers beginning with certain prefixes should be routed.

System components [link]

A PBX often includes:

• The PBX’s internal switching network.
• Microcontroller or microcomputer for arbitrary data processing, control and logic.
• Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
• Stations or telephone sets, sometimes called lines.
• Outside telco trunks that deliver signals to (and carry them from) the PBX.
• Console or switchboard allows the operator to control incoming calls.
• Uninterruptible power supply (UPS) consisting of sensors, power switches and batteries.
• Interconnecting wiring.
• Cabinets, closets, vaults and other housings.

Current trends [link]

One of the latest trends in PBX development is the VoIP PBX, also known as an IP-PBX or IPBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, and programmable caller ID. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.

For some users, the private branch exchange has gone full circle as a term. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, they have evolved into VoIP centres that are hosted by the operators or even hardware manufacturers. These modern IP Centrex systems offer essentially the same service, but they have moved so far from the original concept of the PBX that the term hardly applies at all.

Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrexes. Currently, there are four distinct scenarios in use:

• PBX (Private and Circuit Switched)
• Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
• IP PBX (Private and Packet Switched)
• IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)

Since in reality people want to call from the IP side to the circuit switched PSTN (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.

Home and small business usage [link]

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 are the size of a small cigar box. In Europe these systems for analog phones were followed by consumer-grade PBXs for ISDN. Using small PBXs for ISDN is a logical step, since the ISDN basic rate interface 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 for LittlePhoneBox Server ^[5] ). With the adoption of VoIP by consumers, consumer VoIP PBXs have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches.

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

PBX functions [link]

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 features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include (manufacturers may have a different name for each capability):

• Auto attendant
• Auto dialing
• Automatic call distributor
• Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
• Automatic ring back
• Call accounting
• Call blocking
• Call forwarding on busy or absence
• Call park
• Call pick-up
• Call transfer
• Call waiting
• Camp-on
• Conference call
• Custom greetings
• Customised Abbreviated dialing (Speed Dialing)
• Busy Override
• Direct Inward Dialing
• Direct Inward System Access (DISA) (the ability to access internal features from an outside telephone line)
• Do not disturb (DND)
• Follow-me, 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
• Music on hold
• Night service
• Public address voice paging
• Shared message boxes (where a department can have a shared voicemail box)
• Voice mail
• Voice message broadcasting
• Welcome Message

Interface standards [link]

Interfaces for connecting extensions to a PBX include:

• POTS (plain old telephone service) - the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension.
• proprietary - the manufacturer has defined a protocol. One can only connect the manufacturer's sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons.
• DECT - a standard for connecting cordless phones.
• Internet Protocol - For example, H.323 and SIP.

Interfaces for connecting PBXs to each other include:

• proprietary protocols - if equipment from several manufacturers is on site, the use of a standard protocol is required.
• ISDN PRI - Runs over T1, 23 bearer channels + 1 signalling channel
• QSIG - for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits.
• DPNSS - for connecting PBXs to trunk lines. Standardized by British Telecom, this usually runs over E1 (E-carrier) physical circuits.
• Internet Protocol - H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.

Interfaces for connecting PBXs to trunk lines include:

• standard POTS (plain old telephone service) lines - the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call.
• ISDN - the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections.
• RBS (robbed bit signaling) - delivers 24 digital circuits over a four-wire (T1) interface.
• Internet Protocol - H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers.

Interfaces for collecting data from the PBX:

• Serial interface - historically used to print every call record to a serial printer. Now an application connects via serial cable to this port.
• Network Port (listen mode) - where an external application connects to the TCP or UDP port. The PBX then starts streaming information down to the application.
• Network port (server mode) - the PBX connects to another application or buffer.
• File - the PBX generates a file containing the call records from the PBX.

The call records from the PBX are called SMDR, CDR, or CIL. It is possible to use a Voice modem as FXO card.

Hosted PBX systems [link]

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

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

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

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

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 [link]

An IP PBX handles voice signals under Internet protocol, bringing benefits for computer telephony integration (CTI). An IP-PBX can exist as physical hardware, or can carry out its functions virtually, performing the call-routing activities of the traditional PBX or key system as a software system. The virtual version is also called a " -Soft PBX -".

See also [link]

• List of SIP software – SIP related programs
• Circuit ID
• Cloud communications
• Ground start trunk
• Switchboard operator
• Telephone exchange
• Telephone switchboard
• Centrex
• RJ21

References [link]

External links [link]

• Telecommunications Equipment at the Open Directory Project
• SIP Why isn't anyone drinking? - Article by Andrew Taub 2010

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