Key telephone system
From Wikipedia, the free encyclopedia
A key system or key telephone system is a multiline telephone system typically used in small office environments.
Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.
Key systems can be built using three principal architectures:
- Electromechanical shared-control
- Electronic shared-control
- Independent keysets
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[edit] Electromechanical shared-control key systems
Before the advent of large-scale integrated circuits, key systems were typically built out of the same electromechanical components (relays) as larger telephone switching systems. The system marketed in North America as 1A2 was entirely typical and sold for many decades. This system consisted of a central control unit and a number of specialized telephone sets. Each line to the telephone sets was routed using six wires:
- Two wires (one pair) carried the actual telephone line
- Two wires (a second pair) carried control information for that line
- Two wires (a third pair) carried current to a lamp installed at the telephone
A telephone set could contain five, 12, or many individual telephone lines. A common five-line keyphone would be connected using 25-pair cable and an Amphenol 50-position "MicroRibbon" connector. The lamps installed at the telephone sets allowed the user to instantly determine the status of all of the individual telephone lines that "appeared" at that set:
- Lamp off — The line is idle
- Lamp steady on — The line is in use for a call
- Lamp flashing slowly — The line is ringing with an incoming call
- Lamp winking fast — A call on the line is "on hold"
A user could select any of the lines simply by pressing the appropriate line button and picking up the handset. A caller could place a call "on hold" by pressing the red "hold" button. This would place the call on hold and then mechanically release the depressed line button, allowing the user to select another line.
An individual worker or executive might have a set with one or a few lines "appearing". The system attendant (receptionist) might have a set with many lines appearing so that they could monitor the status of all incoming lines simultaneously.
These systems also supported manual buzzers, intercom lines (with or without selective ringing), music on hold, and other simple features. The features were provided on a line-by-line basis by the selection of particular Key Telephone Units (KTUs) plugged into a pre-wired backplane in the central control unit. The central control unit also provided power for the entire key system (including ringing voltage). A mechanical interrupter in the power supply provided the pulsing voltages for the various lamps, buzzers, and ringers in the system.
Compatible 1A2 equipment was manufactured by a number of vendors including Western Electric, Northern Telecom, and Automatic Electric (GTE).
[edit] 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.
Additionally, these more-modern systems allowed vastly more features including:
- Interactive voice response systems
- Answering machine functions
- Remote supervision of the entire system
- Automatic call accounting
- Speed dialing
- Caller ID
- Etc.
Features could be added or modified simply using software, allowing easy customization of these systems.
[edit] 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.
[edit] PBX systems
The line between the largest key systems and full PBX systems is blurred. In the 1A2 days, the line was clear: 1A2 systems did not allow the sharing of anonymous "trunk" lines and PBX systems did. Modern key systems blur this distinction by often allowing this feature.
[edit] 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 ISDN, analogue handsets (in addition to its own dedicated handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. The fact that they support both analogue and digital signalling types gives rise to the "Hybrid" designation.
The modern key system is usually fully digital (although analogue variants still persist) and with the advent of VOIP, is beginning to embrace this new technology. Indeed, key systems now can be considered to have left their humble roots and are nothing less than a small PBX. Effectively, the only aspects that separate a PBX from a key system is the amount, scope and complexity of the features and facilities offered.
