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
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
A telephone switchboard is a telecommunications system used in the public switched telephone network or in enterprises to interconnect circuits of telephones to establish telephone calls between the subscribers or users, or between other exchanges. The switchboard was an essential component of a manual telephone exchange, was operated by switchboard operators who used electrical cords or switches to establish the connections; the electromechanical automatic telephone exchange, invented by Almon Strowger in 1888 replaced manual switchboards in central telephone exchanges around the world. In 1919, the Bell System in Canada adopted automatic switching as its future technology, after years of reliance on manual systems. Many manual branch exchanges remained operational into the second half of the 20th century in many enterprises. Electronic devices and computer technology gave the operator access to an abundance of features. A private branch exchange in a business has an attendant console for the operator, or an auto-attendant, which bypasses the operator entirely.
Following the invention of the telephone in 1876, the first telephones were rented in pairs which were limited to conversation between the parties operating those two instruments. The use of a central exchange was soon found to be more advantageous than in telegraphy. In January 1878 the Boston Telephone Dispatch company had started hiring boys as telephone operators. Boys had been successful as telegraphy operators, but their attitude, lack of patience, behavior was unacceptable for live telephone contact, so the company began hiring women operators instead. Thus, on September 1, 1878, Boston Telephone Dispatch hired Emma Nutt as the first woman operator. Small towns had the switchboard installed in the operator's home so that he or she could answer calls on a 24-hour basis. In 1894, New England Telephone and Telegraph Company installed the first battery-operated switchboard on January 9 in Lexington, Massachusetts. Early switchboards in large cities were mounted floor to ceiling in order to allow the operators to reach all the lines in the exchange.
The operators were boys. Late in the 1890s this measure failed to keep up with the increasing number of lines, Milo G. Kellogg devised the Divided Multiple Switchboard for operators to work together, with a team on the "A board" and another on the "B"; these operators were always women until the early 1970s, when men were once again hired. Cord switchboards were referred to as "cordboards" by telephone company personnel. Conversion to Panel switch and other automated switching systems first eliminated the "B" operator and usually years the "A". Rural and suburban switchboards for the most part remained simple. In many cases, customers came to know their operator by name; as telephone exchanges converted to automatic service, switchboards continued to serve specialized purposes. Before the advent of direct-dialed long distance calls, a subscriber would need to contact the long-distance operator in order to place a toll call. In large cities, there was a special number, such as 112, which would ring the long-distance operator directly.
Elsewhere, the subscriber would ask the local operator to ring the long-distance operator. The long distance operator would record the name and city of the person to be called, the operator would advise the calling party to hang up and wait for the call to be completed; each toll center had only a limited number of trunks to distant cities, if those circuits were busy, the operator would try alternate routings through intermediate cities. The operator would plug into a trunk for the destination city, the inward operator would answer; the inward operator would obtain the number from the local information operator, ring the call. Once the called party answered, the originating operator would advise him or her to stand by for the calling party, whom she'd ring back, record the starting time, once the conversation began. In the 1940s, with the advent of dial pulse and multi-frequency operator dialing, the operator would plug into a tandem trunk and dial the NPA and operator code for the information operator in the distant city.
For instance, the New York City information operator was 212-131. If the customer knew the number, the point was direct-dialable, the operator would dial the call. If the distant city did not have dialable numbers, the operator would dial the code for the inward operator serving the called party, ask her to ring the number. In the 1960s, once most phone subscribers had direct long-distance dialing, a single type of operator began to serve both the local and long distance functions. A customer might call to request a collect call, a call billed to a third number, or a person-to-person call. All toll calls from coin phones required operator assistance; the operator was available to help complete a local or long-distance number which did not complete. For example, if a customer encountered a reorder tone, it could indicate "all circuits busy," or a problem in the destination exchange; the operator might be able to use a different routing to complete the call. If the operator could not get through by dialing the number, she could call the inward operator in the destination city, ask her to try the number, or to test a line to see if it was busy or out of order.
Cord switchboards used for these purposes were replaced in the 1970s and 1980s by TSPS and similar systems, which reduced operator involvement in calls. The customer would, instead of dialing "0" for the operator, dial 0+NPA+7digits, after which an operator would answer and provide th
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
Telephony is the field of technology involving the development and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the development of the telephone. Telephony is referred to as the construction or operation of telephones and telephonic systems and as a system of telecommunications in which telephonic equipment is employed in the transmission of speech or other sound between points, with or without the use of wires; the term is used to refer to computer hardware and computer network systems, that perform functions traditionally performed by telephone equipment. In this context the technology is referred to as Internet telephony, or voice over Internet Protocol; the first telephones were connected directly in pairs. Each user had a separate telephone wired to the locations he or she might wish to reach; this became inconvenient and unmanageable when people wanted to communicate with more than a few people.
The inventions of the telephone exchange provided the solution for establishing telephone connections with any other telephone in service in the local area. Each telephone was connected to the exchange via the local loop. Nearby exchanges in other service areas were connected with trunk lines and long distance service could be established by relaying the calls through multiple exchanges. Switchboards were manually operated by an attendant referred to as the "switchboard operator"; when a customer cranked a handle on the telephone, it turned on an indicator on the board in front of the operator, who would plug the operator headset into that jack and offer service. The caller had to ask for the called party by name by number, the operator connected one end of a circuit into the called party jack to alert them. If the called station answered, the operator disconnected their headset and completed the station-to-station circuit. Trunk calls were made with the assistance of other operators at other exchangers in the network.
In modern times, most telephones are plugged into telephone jacks. The jacks are connected by inside wiring to a drop wire. Cables bring a large number of drop wires from all over a district access network to one wire center or telephone exchange; when a telephone user wants to make a telephone call, equipment at the exchange examines the dialed telephone number and connects that telephone line to another in the same wire center, or to a trunk to a distant exchange. Most of the exchanges in the world are interconnected through a system of larger switching systems, forming the public switched telephone network. After the middle of the 20th century and data became important secondary users of the network created to carry voices, late in the century, parts of the network were upgraded with ISDN and DSL to improve handling of such traffic. Today, telephony uses digital technology in the provisioning of telephone systems. Telephone calls can be provided digitally, but may be restricted to cases in which the last mile is digital, or where the conversion between digital and analog signals takes place inside the telephone.
This advancement has reduced costs in communication, improved the quality of voice services. The first implementation of this, ISDN, permitted all data transport from end-to-end speedily over telephone lines; this service was made much less important due to the ability to provide digital services based on the IP protocol. Since the advent of personal computer technology in the 1980s, computer telephony integration has progressively provided more sophisticated telephony services and controlled by the computer, such as making and receiving voice and data calls with telephone directory services and caller identification; the integration of telephony software and computer systems is a major development in the evolution of office automation. The term is used in describing the computerized services of call centers, such as those that direct your phone call to the right department at a business you're calling. It's sometimes used for the ability to use your personal computer to initiate and manage phone calls.
CTI is not a new concept and has been used in the past in large telephone networks, but only dedicated call centers could justify the costs of the required equipment installation. Primary telephone service providers are offering information services such as automatic number identification, a telephone service architecture that separates CTI services from call switching and will make it easier to add new services. Dialed Number Identification Service on a scale is wide enough for its implementation to bring real value to business or residential telephone usage. A new generation of applications is being developed as a result of standardization and availability of low cost computer telephony links. Starting with the introduction of the transistor, invented in 1947 by Bell Laboratories, to amplification and switching circuits in the 1950s, through development of computer-based electronic switching systems, the public switched telephone network has evolved towards automation and digitization of signaling and audio transmissions.
Digital telephony is the use of digital electronics in the operation and provisioning of telephony systems and services. Since the 1960s a digital core network has replaced the traditional analog transmission and signaling systems, much of the access network has been digitized. Digital
A computer program is a collection of instructions that performs a specific task when executed by a computer. A computer requires programs to function. A computer program is written by a computer programmer in a programming language. From the program in its human-readable form of source code, a compiler can derive machine code—a form consisting of instructions that the computer can directly execute. Alternatively, a computer program may be executed with the aid of an interpreter. A collection of computer programs and related data are referred to as software. Computer programs may be categorized along functional lines, such as application software and system software; the underlying method used for some calculation or manipulation is known as an algorithm. The earliest programmable machines preceded the invention of the digital computer. In 1801, Joseph-Marie Jacquard devised a loom that would weave a pattern by following a series of perforated cards. Patterns could be repeated by arranging the cards.
In 1837, Charles Babbage was inspired by Jacquard's loom to attempt to build the Analytical Engine. The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled; the device would have had a "store"—memory to hold 1,000 numbers of 40 decimal digits each. Numbers from the "store" would have been transferred to the "mill", for processing, and a "thread" being the execution of programmed instructions by the device. It was programmed using two sets of perforated cards—one to direct the operation and the other for the input variables. However, after more than 17,000 pounds of the British government's money, the thousands of cogged wheels and gears never worked together. During a nine-month period in 1842–43, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea; the memoir covered the Analytical Engine. The translation contained Note G which detailed a method for calculating Bernoulli numbers using the Analytical Engine.
This note is recognized by some historians as the world's first written computer program. In 1936, Alan Turing introduced the Universal Turing machine—a theoretical device that can model every computation that can be performed on a Turing complete computing machine, it is a finite-state machine. The machine can move the tape forth, changing its contents as it performs an algorithm; the machine starts in the initial state, goes through a sequence of steps, halts when it encounters the halt state. This machine is considered by some to be the origin of the stored-program computer—used by John von Neumann for the "Electronic Computing Instrument" that now bears the von Neumann architecture name; the Z3 computer, invented by Konrad Zuse in Germany, was a programmable computer. A digital computer uses electricity as the calculating component; the Z3 contained 2,400 relays to create the circuits. The circuits provided a floating-point, nine-instruction computer. Programming the Z3 was through a specially designed keyboard and punched tape.
The Electronic Numerical Integrator And Computer was a Turing complete, general-purpose computer that used 17,468 vacuum tubes to create the circuits. At its core, it was a series of Pascalines wired together, its 40 units weighed 30 tons, occupied 1,800 square feet, consumed $650 per hour in electricity when idle. It had 20 base-10 accumulators. Programming the ENIAC took up to two months. Three function tables needed to be rolled to fixed function panels. Function tables were connected to function panels using heavy black cables; each function table had 728 rotating knobs. Programming the ENIAC involved setting some of the 3,000 switches. Debugging a program took a week; the programmers of the ENIAC were women who were known collectively as the "ENIAC girls." The ENIAC featured parallel operations. Different sets of accumulators could work on different algorithms, it used punched card machines for input and output, it was controlled with a clock signal. It ran for eight years, calculating hydrogen bomb parameters, predicting weather patterns, producing firing tables to aim artillery guns.
The Manchester Baby was a stored-program computer. Programming transitioned away from setting dials. Only three bits of memory were available to store each instruction, so it was limited to eight instructions. 32 switches were available for programming. Computers manufactured; the computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed; this process was repeated. Computer programs were manually input via paper tape or punched cards. After the medium was loaded, the starting address was set via switches and the execute button pressed. In 1961, the Burroughs B5000 was built to be programmed in the ALGOL 60 language; the hardware featured circuits to ease the compile phase. In 1964, the IBM System/360 was a line of six computers each having the same instruction set architecture; the Model 30 was the least expensive. Customers could retain the same application software; each System/360 model featured multiprogramming.
With operating system support, multiple programs could be in memory at once. When one was waiting for input/output, another could compute; each model could emulate other computers. Customers could upgrade to the System/360 and ret
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