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 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 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
The systems marketed in North America 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 (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.
Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.
Two lesser-known key systems were used at airports for air traffic control communications, the 102 and 302 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.
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 PBX and key system quickly merged. One of the most recognized such systems is the AT&T Merlin.
Features could be added or modified simply using software, allowing easy customization of these systems.
- Additionally, these more modern systems allowed a diverse set of features including:
- Answering machine functions
- Automatic call accounting
- Call Forward
- Call Transfer
- 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.
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.
Key Telephone System
4/
5
Oleh
Haswell