5 a LAN 6 a gateway 7 a modem

Transcription

1 STARTER With the help of this digrm, try to descrie the function of these components of typicl network system: 1 file server 2 ridge 3 router 4 ckone 5 LAN 6 gtewy 7 modem Another Novell LAN Router Internet Bridge Fire-optic ckone Bridge Shred network printer Shred hrd disk Gtewy Unix network Fig 1 Components of typicl LAN Twisted pir cling Computers with network interfce crds Locl printer Now red these definitions to check your nswers. You my lso refer to the Glossry. A ridge is hrdwre nd softwre comintion used to connect the sme type of networks. Bridges cn lso prtition lrge network into two smller ones nd connect two LANs tht re nery ech other. A router is specil computer tht directs communicting messges when severl networks re connected together. High-speed routers cn serve s prt of the Internet ckone. A gtewy is n interfce tht enles dissimilr networks to communicte, such s two LANs sed on different topologies or network operting systems. A ckone is the min trnsmission pth, hndling the mjor dt trffic, connecting different LANs together. A LAN is network contined within smll re, for exmple compny deprtment. A modem is device for converting digitl signls to nlogue signls nd vice vers to enle computer to trnsmit nd receive dt using n ordinry telephone line.

2 READING Now study this text nd the digrm of simple home network setup. Mtch the digrm key to the components of the network. The technology needed to set up home network is here tody. It is just mtter of connecting numer of PCs equipped with Ethernet dpters to hu using twisted-pir cling which uses sockets rther like phone sockets. Specil isoltion dpters cn e fitted to llow existing mins lines to e used insted of twisted-pir cling. Most future home networks, however, re likely to e wireless network systems, using tuned trnsmitter nd receiver devices. The simplest networks llow sic file-shring nd multiplyer gming s well s shring of peripherls such s printers. Most dvnced home networks re likely to hve client/server structure, with low-cost terminls, or 'thin' clients, connected to centrl server which mintins the system's storge cpcity nd, depending on whether the terminls re dum or processorequipped network computers, its processing power. To mke the most of such network, it must ecome prt of n integrted home entertinment nd control system. To the user, the desktop ecomes just one of mny fetures ccessile throughout the house. Tired of working in the study? Pop down to the living room nd relod it into the terminl there. Before you strt work, cll up the hi-fi control progrm nd hve the music of your choice pumped through the living room spekers. Computer nd entertinment networks cn e seprte ut linked y the server to llow control of the ltter from the terminls. Future home networks re more likely to hve the entire system sed on single loop. KEY TO THE DIAGRAM Fig 2 Simple home network

3 74 UNIT 11 Networks LANGUAGE WORK ltive cluses with prticiple Reltive cluses with prticiple re often used in technicl descriptions. They llow you to provide lot of informtion out noun using s few words s possile. Study these exmples from the Tsk 3 text. 1 The technology needed to set up home network 2 PCs equipped with Ethernet dpters 3 Network modem llowing clients to ccess the Internet simultneously 4 Dt line linking client to server We cn use the pssive prticiple s in exmples 1 nd 2. 1 The technology needed to set up home network. = technology which is needed 2 PCs equipped with Ethernet dpters = PCs which re equipped We cn use n ctive prticiple s in exmples 3 nd 4. 3 Network modem llowing clients to ccess the Internet simultneously = modem which llows clients to ccess the Internet simultneously 4 Dt line linking client to server = dt line which links client to server Complete these definitions with the correct prticiple of the ver given in rckets. 1 A gtewy is n interfce (enle) dissimilr networks to communicte. 2 A ridge is hrdwre nd softwre comintion (use) to connect the sme type of networks. 3 A ckone is network trnsmission pth (hndle) mjor dt trffic. 4 A router is specil computer (direct) messges when severl networks re linked. 5 A network is numer of computers nd peripherls (link) together. 6 A LAN is network (connect) computers over smll distnce such s within compny. 7 A server is powerful computer (store) mny progrms (shre) y ll the clients in the network. 8 A client is network computer (use) for ccessing service on server. 9 A thin client is simple computer (comprise) processor nd memory, disply, keyord, mouse nd hrd drives only. 10 A hu is n electronic device (connect) ll the dt cling in network.

4 UNIT 11 Networks 75 Link these sttements using reltive cluse with prticiple. c The technology is here tody. It is needed to set up home network. You only need one network printer. It is connected to the server. Her house hs network. It llows sic file-shring nd multi-plyer gming. There is line receiver in the living room. It delivers home entertinment udio to spekers. Eve hs designed site. It is dedicted to dnce. She hs uilt in links. They connect her site to other dnce sites. She creted the site using progrm clled Netscpe Composer. It is contined in Netscpe Communictor. At the centre of Frnce Telecom's home of tomorrow is network. It is ccessed through Plm Pilot-style control pd. The network cn simulte the owner's presence. This mkes sure vitl tsks re crried out in her sence. The house hs n electronic door-keeper. It is progrmmed to recognise you. This gives ccess to fmily only. PROBLEM-SOLVING Work in two groups, A nd B. Group A, list ll the dvntges of network. Group B, list ll the disdvntges. Then together consider how the disdvntges cn e minimised. Group A Advntges of network Group B Disdvntges of network

5 76 UNIT 11 Networks SPEAKING Trnsmission modes Work in pirs, A nd B. Explin to your prtner how one mode of trnsmission etween computers opertes with the help of the text provided. Your explntion should llow your prtner to lel his/her digrm. Student A Your text is on pge 186. Your explntion should llow your prtner to lel this digrm. Fig 3 Asynchronous trnsmission Student B Your text is on pge 192. Your explntion should llow your prtner to lel this digrm. Fig 4 Synchronous trnsmission

6 UNIT 11 Networks 77 WRITING Using the lists you compiled in Tsk 6, descrie the dvntges nd disdvntges of networks. Try to link some of the dvntges nd disdvntges s in these exmples. Advntges Allow dt to e shred. Users cn shre softwre on the server. Disdvntges Permit viruses to spred quickly. Server filure mens no one cn work. 1 Although networks llow dt to e shred, they permit viruses to spred quickly. 2 Users cn shre softwre on the server; however server filure mens tht no one cn work. WHEN YOU HAVE FINISHED THE READING SECTION ON THE FOLLOWING PAGES, COME BACK TO THESE ADDITIONAL EXERCISES 3 Identify which lyer ttches the following heders to network trnsmission: Specifying the lnguge, the compression nd encryption schemes Identifying ech segment's checksum nd its position in the messge c Contining the sequence of pckets nd the ddress of the receiving computer d Mrking the eginning nd end of the messge nd specifying whether the messges will e sent hlf-duplex or fullduplex e Identifying the sending nd receiving computers 4 Fill in the missing words in the following sentences then put the sentences in the correct order: The checksum is reclculted y the lyer which lso ressemles the messge The messge is nd y the presenttion lyer. c The messge is reconverted into y the physicl lyer. d The session lyer then sends the messge to the next e The ppliction lyer converts the its into chrcters, nd directs the dt to the correct f The incoming re recounted y the network lyer for nd illing purposes. g The lyer confirms the rrivl of the pckets, them in, nd clcultes the for ech pcket. h The prts of the messge re y the lyer until the messge is

7 78 UNIT 11 Networks D IALIST READING Find the nswers to these questions in the following text. Into wht units is dt sudivided y the following lyers? trnsport lyer network lyer Wht is the purpose of trnsmission checksum test? How long does the dt-link lyer keep copy of ech pcket? Wht processes cn e crried out t intermedite nodes? Which network communictions lyer is descried y ech of the following sttements? Mkes sure tht the messge is trnsmitted in lnguge tht the receiving computer cn understnd Protects the dt eing sent c Encodes nd sends the pckets d Supervises the trnsmission e The prt of communictions process tht user sees f Strts communictions nd looks fter communictions mong network nodes g Chooses route for the messge h Mkes ckup copies of the dt if required i Confirms the checksum, then ddresses nd duplictes the pckets Network Communictions The ppliction lyer is the only prt of communictions process tht user sees, nd even then, the user doesn't see most of the work tht the ppliction does to prepre messge for sending over network. The lyer converts messge's dt from humn-redle form into its nd ttches heder identifying the sending nd receiving computers. The presenttion lyer ensures tht the messge is trnsmitted in lnguge tht the receiving computer cn interpret (often ASCII). This lyer trnsltes the lnguge, if necessry, nd then compresses nd perhps encrypts the dt. It dds nother heder specifying the lnguge s well s the compression nd encryption schemes. The session lyer opens communictions nd hs the jo of keeping stright the communictions mong ll nodes on the network. It sets oundries (clled rcketing) for the eginning nd end of the messge, nd estlishes whether the messges will e sent hlf-duplex, with ech computer tking turns sending nd receiving, or full-duplex, with oth computers sending nd receiving t the sme time. The detils of these decisions re plced into session heder. The trnsport lyer protects the dt eing sent. It sudivides the dt into segments, cretes checksum tests - mthemticl sums sed on the contents of dt - tht cn e used lter to determine if the dt ws scrmled. It cn lso mke ckup copies of the dt. The trnsport heder identifies ech segment's checksum nd its position in the messge. The network lyer selects route for the messge. It forms dt into pckets, counts them, nd dds heder contining the sequence of pckets nd the ddress of the receiving computer. The dt-link lyer supervises the trnsmission. It confirms the checksum, then ddresses nd duplictes the pckets. This lyer keeps copy of ech pcket until it receives confirmtion from the next point long the route tht the pcket hs rrived undmged.

8 UNIT 11 Networks 79 sending computer receiving computer B Re-red the text to find the nswers to these questions. 1 Mtch the term in Tle A with the sttement in Tle B. heder heder heder heder heder ppliction lyer presenttion lyer session lyer trnsport lyer network lyer dt-link lyer physicl lyer intermedite node The physicl lyer encodes the pckets into the medium tht will crry them - such s n nlogue signl, if the messge is going cross telephone line - nd sends the pckets long tht medium. An intermedite node clcultes nd verifies the checksum for ech pcket. It my lso reroute the messge to void congestion on the network. At the receiving node, the lyered process tht sent the messge on its wy is reversed. The physicl lyer reconverts the messge into its. The dt-link lyer reclcultes the checksum, confirms rrivl, nd logs in the pckets. The network lyer recounts incoming pckets for security nd illing purposes. The trnsport lyer reclcultes the checksum nd ressemles the messge segments. The session lyer holds the prts of the messge until the messge is complete nd sends it to the next lyer. The presenttion lyer expnds nd decrypts the messge. The ppliction lyer converts the its into redle chrcters, nd directs the dt to the correct ppliction. Tle A Brcketing Hlf-duplex c Full-duplex d Tle B i ii Checksum Trnsmission mode in which ech computer tkes turns sending nd receiving Mthemticl clcultions sed on the contents of dt iii Set oundries for the eginning nd end of messge iv Trnsmission mode in which oth computers send nd receive t the sme time 2 Mrk the following sttements s True or Flse: Most of the work tht n ppliction does to prepre messge for sending over network is not seen y the user. ASCII is lwys used to trnsmit dt. c The encryption lyer compresses the messge. d The network lyer keeps trck of how mny pckets re in ech messge. e The network lyer keeps copy of ech pcket until it rrives t the next node undmged. f Anlogue signls re used on ordinry telephone lines. g When messge rrives t its destintion, it psses through the sme seven network communictions lyers s when it ws sent, ut in reverse order. Additionl exercises on pge 77 ['How Computers Work' y Ron White nd Timothy Edwrd Downs (Ziff-Dvis Press) - Extrct in PC Mgzine, Ferury 1993]