LANLAN Local Area Network, LAN, in computer science, a group of computers and other devices dispersed over a relatively limited area and connected by . A communications link that enables any device to Interact with any other on the network. LANs commonly include microcomputers and shared (often expensive) resources such as laser printers and large hard disks.

MODERN LANMODERN LAN Most (modern) LANs can support a wide variety of Most (modern) LANs can support a wide variety of computers and other devices. Each device computers and other devices. Each device must use the proper physical and data-linkmust use the proper physical and data-link protocols for the particular LAN, and all devicesprotocols for the particular LAN, and all devices that want to communicate with each other on the that want to communicate with each other on the LAN must use the same upper-level LAN must use the same upper-level communications protocol.communications protocol. Although single LANs are geographically limited Although single LANs are geographically limited (to a department or an office building, for example),(to a department or an office building, for example), separate LANs can be connected to form larger networks. separate LANs can be connected to form larger networks. Similar LANs are linked by bridgesSimilar LANs are linked by bridgeswhich act as transfer points between networks;which act as transfer points between networks;

LAN MANAGERLAN MANAGER LAN Manager, in computer science, a local area network technology developed by Microsoft Corporation and distributed by Microsoft, IBM (as IBM LAN Server), and other original equipment manufacturers (OEMs). LAN Manager connects computers running the MS-DOS; OS/2, and UNIX operating systems and allows users to share files and system resources such as hard disks and printers and to run distributed applications using a client-server architecture. See also Client-Server Architecture; LAN; Network;

Management Management

It is a full-time job to keep a LAN operating as it It is a full-time job to keep a LAN operating as it should.should.

Keeping a computer network that is distributed Keeping a computer network that is distributed across the world running smoothly takes the across the world running smoothly takes the challenge of network management a step further. challenge of network management a step further.

The essential concepts for managing distributed The essential concepts for managing distributed and diverse networks have received a lot of attention and diverse networks have received a lot of attention lately.lately.

There are now enough tools and standards for There are now enough tools and standards for this important aspect of computer networks to allow this important aspect of computer networks to allow global networks to be supervised effectively.global networks to be supervised effectively.

SecuritySecurity

With ever increasing amounts of important With ever increasing amounts of important information being entrusted to ever more distributed information being entrusted to ever more distributed computers, computer security becomes ever more computers, computer security becomes ever more important. important.

In a highly distributed system it would be all too easy In a highly distributed system it would be all too easy for an informed superhighway man to access for an informed superhighway man to access confidential information without being seen. confidential information without being seen.

The Data Encryption System (DES) standard for The Data Encryption System (DES) standard for protecting computer data, introduced in the late 1970s, protecting computer data, introduced in the late 1970s, has more recently been supplemented by “public key” has more recently been supplemented by “public key” systems that allow users to scramble and unscramble systems that allow users to scramble and unscramble their messages easily without a third party intruding.their messages easily without a third party intruding.

SECURITY AND MANAGEMENTSECURITY AND MANAGEMENT

Having fast computer networks built of Having fast computer networks built of machines that can talk to each other is not machines that can talk to each other is not the end of the story. the end of the story.

The specters of the “information The specters of the “information superhighwayman” and the “information superhighwayman” and the “information superroadworks” have yet to be dealt with.superroadworks” have yet to be dealt with.

Open SystemsOpen Systems

This term covers the general aim of building computer systems so that This term covers the general aim of building computer systems so that they can readily be interconnected, and hence distributed. In practice, they can readily be interconnected, and hence distributed. In practice, open systems is all about unbundling all the complexities of a open systems is all about unbundling all the complexities of a computer system and using similar structure across different systems. computer system and using similar structure across different systems. And this entails a mixture of standards (which tell the manufacturers And this entails a mixture of standards (which tell the manufacturers what they should be doing) and consortia (groups of like-minded what they should be doing) and consortia (groups of like-minded people who help them to do it). The overall effect is that they can talk people who help them to do it). The overall effect is that they can talk to each other.to each other.

The ultimate aim of all of the work in distributed systems is to allow The ultimate aim of all of the work in distributed systems is to allow anyone to buy computers from a number of different manufacturers, to anyone to buy computers from a number of different manufacturers, to site them wherever is convenient, to use broadband connections to link site them wherever is convenient, to use broadband connections to link them, and to operate them as one cooperating machine that takes full them, and to operate them as one cooperating machine that takes full advantage of the fast links.advantage of the fast links.

Object TechnologyObject Technology

Another way to build computer systems Another way to build computer systems works from the premise that they should be works from the premise that they should be built from well-defined parts—objects which built from well-defined parts—objects which are encapsulated, defined, and implemented are encapsulated, defined, and implemented so that they can be independent agents. so that they can be independent agents.

The adoption of objects as a means of The adoption of objects as a means of building computer systems has helped to building computer systems has helped to allow interchangeability of parts.allow interchangeability of parts.

Client ServerClient Server

Instead of building computer systems as monolithic Instead of building computer systems as monolithic systems, there is now general agreement that they should systems, there is now general agreement that they should be constructed as client/server systems. be constructed as client/server systems.

The client (a PC user) requests a service (such as The client (a PC user) requests a service (such as printing) and the server (a LAN-connected processor) printing) and the server (a LAN-connected processor) provides it. provides it.

This consensus view on the structure of a computer This consensus view on the structure of a computer system means that there is a separation of functions system means that there is a separation of functions previously bundled together. previously bundled together.

The implementation details that flow from a simple The implementation details that flow from a simple concept go a long way to enabling all computers to be concept go a long way to enabling all computers to be treated uniformly.treated uniformly.

The facilitiesThe facilities    On most LANs are very powerful. Most organizations do not wish On most LANs are very powerful. Most organizations do not wish

to have small isolated islands of computing facilities. to have small isolated islands of computing facilities. They usually want to extend facilities over a wider area so that They usually want to extend facilities over a wider area so that

groups can work without having to be located. groups can work without having to be located. Routers and bridges are specialized devices that allow two or Routers and bridges are specialized devices that allow two or

more LANs to be connected. The bridge is the more basic device more LANs to be connected. The bridge is the more basic device and can only connect LANs of the same type.and can only connect LANs of the same type.

The router is a more intelligent component that can interconnect The router is a more intelligent component that can interconnect many different types of computer network.many different types of computer network.

LOCAL AREA NETWORKSLOCAL AREA NETWORKS

1.One of the most dramatic events in computer networking 1.One of the most dramatic events in computer networking has been the introduction and rapid growth of the local has been the introduction and rapid growth of the local area network (LAN) as a way to standardize the system area network (LAN) as a way to standardize the system of linking computers used in office systems. of linking computers used in office systems.

As the name suggests, this is a means of connecting a As the name suggests, this is a means of connecting a number of computing elements together. At the simplest number of computing elements together. At the simplest level, a LAN provides no more than a shared medium level, a LAN provides no more than a shared medium along with a set of rules that govern the access to that along with a set of rules that govern the access to that medium.medium.

The most widely used LAN, Ethernet, uses a The most widely used LAN, Ethernet, uses a mechanism . This means that each connected device mechanism . This means that each connected device can only use the cable when it has established that no can only use the cable when it has established that no other device is using it. If there is contention directlother device is using it. If there is contention directl

2.In addition to providing shared access, modern LANs can 2.In addition to providing shared access, modern LANs can also give users a wide range of sophisticated facilities.also give users a wide range of sophisticated facilities.

Management software packages are available to control Management software packages are available to control the way in which devices are configured on the LAN, how the way in which devices are configured on the LAN, how users are administered, and how network resources are users are administered, and how network resources are controlled. controlled.

A widely adopted structure on local networks is to have a A widely adopted structure on local networks is to have a number of servers that are available to a (usually much number of servers that are available to a (usually much greater) number of clients. greater) number of clients.

The former, usually powerful computers, provide services The former, usually powerful computers, provide services such as print control, file sharing, and mail to the latter, such as print control, file sharing, and mail to the latter, which are usually personal computers.which are usually personal computers.

3.Ethernet and CSMA-CD are examples of LANs. 3.Ethernet and CSMA-CD are examples of LANs. There are many different layouts (such as bus, There are many different layouts (such as bus, star, ring) and a number of different access star, ring) and a number of different access protocols.protocols.

Despite this variety, all LANs share the feature that Despite this variety, all LANs share the feature that they are limited in range (typically they cover one they are limited in range (typically they cover one building) andbuilding) and

are fast enough to make the connecting network are fast enough to make the connecting network invisible to the devices that use itinvisible to the devices that use it

TYPES OF LANTYPES OF LAN LANs are linked by gateways, which both transfer data andLANs are linked by gateways, which both transfer data andconvert it according to the protocols used by the receivingconvert it according to the protocols used by the receivingnetwork.network. The devices on a LAN are known as nodes, and the nodes areThe devices on a LAN are known as nodes, and the nodes areconnected by cabling through which messages are transmitted. Types of cables connected by cabling through which messages are transmitted. Types of cables

include twisted-pair wiring, coaxial cable, orinclude twisted-pair wiring, coaxial cable, orfibre-optic cable. fibre-optic cable. Nodes on a LAN can be wired together in any of three basic Nodes on a LAN can be wired together in any of three basic topologies, known as bus, ring, and star. topologies, known as bus, ring, and star. As implied by their names, a bus network is more or lessAs implied by their names, a bus network is more or lesslinear, a ring, network forms a loop, and a star networklinear, a ring, network forms a loop, and a star networkradiates from a central hub.radiates from a central hub. To avoid potential collisions when two or more nodesTo avoid potential collisions when two or more nodesattempt to transmit at the same time, LANs use either attempt to transmit at the same time, LANs use either contention and collision detection or token passing to regulatecontention and collision detection or token passing to regulatetraffic.traffic.

MODEMS AND COMPUTER BUREAUXMODEMS AND COMPUTER BUREAUX

As recently as the 1970s, computers were expensive, fragile As recently as the 1970s, computers were expensive, fragile machines that had to be looked after by specialists and machines that had to be looked after by specialists and kept in a controlled environment. They could be used either kept in a controlled environment. They could be used either by plugging in a terminal directly or by using a phone line by plugging in a terminal directly or by using a phone line and modem to gain access from a distance.and modem to gain access from a distance.

Computer networks during this period were not commercially Computer networks during this period were not commercially available. Even so, one of the most significant available. Even so, one of the most significant developments to shape the modern world of technology developments to shape the modern world of technology was initiated at this time: experimentation by the US was initiated at this time: experimentation by the US Defence Department in distributing computer resources to Defence Department in distributing computer resources to provide resilience against failure. provide resilience against failure.

This work is now known as the Internet.This work is now known as the Internet.

COMPUTER NETWORKCOMPUTER NETWORK Computer Networks, the widespread sharing of information among Computer Networks, the widespread sharing of information among

groups of computers and their users, a central part of the information groups of computers and their users, a central part of the information age. The popular adoption of the personal computer (PC) and the local age. The popular adoption of the personal computer (PC) and the local area network (LAN) during the 1980s has led to the capacity to access area network (LAN) during the 1980s has led to the capacity to access information on a distant database; download an application from information on a distant database; download an application from overseas; send a message to a friend in a different country; and share overseas; send a message to a friend in a different country; and share files with a colleague—all from a personal computer.files with a colleague—all from a personal computer.

The networks that allow all this to be done so easily are sophisticated The networks that allow all this to be done so easily are sophisticated and complex entities. They rely for their effectiveness on many and complex entities. They rely for their effectiveness on many cooperating components. The design and deployment of the worldwide cooperating components. The design and deployment of the worldwide computer network can be viewed as one of the great technological computer network can be viewed as one of the great technological wonders of recent decades.wonders of recent decades.

Token Ring NetworkToken Ring Network

Token Ring NetworkToken Ring Network, in computer science, a local area , in computer science, a local area network formed in a ring (closed loop) topology that uses network formed in a ring (closed loop) topology that uses token passing as a means of regulating traffic on the line.token passing as a means of regulating traffic on the line.

On a token ring network, a token governing the right to On a token ring network, a token governing the right to transmit is passed from one station to the next in a transmit is passed from one station to the next in a physical circle. If a station has information to transmit, it physical circle. If a station has information to transmit, it “seizes” the token, marks it as being in use, and inserts “seizes” the token, marks it as being in use, and inserts the information. The “busy” token, plus message, is then the information. The “busy” token, plus message, is then passed around the circle, copied when it arrives at its passed around the circle, copied when it arrives at its destination, and eventually returned to the sender.destination, and eventually returned to the sender.

The sender removes the attached message and then The sender removes the attached message and then passes the freed token to the next station in line.passes the freed token to the next station in line.

Token ring networks are defined in the IEEE 802.5 Token ring networks are defined in the IEEE 802.5 standards. standards. See also See also LANLAN; ; Star NetworkStar Networks.s.

World Wide WebWorld Wide Web

1.World Wide Web, library of resources available to 1.World Wide Web, library of resources available to computer users through the global Internet. It computer users through the global Internet. It

enables users to view a wide variety of enables users to view a wide variety of information, including magazine archives, public information, including magazine archives, public and college library resources, and current world and college library resources, and current world

and business news. and business news. 2.World Wide Web (WWW) resources are organized 2.World Wide Web (WWW) resources are organized

so that users can easily move from one resource so that users can easily move from one resource to another. The connections to different source to another. The connections to different source

computers, or servers.computers, or servers.

3.WWW pages are formatted using Hypertext 3.WWW pages are formatted using Hypertext Markup Language (HTML), and WWW Markup Language (HTML), and WWW

communication among computers uses the communication among computers uses the Hypertext Transfer Protocol (HTTP), or Wireless Hypertext Transfer Protocol (HTTP), or Wireless Access Protocol (WAP) for mobile phones. This Access Protocol (WAP) for mobile phones. This

communication is usually through the Internet via communication is usually through the Internet via Transmission Control Protocol (TCP) connections, Transmission Control Protocol (TCP) connections,

but almost any kind of connection can be used.but almost any kind of connection can be used.

4.The WWW was developed in 1989 by Timothy 4.The WWW was developed in 1989 by Timothy Berners-Lee, a British computer scientist at the Berners-Lee, a British computer scientist at the CERN research facility near Geneva, Switzerland, CERN research facility near Geneva, Switzerland, to allow information-sharing among internationally to allow information-sharing among internationally dispersed teams of high-energy physics dispersed teams of high-energy physics researchers. It subsequently became a platform researchers. It subsequently became a platform for related software development, and the for related software development, and the numbers of linked computersnumbers of linked computers

TelegraphTelegraph

Telegraph system of communication Telegraph system of communication employing electrical apparatus to transmit employing electrical apparatus to transmit and receive signals in accordance with a and receive signals in accordance with a code of electrical pulses. code of electrical pulses.

Originally the term Originally the term telegraphytelegraphy referred to referred to any form of communication over long any form of communication over long distances in which messages were distances in which messages were transmitted by signs or sounds.transmitted by signs or sounds.

THE MORSE TELEGRAPHTHE MORSE TELEGRAPH The first electrical instruments for telegraphic transmission The first electrical instruments for telegraphic transmission

were invented in the United States by the American were invented in the United States by the American inventor Samuel F. B. Morse in 1837 and in Britain the inventor Samuel F. B. Morse in 1837 and in Britain the same year by the British physicist Sir Charles Wheatstone same year by the British physicist Sir Charles Wheatstone in collaboration with the British engineer Sir William F. in collaboration with the British engineer Sir William F. Cooke. Cooke.

The simple code that became known as Morse code The simple code that became known as Morse code transmitted messages by electric pulses passing over a transmitted messages by electric pulses passing over a single wire. Morse's apparatus, which sent the first public single wire. Morse's apparatus, which sent the first public telegram in 1844, resembled a simple electric switch.telegram in 1844, resembled a simple electric switch.

It allowed current to pass for a prescribed length of time It allowed current to pass for a prescribed length of time and then shut it off, all at the pressure of a finger. The and then shut it off, all at the pressure of a finger. The original Morse receiver had an electromagnetically original Morse receiver had an electromagnetically controlled pencil that made marks on paper tape moving controlled pencil that made marks on paper tape moving over a clockwork-operated cylinder. over a clockwork-operated cylinder.

Type MODERN TELEGRAPH SERVICESType MODERN TELEGRAPH SERVICES

1.TELEX-1.TELEX- In 1958 a system of direct-dial teleprinter exchange, called In 1958 a system of direct-dial teleprinter exchange, called Telex, was introduced, and within ten years it had more than Telex, was introduced, and within ten years it had more than 25,000 subscribers. 25,000 subscribers. The Telex system enabled subscribers to send messages The Telex system enabled subscribers to send messages and data directly to other subscribers in North America and, and data directly to other subscribers in North America and, through the facilities of international carriers, in many other through the facilities of international carriers, in many other parts of the world. parts of the world. In some areas Telex subscribers could also send In some areas Telex subscribers could also send messages to non-subscribers by dialing special messages to non-subscribers by dialing special communications centre that delivered the messages as communications centre that delivered the messages as telegrams.telegrams.

2. Facsimile Reproduction2. Facsimile Reproduction Facsimile telegraph systems, which send and receive Facsimile telegraph systems, which send and receive images and text, have been rendered largely images and text, have been rendered largely obsolete by fax (facsimile transmission).obsolete by fax (facsimile transmission).

AUTOMATIC TELEGRAPH SYSTEMSAUTOMATIC TELEGRAPH SYSTEMS

1 In teleprinting, the message is received in 1 In teleprinting, the message is received in the form of typed words on a paper form. In the form of typed words on a paper form. In this system each letter of the alphabet is this system each letter of the alphabet is represented by one of 31 combinations of five represented by one of 31 combinations of five equal-interval electronic impulses, with the equal-interval electronic impulses, with the sequence of used and unused intervals sequence of used and unused intervals determining the letter. The start-stop printing determining the letter. The start-stop printing code uses seven pulses for each character, code uses seven pulses for each character, the first pulse indicating the beginning and the the first pulse indicating the beginning and the seventh pulse the end of the letter.seventh pulse the end of the letter.

.

..

2 . system of communication employing electrical 2 . system of communication employing electrical apparatus to transmit and receive signals in apparatus to transmit and receive signals in accordance with a code of electrical pulses. accordance with a code of electrical pulses. Originally the term Originally the term telegraphytelegraphy referred to any referred to any form of communication over long distances form of communication over long distances in which messages were transmitted by signs or in which messages were transmitted by signs or soundssounds

WAP Mobile PhoneWAP Mobile Phone

Internet services became available to mobile Internet services became available to mobile phone users in the United Kingdom in 2000 phone users in the United Kingdom in 2000 with the introduction of WAP (Wireless with the introduction of WAP (Wireless Access Protocol) enabled handsets.Access Protocol) enabled handsets.

dpa Deutsche Presse-AgentuGmbH/Marcus dpa Deutsche Presse-AgentuGmbH/Marcus KrügerKrüger

Types of cables Types of cables The devices on a LAN are known as nodes, The devices on a LAN are known as nodes, and the nodes are connected by cabling through and the nodes are connected by cabling through which messages are transmitted.which messages are transmitted. Types of cables Types of cables include twisted-pair wiring, coaxial cable, or fibre-optic include twisted-pair wiring, coaxial cable, or fibre-optic cable. Nodes on a LAN can be wired together in any of cable. Nodes on a LAN can be wired together in any of three basic topologies, known as bus, ring, and star. three basic topologies, known as bus, ring, and star. As implied by their names, a bus network is moreAs implied by their names, a bus network is more or less linear, a ring network forms a loop, and or less linear, a ring network forms a loop, and a star network radiates from a central hub.a star network radiates from a central hub. To avoid potential collisions when two or more nodes To avoid potential collisions when two or more nodes attempt to transmit at the same time, LANs use either attempt to transmit at the same time, LANs use either contention and collision detection orcontention and collision detection or token passing to regulate traffic.token passing to regulate traffic.

Children's TelevisionChildren's Television◊ historical development of broadcasting aimed at young viewers. historical development of broadcasting aimed at young viewers. ◊ Children’s television in the United Kingdom owes much to the public Children’s television in the United Kingdom owes much to the public

service ideals of the BBC, where John Reith, as the first director-service ideals of the BBC, where John Reith, as the first director-general, made a commitment to provide programmes for children on general, made a commitment to provide programmes for children on the new “wireless”: the new “wireless”:

◊ ““It is not to be like school. It is not to be like school. ◊ They’ve been at school all day. You will need to devise something to They’ve been at school all day. You will need to devise something to

entertain and inform children and, if possible, to delight them.entertain and inform children and, if possible, to delight them.◊ Children’s programmes must become a wonderment.” The daily radio Children’s programmes must become a wonderment.” The daily radio

programme programme Children’s HourChildren’s Hour became a popular tradition in many became a popular tradition in many households at teatime as it provided a carefully planned mixture of households at teatime as it provided a carefully planned mixture of information and entertainment. information and entertainment.

◊ When television became available throughout the United Kingdom When television became available throughout the United Kingdom after World War II it was a natural progression to build on this legacy.after World War II it was a natural progression to build on this legacy.

◊ The new visual medium attracted even the youngest children to string The new visual medium attracted even the youngest children to string puppets, such as puppets, such as Muffin the MuleMuffin the Mule and and Andy PandyAndy Pandy, and many other , and many other programmes became firm favouritesprogrammes became firm favourites

ClientClient//Server ArchitectureServer Architecture IIn computer science, an arrangement used on local area networks n computer science, an arrangement used on local area networks

that makes use of “distributed intelligence” to treat both the server that makes use of “distributed intelligence” to treat both the server the individual workstations as intelligent, programmable devices, the individual workstations as intelligent, programmable devices,

thus exploiting the full computing power of each. thus exploiting the full computing power of each. This is done by splitting the processing of an application between This is done by splitting the processing of an application between

two distinct components: a “front-end” client and a “back-end” two distinct components: a “front-end” client and a “back-end” server. server.

The client component, itself a complete, stand-alone personal The client component, itself a complete, stand-alone personal computer (versus the “dumb” terminal found in older architectures computer (versus the “dumb” terminal found in older architectures such as the time-sharing used on a mainframe) offers the user its such as the time-sharing used on a mainframe) offers the user its full range of power and features for running applications. full range of power and features for running applications.

The server component, which can be another personal computer, The server component, which can be another personal computer, minicomputer, or a mainframe, enhances the client component by minicomputer, or a mainframe, enhances the client component by providing the traditional strengths offered by minicomputers and providing the traditional strengths offered by minicomputers and mainframes in a time-sharing environment: mainframes in a time-sharing environment:

data management, information sharing between clients, and data management, information sharing between clients, and sophisticated network administration and security features. sophisticated network administration and security features.

The advantage of the client/server The advantage of the client/server

architecturearchitecture The advantage of the client/server architecture The advantage of the client/server architecture

over older architectures is that the client and over older architectures is that the client and server machines work together to accomplish the server machines work together to accomplish the processing of the application being used. processing of the application being used.

Not only does this increase the processing Not only does this increase the processing power available, but it also uses that power more power available, but it also uses that power more efficiently. efficiently.

The client portion of the application is typically The client portion of the application is typically optimized for user interaction, whereas the server optimized for user interaction, whereas the server portion provides the centralized, multi-user portion provides the centralized, multi-user functionality. functionality.

Application Program InterfaceApplication Program Interface IIn computer science, a set of routines that an n computer science, a set of routines that an applicationapplication program uses program uses

to request and carry out lower-level services performed by a to request and carry out lower-level services performed by a computer's computer's operating systemoperating system. .

An application program carries out two types of tasks: those related to An application program carries out two types of tasks: those related to work being performed, such as accepting text or numbers input to a work being performed, such as accepting text or numbers input to a document or spreadsheet, and those related to maintenance chores, document or spreadsheet, and those related to maintenance chores, such as managing files and displaying information on the screen. such as managing files and displaying information on the screen.

These maintenance chores are performed by the computer's operating These maintenance chores are performed by the computer's operating systemsystem..

On local area networks, an API, such as IBM's NetBIOS, provides On local area networks, an API, such as IBM's NetBIOS, provides applications with a uniform means of requesting services from the applications with a uniform means of requesting services from the lower levels of the network.lower levels of the network.

TelecommunicationsTelecommunications

Dumb TerminalDumb Terminal, in computer science, a , in computer science, a terminal that does not contain an terminal that does not contain an internal internal microprocessormicroprocessor. Dumb . Dumb terminals are typically capable of terminals are typically capable of displaying only characters and numbers displaying only characters and numbers and responding to simple control codes. and responding to simple control codes. See also See also LANLAN; ; Smart TerminalSmart Terminal;;

UES OF WIRELESS LANUES OF WIRELESS LAN

The reasons why electricity is universally The reasons why electricity is universally employed as a medium of energy transfer employed as a medium of energy transfer and use are thatand use are that: (: (11) ) It can be efficiently It can be efficiently transported from transported from generatorsgenerators to the point of to the point of use in the consumer's premises through a use in the consumer's premises through a simplesimple--toto--install network of wiresinstall network of wires. (. (22) ) It can It can be converted at high efficiency into heat, be converted at high efficiency into heat, mechanical, and chemical energymechanical, and chemical energy. . It powers It powers electronic deviceselectronic devices. . It provides It provides light. (light. (33) ) It is It is instantly controllable at the point of use—it instantly controllable at the point of use—it takes only a flick of a switch to turn an takes only a flick of a switch to turn an electrical device on or offelectrical device on or off..

advantageadvantage

1.  television after World War II, many 1.  television after World War II, many broadcasters predicted the complete demise broadcasters predicted the complete demise of radioof radio. . A reliance on sound, when images A reliance on sound, when images were available, seemed an anachronismwere available, seemed an anachronism. .

In time, it became apparent that the medium In time, it became apparent that the medium had many unique characteristics that helped had many unique characteristics that helped distinguish it from both the press and distinguish it from both the press and televisiontelevision. .

Some of these characteristics seem more like Some of these characteristics seem more like limitations, but many can also be turned to limitations, but many can also be turned to the medium’s.the medium’s.

2 .Food processors and manufacturers2 .Food processors and manufacturers, who will , who will benefit from produce with a longer shelf-life, benefit from produce with a longer shelf-life, and better properties for processing and and better properties for processing and manufacture.manufacture.

3 .3 . ConsumersConsumers, who will have cheaper and , who will have cheaper and more plentiful food as a result of the more plentiful food as a result of the advantages to growers and processors, as advantages to growers and processors, as well as possibly better rape) have been well as possibly better rape) have been designed to modify the proportions of designed to modify the proportions of different polyunsaturated fatty acids, and so different polyunsaturated fatty acids, and so improve the nutritional quality of the oilimprove the nutritional quality of the oil

DisadvantageDisadvantage

GrowersGrowers, who will benefit from the , who will benefit from the resistance of crops to insect pests, resistance of crops to insect pests, viruses, and fungi, by the viruses, and fungi, by the introduction of natural insecticides introduction of natural insecticides or fungicides from other species, so or fungicides from other species, so reducing the need for application of reducing the need for application of agricultural chemicals, hence also agricultural chemicals, hence also an environmental gain;an environmental gain;

Local governmentLocal government

Each of the 24 counties Each of the 24 counties ((länlän) ) in Sweden in Sweden is governed by an administrative board is governed by an administrative board appointed by the central government, in appointed by the central government, in conjunction with a popularly elected conjunction with a popularly elected county councilcounty council. . Towns, cities, and rural Towns, cities, and rural districts within the counties constitute districts within the counties constitute communes and also have popularly communes and also have popularly elected councilselected councils..

communicationcommunication

Early telegraph and Early telegraph and telephonetelephone systems relied upon a direct systems relied upon a direct connection between source and destination, usually through connection between source and destination, usually through copper wires or cablescopper wires or cables. . The development of The development of radioradio made made possible “wireless” possible “wireless” communicationcommunication, and, later, the development , and, later, the development of of televisiontelevision transmission transmission..

These inventions have also enabled the development of the These inventions have also enabled the development of the InternetInternet, and digital systems combined with radio have , and digital systems combined with radio have produced produced cellularcellular mobile telephone networks mobile telephone networks. .

They have also made possible They have also made possible communications satellitecommunications satellite systems, the satellitesystems, the satellite--based based global positioning system (global positioning system (GPSGPS)), , and advanced military and commercial and advanced military and commercial radarradar systems systems. . The The latest application is multichannel digital radio and television latest application is multichannel digital radio and television broadcasting systems, which will soon replace the old analogue broadcasting systems, which will soon replace the old analogue systemssystems..

Cellular RadioCellular Radio

mobile radio mobile radio telephonetelephone system, which has rapidly system, which has rapidly supplementedlandlinetelecommunications as a supplementedlandlinetelecommunications as a means of two-way personal communications. Cellular means of two-way personal communications. Cellular radio works on the principle and uses the physics of radio works on the principle and uses the physics of two-way two-way radioradio communications and is named after communications and is named after the unit “cell” into which an area is divided the unit “cell” into which an area is divided

Each cell has a radius of about 1.5 to 2.4 km (1 to 2.5 Each cell has a radius of about 1.5 to 2.4 km (1 to 2.5 mi) and is equipped with a radiomi) and is equipped with a radio transmitter that transmitter that employs its own range of radio frequenciesemploys its own range of radio frequencies..

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