Бизнес ортасындағы телефон жүйелері

Telephone system typically used in business environments

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 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 costs and greater functionality.

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.

Into the 21st century, the distinction between key systems and PBX systems has become increasingly blurred. 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 handsetsusually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signaling, and of some PBX functionality gives rise to the hybrid designation. A hybrid system typically has some call appearance buttons that directly correspond to individual lines and/or stations, but may also support direct 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 implement VOIP services. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope, and complexity of the features and facilities offered.

A PBX is a telephone exchange or switching system that serves a private organization. A PBX permits the sharing of central office trunks between internally installed telephones, and provides intercommunication between those internal telephones within the organization without the use of external lines. The central office lines provide connections to the public switched telephone network (PSTN) and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization. Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network. This method reduces the number of lines needed from the organization to the public switched telephone network. Each device connected to the PBX, such as a telephone, a fax machine, or a computer modem, is referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of the central office and the telephone number block allocated to the PBX. Initially, PBX systems offered the primary advantage of cost savings for internal phone calls: handling the circuit switching locally reduced charges for telephone service via central office lines. As PBX systems gained popularity, they began to feature services not available in the public network, such as hunt groups, call forwarding, and extension dialing. From the 1960s, a simulated PBX, known as Centrex, provided similar features from the central telephone exchange. A PBX differs from a key telephone system (KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select the outgoing line automatically. The telephone sets connected to a PBX do not normally have special keys for central office line control, but it is not uncommon for key systems to be connected to a PBX to extend its services. A PBX, in contrast to a key system, employs an organizational numbering plan for its stations. In addition, a dial plan determines whether additional digit sequences must be prefixed when dialing to obtain access to a central office trunk. Modern number analysis systems permit users to dial internal and external telephone numbers without special codes to distinguish the intended destination.

The term PBX originated when switchboard operators managed company switchboards manually using cord circuits. As automated electromechanical switches and later electronic switching systems gradually replaced the manual systems, the terms private automatic branch exchange (PABX) and private manual branch exchange (PMBX) differentiated them. Solid state digital systems were sometimes referred to as electronic private automatic branch exchanges (EPABX). , the term PBX is by far the most widely recognized. The abbreviation now applies to all types of complex, in house telephony switching systems. 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 proved 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 voice over IP PBX, or IP PBX. The other trend involved 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 the hosted PBX. In wireline telephony, the original hosted PBX was the Centrex service provided by telcos since the 1960s; later competitive offerings evolved into the modern competitive local exchange carrier. In voice over IP, hosted solutions are easier to implement as the PBX may be located at and managed by any telephone service provider, connecting to the individual extensions via the Internet. The upstream provider no longer needs to run direct, local leased lines to the served premises.

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 were 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) that can be used in parallel. 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. Additionally, many telecommunications providers now offer hosted PBX systems where the provider actually hosts the PBX and the phone handsets are connected to it through an internet connection. Open source projects have provided PBX style features since the 1990s. These projects provide flexibility, features, and programmability.

The ongoing migration of most major telecommunication carriers to IP based networks, coupled with the rise in Cloud Communications has resulted in a significant rise in the uptake of hosted PBX solutions.

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.

An IP PBX handles voice calls over the Internet Protocol (IP), 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".