A KSU-less telephone system is a type of electronic

key 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 ...

that puts the switching circuitry inside of each individual phone. This eliminates the need for a key service unit (KSU), which in traditional key systems is a centralized cabinet that contains the switching circuitry, and allows phones to be wired together in series while retaining certain advanced key system features such as

intercom An intercom, also called an intercommunication device, intercommunicator, or interphone, is a stand-alone voice communications system for use within a building, small collection of buildings or portably within a small coverage area, which funct ...

paging In computer operating systems, memory paging is a memory management scheme that allows the physical Computer memory, memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compact ...

, and hold. Unlike traditional key systems, however, KSU-less systems support a limited number of

extensions Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (proof theory) * Extension (predicate logic), the set of tuples of values t ...

and

trunk line In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each clie ...

s and are typically not very scalable. The first KSU-less systems were introduced in the 1970s by the

AT&T Corporation AT&T Corporation, an abbreviation for its former name, the American Telephone and Telegraph Company, was an American telecommunications company that provided voice, video, data, and Internet telecommunications and professional services to busi ...

History

In traditional key telephone systems, the key service unit (KSU) is a centralized cabinet containing the electronics that handle all of the switching of calls, with each phone connecting to the KSU with its own cable in a

star topology A star network is an implementation of a spoke–hub distribution paradigm in computer networks. In a star network, every host is connected to a central hub. In its simplest form, one central hub acts as a conduit to transmit messages. The ...

. In KSU-less systems, the electronic circuitry is located inside each phone in the system, and the phones are wired in series, as is typical in standard in-wall installations. The advent of large-scale

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s in the mid-1970s made the incorporation of such switching circuitry inside of the phone itself possible, and the first KSU-less system was introduced in 1975 with the Com Key 416 by the

. The Com Key 416 differed from more modern KSU-less systems, in that the switching circuitry was located only within one phone, designated the ''master''. One master phone supported two phone lines and eight extensions; two masters can be hooked together to support four phone lines and sixteen extensions. As with contemporaneous key system units, the switching circuitry inside the Com Key 416 still contained

electromechanical Electromechanics combine processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focus on the interaction of electrical and mechanical systems as a whole and how the two systems interact with each ...

components. With the progression of VLSI technology, newer KSU-less systems were designed with the ICs handling the switching located inside each phone in the system. This eliminated the need for a master phone and increased the reliability of the system.

Wiring

Because of the in-series wiring scheme of most KSU-less systems, installation is typically easier than systems using a key service unit, and standard in-wall installations may be used. Unlike KSU systems or standard POTS lines, KSU-less systems typically require an external power supply unit for each extension. Some KSU-less units sacrifice a trunk to transmit power to each extension on the line, eliminating the need for external PSUs while reducing the number of connections to central offices. Other systems still have a recharable battery backup allowing phone service to continue in case mains power is lost. Because of their limited amount of trunk connections and extensions, KSU-less systems are not as scalable and are mainly marketed at the

small office/home office Small office/home office (or single office/home office; sometimes short SOHO) refers to the category of business or cottage industry that involves from 1 to 1000 workers. In New Zealand, the Ministry of Business, Innovation and Employment (MBI ...

segment. A basic KSU-less system in the 1990s supported up to three lines and eight extensions, although some more advanced units supported up to four lines and sixteen extensions, and by the early 2000s units were available supporting up to 24 extensions. More advanced KSU-less units may also support a "non-square" topology, in which a private trunk line that cannot be accessed from all extensions can be wired up to only one phone.

Features

The advanced features of KSU-less systems, such as

, hold, and transfer, are able to function on the same wire as normal phone service because the switching circuitry moves the inter-office voice path, as well as the signaling necessary to establish such paths, to the

high-frequency High frequency (HF) is the International Telecommunication Union, ITU designation for the radio band, band of radio waves with frequency between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengt ...

RF band. Concurrent intercoms are typically not possible on basic KSU-less systems; some advanced systems support up to two simultaneous intercoms. KSU-less systems before the late 1990s typically shared the

voicemail A voicemail system (also known as voice message or voice bank) is a computer-based system that allows callers to leave a recorded message when the recipient has been unable (or unwilling) to answer the phone. Calls may be directed to voicemail m ...

between all phones, presenting a potential privacy issue. With the advent of affordable flash storage in the turn of the millennium, KSU-less systems began supporting per-extension voicemail, as well as auto attendants, which presents outside callers with a prerecorded voice menu to dial specific extensions. Due to incompatible signaling schemes, different vendor's KSU-less phones typically cannot be mixed together in the same system.

References

{{reflist, colwidth=30em Telephone exchange equipment Computer telephony integration Telephony equipment