A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems human operators that interconnect telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time; the term telephone exchange is used synonymously with central office, a Bell System term. A central office is defined as a building used to house the inside plant equipment of several telephone exchanges, each serving a certain geographical area; such an area has been referred to as the exchange. Central office locations may be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying central offices with a three-digit central office code, used as a prefix to subscriber telephone numbers. All central offices within a larger region aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone trunks driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is referred to as a private branch exchange, when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers, stock exchanges few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone. A telephone exchange is a telephone system located at service centers responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations; this made it possible for subscribers to call each other at businesses, or public spaces.
These made telephony an available and comfortable communication tool for everyday use, it gave the impetus for the creation of a whole new industrial sector. As with the invention of the telephone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison; the first experimental telephone exchange was based on the ideas of Puskás, it was built by the Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878; the switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.
In Europe other early telephone exchanges were based in London and Manchester, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange a year later. In 1887 Puskás introduced the multiplex switchboard.. Exchanges consisted of one to several hundred plug boards staffed by switchboard operators; each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve jacks, each of, the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit; when a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair into the called party's local jack and started the ringing cycle. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.
In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatic
Business telephone system
A business telephone system is a multiline telephone system used in business environments, encompassing systems ranging from the small key telephone system to the large private branch exchange. A business telephone system differs from an installation of several telephones with multiple central office lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, that such a system provides additional features related to call handling. Business telephone systems are broadly classified into key telephone systems, private branch exchanges, but many hybrid systems exist. A key telephone system was distinguished from a private branch exchange in that it did not require an operator or attendant at the 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, 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 defined by arrangements with individual line selection buttons for each available telephone line; the earliest systems were known as wiring plans and consisted of telephone sets, keys 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.
Before the advent of large-scale integrated circuits, key systems were composed of electromechanical components as were larger telephone switching systems. 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 key telephone units were introduced in the late 1930s and remained in use to the 1950s. 1A equipment required at least two KTUs per line. The telephone instrument 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, 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 installed as the dial intercom in a 1A1 or 1A2 key system.
The 6A systems were complex and expensive, never became popular. The advent of 1A2 technology in the 1964 simplified key system 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 or ground control approach, included radio line connections. Automatic Electric Company 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. 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 that used by non-key systems.
Electronic shared-control systems led to the modern hybrid telephone system, as the features of PBX and key system 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 Features could be added or modified 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 allowed smaller systems to distribute the control into individual telephone sets that don't require any single shared control unit; these systems are used with a few telephone sets and it is more difficult to keep the feature set in synchrony between the various sets. Into the 21st century, the distinction between key systems and PBX systems has become 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 it
Integrated Services Digital Network
Integrated Services Digital Network is a set of communication standards for simultaneous digital transmission of voice, video and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data; the key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface, Primary Rate Interface, Narrowband ISDN, Broadband ISDN. ISDN is a circuit-switched telephone network system, which provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone can provide, it offers circuit-switched connections, packet-switched connections, in increments of 64 kilobit/s.
In some countries, ISDN found major market application for Internet access, in which ISDN provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Channel bonding can achieve a greater data rate. ISDN is employed as data-link and physical layers in the context of the OSI model. In common use, ISDN is limited to usage to Q.931 and related protocols, which are a set of signaling protocols establishing and breaking circuit-switched connections, for advanced calling features for the user. They were introduced in 1986. In a videoconference, ISDN provides simultaneous voice and text transmission between individual desktop videoconferencing systems and group videoconferencing systems. Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice and fax, over a single line. Multiple devices can be attached to the line, used as needed; that means an ISDN line can take care of what were expected to be most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
It refers to integrated switching and transmission in that telephone switching and carrier wave transmission are integrated rather than separate as in earlier technology. The entry level interface to ISDN is the Basic Rate Interface, a 128 kbit/s service delivered over a pair of standard telephone copper wires; the 144 kbit/s overall payload rate is divided into two 64 kbit/s bearer channels and one 16 kbit/s signaling channel. This is sometimes referred to as 2B+D; the interface specifies the following network interfaces: The U interface is a two-wire interface between the exchange and a network terminating unit, the demarcation point in non-North American networks. The T interface is a serial interface between a computing device and a terminal adapter, the digital equivalent of a modem; the S interface is a four-wire bus. The R interface defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device. BRI-ISDN is popular in Europe but is much less common in North America.
It is common in Japan — where it is known as INS64. The other ISDN access available is the Primary Rate Interface, carried over T-carrier with 24 time slots in North America, over E-carrier with 32 channels in most other countries; each channel provides transmission at a 64 kbit/s data rate. With the E1 carrier, the available channels are divided into 30 bearer channels, one data channel, one timing and alarm channel; this scheme is referred to as 30B+2D. In North America, PRI service is delivered via T1 carriers with only one data channel referred to as 23B+D, a total data rate of 1544 kbit/s. Non-Facility Associated Signalling allows two or more PRI circuits to be controlled by a single D channel, sometimes called 23B+D + n*24B. D-channel backup allows for a second D channel in case the primary fails. NFAS is used on a Digital Signal 3. PRI-ISDN is popular throughout the world for connecting private branch exchanges to the public switched telephone network. Though many network professionals use the term ISDN to refer to the lower-bandwidth BRI circuit, in North America BRI is uncommon whilst PRI circuits serving PBXs are commonplace.
The bearer channel is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-channels can be used to carry data, since they are nothing more than digital channels; each one of these channels is known as a DS0. Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines; this has since become less so. X.25 can be carried over the B or D channels of a BRI line, over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale terminals because it eliminates the modem setup, because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines. X.25 was part of an ISDN protocol
International Organization for Standardization
The International Organization for Standardization is an international standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization promotes worldwide proprietary and commercial standards, it is headquartered in Geneva and works in 164 countries. It was one of the first organizations granted general consultative status with the United Nations Economic and Social Council; the International Organization for Standardization is an independent, non-governmental organization, the members of which are the standards organizations of the 164 member countries. It is the world's largest developer of voluntary international standards and facilitates world trade by providing common standards between nations. Over twenty thousand standards have been set covering everything from manufactured products and technology to food safety and healthcare. Use of the standards aids in the creation of products and services that are safe, reliable and of good quality.
The standards help businesses increase productivity while minimizing errors and waste. By enabling products from different markets to be directly compared, they facilitate companies in entering new markets and assist in the development of global trade on a fair basis; the standards serve to safeguard consumers and the end-users of products and services, ensuring that certified products conform to the minimum standards set internationally. The three official languages of the ISO are English and Russian; the name of the organization in French is Organisation internationale de normalisation, in Russian, Международная организация по стандартизации. ISO is not an acronym; the organization adopted ISO as its abbreviated name in reference to the Greek word isos, as its name in the three official languages would have different acronyms. During the founding meetings of the new organization, the Greek word explanation was not invoked, so this meaning may have been made public later. ISO gives this explanation of the name: "Because'International Organization for Standardization' would have different acronyms in different languages, our founders decided to give it the short form ISO.
ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, the short form of our name is always ISO."Both the name ISO and the ISO logo are registered trademarks, their use is restricted. The organization today known as ISO began in 1928 as the International Federation of the National Standardizing Associations, it was suspended in 1942 during World War II, but after the war ISA was approached by the formed United Nations Standards Coordinating Committee with a proposal to form a new global standards body. In October 1946, ISA and UNSCC delegates from 25 countries met in London and agreed to join forces to create the new International Organization for Standardization. ISO is a voluntary organization whose members are recognized authorities on standards, each one representing one country. Members meet annually at a General Assembly to discuss ISO's strategic objectives; the organization is coordinated by a Central Secretariat based in Geneva. A Council with a rotating membership of 20 member bodies provides guidance and governance, including setting the Central Secretariat's annual budget.
The Technical Management Board is responsible for over 250 technical committees, who develop the ISO standards. ISO has formed two joint committees with the International Electrotechnical Commission to develop standards and terminology in the areas of electrical and electronic related technologies. ISO/IEC Joint Technical Committee 1 was created in 1987 to "evelop, maintain and facilitate IT standards", where IT refers to information technology. ISO/IEC Joint Technical Committee 2 was created in 2009 for the purpose of "tandardization in the field of energy efficiency and renewable energy sources". ISO has 163 national members. ISO has three membership categories: Member bodies are national bodies considered the most representative standards body in each country; these are the only members of ISO. Correspondent members are countries; these members do not participate in standards promulgation. Subscriber members are countries with small economies, they can follow the development of standards. Participating members are called "P" members, as opposed to observing members, who are called "O" members.
ISO is funded by a combination of: Organizations that manage the specific projects or loan experts to participate in the technical work. Subscriptions from member bodies; these subscriptions are in proportion to each country's gross national trade figures. Sale of standards. ISO's main products are international standards. ISO publishes technical reports, technical specifications, publicly available specifications, technical corrigenda, guides. International standards These are designated using the format ISO nnnnn: Title, where nnnnn is the number of the standard, p is an optional part number, yyyy is the year published, Title describes the subject. IEC for International Electrotechnical Commission is included if the standard results from the work of ISO/IEC JTC1. ASTM is used for standards developed in cooperation with ASTM International. Yyyy and IS are not used for an incomplete or unpublished standard and may under some