In telecommunications, transmission is the process of sending and propagating an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless. One example of transmission is the sending of a signal with limited duration, for example a block or packet of data, a phone call, or an email. Transmission technologies and schemes refer to physical layer protocol duties such as modulation, line coding, error control, bit synchronization and multiplexing, but the term may involve higher-layer protocol duties, for example, digitizing an analog message signal, data compression. Transmission of a digital message, or of a digitized analog signal, is known as digital communication
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
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
An electronic circuit is composed of individual electronic components, such as resistors, capacitors and diodes, connected by conductive wires or traces through which electric current can flow. To be referred to as electronic, rather than electrical at least one active component must be present; the combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, data can be moved from one place to another. Circuits can be constructed of discrete components connected by individual pieces of wire, but today it is much more common to create interconnections by photolithographic techniques on a laminated substrate and solder the components to these interconnections to create a finished circuit. In an integrated circuit or IC, the components and interconnections are formed on the same substrate a semiconductor such as silicon or gallium arsenide. An electronic circuit can be categorized as an analog circuit, a digital circuit, or a mixed-signal circuit.
Breadboards and stripboards are common for testing new designs. They allow the designer to make quick changes to the circuit during development. Analog electronic circuits are those in which current or voltage may vary continuously with time to correspond to the information being represented. Analog circuitry is constructed from two fundamental building blocks: parallel circuits. In a series circuit, the same current passes through a series of components. A string of Christmas lights is a good example of a series circuit: if one goes out, they all do. In a parallel circuit, all the components are connected to the same voltage, the current divides between the various components according to their resistance; the basic components of analog circuits are wires, capacitors, inductors and transistors. Analog circuits are commonly represented in schematic diagrams, in which wires are shown as lines, each component has a unique symbol. Analog circuit analysis employs Kirchhoff's circuit laws: all the currents at a node, the voltage around a closed loop of wires is 0.
Wires are treated as ideal zero-voltage interconnections. Active components such as transistors are treated as controlled current or voltage sources: for example, a field-effect transistor can be modeled as a current source from the source to the drain, with the current controlled by the gate-source voltage. An alternative model is to take independent power sources and induction as basic electronic units; when the circuit size is comparable to a wavelength of the relevant signal frequency, a more sophisticated approach must be used, the distributed element model. Wires are treated as transmission lines, with nominally constant characteristic impedance, the impedances at the start and end determine transmitted and reflected waves on the line. Circuits designed according to this approach are distributed element circuits; such considerations become important for circuit boards at frequencies above a GHz. In digital electronic circuits, electric signals take on discrete values, to represent logical and numeric values.
These values represent the information, being processed. In the vast majority of cases, binary encoding is used: one voltage represents a binary'1' and another voltage represents a binary'0'. Digital circuits make extensive use of transistors, interconnected to create logic gates that provide the functions of Boolean logic: AND, NAND, OR, NOR, XOR and all possible combinations thereof. Transistors interconnected so as to provide positive feedback are used as latches and flip flops, circuits that have two or more metastable states, remain in one of these states until changed by an external input. Digital circuits therefore can provide both logic and memory, enabling them to perform arbitrary computational functions; the design process for digital circuits is fundamentally different from the process for analog circuits. Each logic gate regenerates the binary signal, so the designer need not account for distortion, gain control, offset voltages, other concerns faced in an analog design; as a consequence complex digital circuits, with billions of logic elements integrated on a single silicon chip, can be fabricated at low cost.
Such digital integrated circuits are ubiquitous in modern electronic devices, such as calculators, mobile phone handsets, computers. As digital circuits become more complex, issues of time delay, logic races, power dissipation, non-ideal switching, on-chip and inter-chip loading, leakage currents, become limitations to the density and performance. Digital circuitry is used to create general purpose computing chips, such as microprocessors, custom-designed logic circuits, known as application-specific integrated circuit. Field-programmable gate arrays, chips with logic circuitry
A telephone hook or switchhook is an electrical switch which indicates when the phone is hung up with a lever or magnetic button inside the cradle or base where a telephone handset resides. It takes its name from old wooden wall telephones and candlestick telephones, where the mouthpiece was mounted on the telephone box and, due to sidetone considerations, the receiver was separate, on a cable; when the telephone was not in use, the receiver was hung on a spring-loaded hook. When the handset is on the cradle, the telephone is said to be ready for a call; when the handset is off the cradle, the telephone is said to be "off-hook", or unable to receive any calls. Pushing the switchhook is termed a "hook flash". "I tried calling you all day, but your phone must have been off the hook, because all I got was a busy signal." A phone receiving many calls in rapid succession can be said to be "ringing off the hook". This most led to the present use of "off the hook" as a synonym for "crazy" or "exciting".
"Last night was off-the-hook" may mean "last night was awesome". "Off the hook", meaning "freed from responsibility or culpability" is not related to the telephone, but most derives from the image of a worm or fish dislodging itself from a fish hook. Hook flash Off the Hook: radio show Permanent signal
Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. The term complex impedance may be used interchangeably. Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of a sinusoidal voltage between its terminals to the complex representation of the current flowing through it. In general, it depends upon the frequency of the sinusoidal voltage. Impedance extends the concept of resistance to AC circuits, possesses both magnitude and phase, unlike resistance, which has only magnitude; when a circuit is driven with direct current, there is no distinction between impedance and resistance. The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law. In multiple port networks, the two-terminal definition of impedance is inadequate, but the complex voltages at the ports and the currents flowing through them are still linearly related by the impedance matrix.
Impedance is a complex number, with the same units as resistance. Its symbol is Z, it may be represented by writing its magnitude and phase in the form |Z|∠θ. However, cartesian complex number representation is more powerful for circuit analysis purposes; the reciprocal of impedance is admittance, whose SI unit is the siemens called mho. Instruments used to measure the electrical impedance are called impedance analyzers; the term impedance was coined by Oliver Heaviside in July 1886. Arthur Kennelly was the first to represent impedance with complex numbers in 1893. In addition to resistance as seen in DC circuits, impedance in AC circuits includes the effects of the induction of voltages in conductors by the magnetic fields, the electrostatic storage of charge induced by voltages between conductors; the impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part. Impedance is defined as the frequency domain ratio of the voltage to the current.
In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency ω. For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular: The magnitude of the complex impedance is the ratio of the voltage amplitude to the current amplitude; the impedance of a two-terminal circuit element is represented as a complex quantity Z. The polar form conveniently captures both magnitude and phase characteristics as Z = | Z | e j arg where the magnitude | Z | represents the ratio of the voltage difference amplitude to the current amplitude, while the argument arg gives the phase difference between voltage and current. J is the imaginary unit, is used instead of i in this context to avoid confusion with the symbol for electric current. In Cartesian form, impedance is defined as Z = R + j X where the real part of impedance is the resistance R and the imaginary part is the reactance X.
Where it is needed to add or subtract impedances, the cartesian form is more convenient. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers. To simplify calculations, sinusoidal voltage and current waves are represented as complex-valued functions of time denoted as V and I. V = | V | e j; the impedance of a bipolar circuit is defined as the ratio of these quantities: Z = V I = | V | | I | e j ( ϕ V − ϕ I
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