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d the basis of experimental broadcasts done by the british broadcasting corporation beginning on 30 september 1929 34 vacuum tubes edit main article vacuum tube vacuum tubes use thermionic emission of electrons from a heated cathode for a number of fundamental electronic functions such as signal amplification and current rectification 35 the simplest vacuum tube the diode invented in 1904 by john ambrose fleming contains only a heated electron emitting cathode and an anode electrons can only flow in one direction through the device from the cathode to the anode adding one or more control grids within the tube enables the current between the cathode and anode to be controlled by the voltage on the grid or grids 36 these devices became a key component of electronic circuits for the first half of the 20th century and were crucial to the development of radio television radar sound recording and reproduction long distance telephone networks and analogue and early digital computers while some applications had used earlier technologies such as the spark gap transmitter for radio or mechanical computers for computing it was the invention of the thermionic vacuum tube that made these technologies widespread and practical leading to the creation of electronics 37 for most of the 20th century televisions depended on a kind of vacuum tube the cathode ray tube invented by karl ferdinand braun the first version of such a television to show promise was produced by philo farnsworth and demonstrated to his family on 7 september 1927 38 after world war ii interrupted experiments resumed and television became an important home entertainment broadcast medium also in the 1940s the invention of semiconductor devices made it possible to produce solid state devices which are smaller cheaper and more efficient reliable and durable than vacuum tubes starting in the mid 1960s vacuum tubes were replaced with the transistor vacuum tubes still have some applications for certain high frequency amplifiers computer networks and the internet edit main articles computer network and internet on 11 september 1940 george stibitz transmitted problems for his complex number calculator in new york using a teletype and received the computed results back at dartmouth college in new hampshire 39 this configuration of a centralized computer mainframe with remote dumb terminals remained popular well into the 1970s in the 1960s paul baran and independently donald davies started to investigate packet switching a technology that sends a message in portions to its destination asynchronously without passing it through a centralized mainframe a four node network emerged on 5 december 1969 constituting the beginnings of the arpanet which by 1981 had grown to 213 nodes 40 arpanet eventually merged with other networks to form the internet while internet development was a focus of the internet engineering task force ietf who published a series of request for comments documents other networking advancements occurred in industrial laboratories such as the local area network lan developments of ethernet 1983 token ring 1984 citation needed and star network topology growth of transmission capacity edit the effective capacity to exchange information worldwide through two way telecommunication networks grew from 281 petabytes pb of optimally compressed information in 1986 to 471 pb in 1993 to 2 2 exabytes eb in 2000 to 65 eb in 2007 41 this is the informational equivalent of two newspaper pages per person per day in 1986 and six entire newspapers per person per day by 2007 42 given this growth telecommunications play an increasingly important role in the world economy and the global telecommunications industry was about a 4 7 trillion sector in 2012 43 44 the service revenue of the global telecommunications industry was estimated to be 1 5 trillion in 2010 corresponding to 2 4 of the world s gross domestic product gdp 43 technical concepts edit modern telecommunication is founded on a series of key concepts that experienced progressive development and refinement in a period of well over a century basic elements edit telecommunication technologies may primarily be divided into wired and wireless methods overall a basic telecommunication system consists of three main parts that are always present in some form or another a transmitter that takes information and converts it to a signal a transmission medium also called the physical channel that carries the signal e g the free space channel a receiver that takes the signal from the channel and converts it back into usable information for the recipient in a radio broadcasting station the station s large power amplifier is the transmitter and the broadcasting antenna is the interface between the power amplifier and the free space channel the free space channel is the transmission medium and the receiver s antenna is the interface between the free space channel and the receiver next the radio receiver is the destination of the radio signal where it is converted from electricity to sound telecommunication systems are occasionally duplex two way systems with a single box of electronics working as both the transmitter and a receiver or a transceiver e g a mobile phone 45 the transmission electronics and the receiver electronics within a transceiver are quite independent of one another this can be explained by the fact that radio transmitters contain power amplifiers that operate with electrical powers measured in watts or kilowatts but radio receivers deal with radio powers measured in microwatts or nanowatts hence transceivers have to be carefully designed and built to isolate their high power circuitry and their low power circuitry from each other to avoid interference telecommunication over fixed lines is called point to point communication because it occurs between a transmitter and a receiver telecommunication through radio broadcasts is called broadcast communication because it occurs between a powerful transmitter and numerous low power but sensitive radio receivers 45 telecommunications in which multiple transmitters and multiple receivers have been designed to cooperate and share the same physical channel are called multiplex systems the sharing of physical channels using multiplexing often results in significant cost reduction multiplexed systems are laid out in telecommunication networks and multiplexed signals are switched at nodes to reach the correct destination terminal receiver analogue versus digital communications edit communications can be encoded as analogue or digital signals which may in turn be carried by analogue or digital communication systems analogue signals vary continuously with respect to the information while digital signals encode information as a set of discrete values e g a set of ones and zeroes 46 during propagation and reception information contained in analogue signals is degraded by undesirable noise commonly the noise in a communication system can be expressed as adding or subtracting from the desirable signal via a random process this form of noise is called additive noise with the understanding that the noise can be negative or positive at different instances unless the additive noise disturbance exceeds a certain threshold the information contained in digital signals will remain intact their resistance to noise represents a key advantage of digital signals over analogue signals however digital systems fail catastrophically when noise exceeds the system s ability to autocorrect on the other hand analogue systems fail gracefully as noise increases the signal becomes progressively more degraded but still usable also digital transmission of continuous data unavoidably adds quantization noise to the output this can be reduced but not eliminated only at the expense of increasing the channel bandwidth requirement communication channels edit main article communication channel the term channel has two different meanings in one meaning a channel is the physical medium that carries a signal between the transmitter and the receiver examples of this include the atmosphere for sound communications glass optical fibres for some kinds of optical communications coaxial cables for communications by way of the voltages and electric currents in them and free space for communications using visible light infrared waves ultraviolet light and radio waves coaxial cable types are classified by rg type or radio guide terminology derived from world war ii the various rg designations are used to classify the specific signal transmission applications 47 this last channel is called the free space channel the sending of radio waves from one place to another has nothing to do with the presence or absence of an atmosphere between the two radio waves travel through a perfect vacuum just as easily as they travel through air fog clouds or any other kind of gas the other meaning of the term channel in telecommunications is seen in the phrase communications channel which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously for example one radio station can broadcast radio waves into free space at frequencies in the neighbourhood of 94 5 mhz megahertz while another radio station can simultaneously broadcast radio waves at frequencies in the neighbourhood of 96 1 mhz each radio station would transmit radio waves over a frequency bandwidth of about 180 khz kilohertz centred at frequencies such as the above which are called the carrier frequencies each station in this example is separated from its adjacent stations by 200 khz and the difference between 200 khz and 180 khz 20 khz is an engineering allowance for the imperfections in the communication system in the example above the free space channel has been divided into communications channels according to frequencies and each channel is assigned a separate frequency bandwidth in which to broadcast radio waves this system of dividing the medium into channels according to frequency is called frequency division multiplexing another term for the same concept is wavelength division multiplexing which is more commonly used in optical communications when multiple transmitters share the same physical medium another way of dividing a communications medium into channels is to allocate each sender a recurring segment of time a time slot for example 20 milliseconds out of each second and to allow each sender to send messages only within its own time slot this method of dividing the medium into communication channels is called time division multiplexing tdm and is used in optical fibre communication some radio communication systems use tdm within an allocated fdm channel hence these systems use a hybrid of tdm and fdm modulation edit main article modulation the shaping of a signal to convey information is known as modulation modulation can be used to represent a digital message as an analogue waveform this is commonly called keying a term derived from the older use of morse code in telecommunications and several keying techniques exist these include phase shift keying frequency shift keying and amplitude shift keying the bluetooth system for example uses phase shift keying to exchange information between various devices 48 49 in addition there are combinations of phase shift keying and amplitude shift keying which is called in the jargon of the field quadrature amplitude modulation qam that are used in high capacity digital radio communication systems modulation can also be used to transmit the information of low frequency analogue signals at higher frequencies this is helpful because low frequency analogue signals cannot be effectively transmitted over free space hence the information from a low frequency analogue signal must be impressed into a higher frequency signal known as the carrier wave before transmission there are several different modulation schemes available to achieve this two of the most basic being amplitude modulation am and frequency modulation fm an example of this process is a disc jockey s voice being impressed into a 96 mhz carrier wave using frequency modulation the voice would then be received on a radio as the channel 96 fm 50 in addition modulation has the advantage that it may use frequency division multiplexing fdm telecommunication networks edit main article telecommunications network a telecommunications network is a collection of transmitters receivers and communications channels that send messages to one another some digital communications networks contain one or more routers that work together to transmit information to the correct user an analogue communications network consists of one or more switches that establish a connection between two or more users for both types of networks repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances this is to combat attenuation that can render the signal indistinguishable from the noise 51 another advantage of digital systems over analogue is that their output is easier to store in memory i e two voltage states high and low are easier to store than a continuous range of states societal impact edit telecommunication has a significant social cultural and economic impact on modern society in 2008 estimates placed the telecommunication industry s revenue at us 4 7 trillion or just under three per cent of the gross world product official exchange rate 43 several of the following sections discuss the impact of telecommunication on society microeconomics edit on the microeconomic scale companies have used telecommunications to help build global business empires this is self evident in the case of online retailer amazon com but according to academic edward lenert even the conventional retailer walmart has benefited from better telecommunication infrastructure compared to its competitors 52 in cities throughout the world home owners use their telephones to order and arrange a variety of home services ranging from pizza deliveries to electricians even relatively poor communities have been noted to use telecommunication to their advantage in bangladesh s narsingdi district isolated villagers use cellular phones to speak directly to wholesalers and arrange a better price for their goods in côte d ivoire coffee growers share mobile phones to follow hourly variations in coffee prices and sell at the best price 53 macroeconomics edit on the macroeconomic scale lars hendrik röller and leonard waverman suggested a causal link between good telecommunication infrastructure and economic growth 54 55 few dispute the existence of a correlation although some argue it is wrong to view the relationship as causal 56 because of the economic benefits of good telecommunication infrastructure there is increasing worry about the inequitable access to telecommunication services amongst various countries of the world this is known as t...