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Group A, list all the advantagesof a network. Group B, list all the disadvantages. Then togetherconsider how the disadvantages can be minimised.Advantages of a networkGroup BDisadvantages of a network76 UNIT 11 NetworksSPEAKINGTransmission modes Work in pairs, A and B. Explain to yourpartner how one mode of transmission between computers operateswith the help of the text provided. Your explanation should allowyour partner to label his/her diagram.Student AYour text is on page 186. Your explanation should allow your partnerto label this diagram.Fig 3Asynchronous transmissionStudent BYour text is on page 192.
Your explanation should allow your partnerto label this diagram.Fig 4Synchronous transmissionUNIT 11 Networks 77WRITINGUsing the lists you compiled in Task 6, describe theadvantages and disadvantages of networks. Try to link some of theadvantages and disadvantages as in these examples.AdvantagesDisadvantagesAllow data to be shared.Permit viruses to spreadquickly.1Users can share software onServer failure means no onethe server.can work.Although networks allow data to be shared, they permit virusesto spread quickly.2Users can share software on the server; however server failuremeans that no one can work.WHEN YOU HAVE FINISHED THE READING SECTION ON THE FOLLOWING PAGES,COME BACK TO THESE ADDITIONAL EXERCISES3 Identify which layer attaches the followingheaders to a network transmission:aSpecifying the language, the compressionand encryption schemesb Identifying each segment's checksum and itsposition in the messagec Containing the sequence of packets and theaddress of the receiving computerd Marking the beginning and end of themessage and specifying whether themessages will be sent half-duplex or fullduplexe Identifying the sending and receivingcomputers4 Fill in the missing words in the followingsentences then put the sentences in the correctorder:aThe checksum is recalculated by thelayer which also reassembles the messagebThe message isandby thepresentation layer.The message is reconverted intobythe physical layer.The session layer then sends the message tothe nextThe application layer converts the bits intocharacters, and directs the data tothe correctThe incomingare recounted by thenetwork layer forand billingpurposes.Thelayer confirms the arrival of thepackets,them in, and calculates thefor each packet.The parts of the message areby thelayer until the message iscdefgh78UNIT 11 NetworksIALIST READINGDFind the answers to these questions in thefollowing text.Into what units is data subdivided by thefollowing layers?a transport layerb network layerWhat is the purpose of a transmissionchecksum test?How long does the data-link layer keep acopy of each packet?What processes can be carried out atintermediate nodes?Which network communications layer isdescribed by each of the followingstatements?a Makes sure that the message istransmitted in a language that thereceiving computer can understandb Protects the data being sentc Encodes and sends the packetsd Supervises the transmissione The part of a communications processthat a user seesf Starts communications and looks aftercommunications among network nodesg Chooses a route for the messageh Makes backup copies of the data ifrequiredi Confirms the checksum, then addressesand duplicates the packetsNetworkCommunicationsThe application layer is the only part of acommunications process that a user sees, andeven then, the user doesn't see most of the workthat the application does to prepare a messagefor sending over a network.
The layer converts amessage's data from human-readable form intobits and attaches a header identifying thesending and receiving computers.The presentation layer ensures that themessage is transmitted in a language that thereceiving computer can interpret (often ASCII).This layer translates the language, if necessary,and then compresses and perhaps encrypts thedata. It adds another header specifying thelanguage as well as the compression andencryption schemes.The session layer opens communications andhas the job of keeping straight thecommunications among all nodes on the network.It sets boundaries (called bracketing) for thebeginning and end of the message, andestablishes whether the messages will be senthalf-duplex, with each computer taking turnssending and receiving, or full-duplex, with bothcomputers sending and receiving at the sametime.
The details of these decisions are placedinto a session header.The transport layer protects the data beingsent. It subdivides the data into segments,creates checksum tests - mathematical sumsbased on the contents of data - that can be usedlater to determine if the data was scrambled. Itcan also make backup copies of the data. Thetransport header identifies each segment'schecksum and its position in the message.The network layer selects a route for themessage. It forms data into packets, counts them,and adds a header containing the sequence ofpackets and the address of the receivingcomputer.The data-link layer supervises thetransmission. It confirms the checksum, thenaddresses and duplicates the packets.
This layerkeeps a copy of each packet until it receivesconfirmation from the next point along the routethat the packet has arrived undamaged.UNIT 11 Networkssending computer79receiving computerBRe-read the text to find the answers tothese questions.1 Match the term in Table A with thestatement in Table B.Table Aapplication layerheaderpresentation layeraBracketingb Half-duplexc Full-duplexheadersession layerdChecksumheadertransport layerheaderTable BiTransmission mode in which eachcomputer takes turns sending andreceivingiiMathematical calculations based on thecontents of datanetwork layerheaderdata-link layerphysical layerintermediatenodeThe physical layer encodes the packets intothe medium that will carry them - such as ananalogue signal, if the message is going across atelephone line - and sends the packets along thatmedium.An intermediate node calculates and verifiesthe checksum for each packet.
It may also reroutethe message to avoid congestion on the network.At the receiving node, the layered processthat sent the message on its way is reversed.The physical layer reconverts the message intobits. The data-link layer recalculates thechecksum, confirms arrival, and logs in thepackets. The network layer recounts incomingpackets for security and billing purposes. Thetransport layer recalculates the checksum andreassembles the message segments. The sessionlayer holds the parts of the message until themessage is complete and sends it to the nextlayer.
The presentation layer expands anddecrypts the message. The application layerconverts the bits into readable characters, anddirects the data to the correct application.['How Computers Work' by Ron White and Timothy Edward Downs(Ziff-Davis Press) - Extract in PC Magazine, February 1993]iii Set boundaries for the beginning and endof a messageiv Transmission mode in which bothcomputers send and receive at the sametime2 Mark the following statements as True orFalse:a Most of the work that an application does toprepare a message for sending over anetwork is not seen by the user.b ASCII is always used to transmit data.c The encryption layer compresses themessage.d The network layer keeps track of how manypackets are in each message.e The network layer keeps a copy of eachpacket until it arrives at the next nodeundamaged.f Analogue signals are used on ordinarytelephone lines.g When a message arrives at its destination, itpasses through the same seven networkcommunications layers as when it was sent,but in reverse order.Additional exercises on page 77UNIT 12The InternetMatch each of the Internet services in Column A with the usesin Column B.STARTERColumn AColumn B1IRCalogging on to your computer at a distance2MOOsbsending and receiving messages3emailcdownloading a file from a server4FTPdchatting to other users in real-time5WWWeaccessing newsgroups6Telnetfbrowsing webpages7Usenetg taking part in simulations in a shared environmentof three, A, B, and C.
Read one of these examples of CMC andcomplete this table.ExtractABCType of CMCNumber of participantsTopicsSynchronous or asynchronousSpecial features of this type of CMCInside the lounge of the House of Language.There is a television in the corner.You see the following exits: east and northHank, Spartacus, Diana, Tony (resting)Hank says, 'have any of you guys tried batmud?: )'Diana says, ' no'Spartacus says, 'what is it?'Hank says, 'it's a virtual reality game. you'll find it at: bat.
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