Business telephone system

A business telephone system is a telephone system typically used in business environments, encompassing the range of 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 for call handling. Business telephone systems are often broadly classified into key telephone systems and private branch exchanges, but many combinations (hybrid telephone 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 a 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

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 for a customer-controlled switching system^[1] such as the line buttons on the phones associated with such systems. The electrical components that allow for the selection of lines and features such as hold and intercom are housed in a panel or cabinet, called the key service unit or key system unit (KSU).^[2]

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.^[3]

Key systems can be built using three principal architectures: electromechanical 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 costs and greater functionality.

Electromechanical shared-control key system

Before the development of large-scale integrated circuits, key systems typically consisted of electromechanical components, such as relays, as were larger telephone switching systems.

The systems marketed in North America as the 1A, 1A1, 1A2 Key System, and the 6A, are typical examples and were sold for many decades. The Western Electric 1A family of key telephone units (KTUs) was introduced in the late 1930s and remained in use until 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 300/400-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 1964 simplified key system setup 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) or ground control approach (GCA) and included radio line connections.

Automatic Electric Company 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 PBXs and key systems quickly merged. One of the most recognized such systems is the AT&T Merlin.

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

Answering machine functions
Automatic call accounting
Caller ID
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 LEDs instead of incandescent light bulbs 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. Such systems were dubbed

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