<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=us-ascii"><meta name=Generator content="Microsoft Word 12 (filtered medium)"><style><!--
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</o:shapelayout></xml><![endif]--></head><body lang=EN-AU link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Hi Mark,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> I continue to be troubled by this question. Evidently the writers of the question expect there be no role for the physical layer in error detection/control/correction.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Here are some sources that describe extensive error handling capability at the physical layer:<o:p></o:p></span></p><p class=MsoNormal><a href="http://www.oohoo.org/Book/MindShare-PCIe-System/0321156307_ch11lev1sec7.html">http://www.oohoo.org/Book/MindShare-PCIe-System/0321156307_ch11lev1sec7.html</a><o:p></o:p></p><p class=MsoNormal><a href="http://my.safaribooksonline.com/book/hardware/0321156307/physical-layer-logic/ch11lev1sec7">http://my.safaribooksonline.com/book/hardware/0321156307/physical-layer-logic/ch11lev1sec7</a><o:p></o:p></p><p class=MsoNormal><a href="http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4784785">http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4784785</a> <o:p></o:p></p><p class=MsoNormal><span lang=EN style='font-size:10.5pt;font-family:"Verdana","sans-serif";color:#333333'>This covers fundamental behavior of a 802.11g network which already has embedded error correction at the physical layer, under unicast and broadcast modes</span><o:p></o:p></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Error detection and handling appears at a number of layers in the OSI. When the physical layer reports an error either “caused by the medium” or during transmission over the medium, the data link layer then organizes to attempt to deliver/dispatch again. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>So as with the question on the worm example - it depends what you mean by the word “infect” - we now need to ask what we mean with “ensure” in:<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>to <u>ensure</u> that packets of data received from another point in the network are free of any errors caused by the medium connecting the two points<span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Without the physical layer reporting problems that insurance lapses. The fact both layers ( physical and data link ) are involved in this does not remove the role of the physical layer. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Take Care<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Kevork Krozian<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Edulists Creator Administrator<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>www.edulists.com.au<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>tel: 0419 356 034<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> sofdev-bounces@edulists.com.au [mailto:sofdev-bounces@edulists.com.au] <b>On Behalf Of </b>Mark KELLY<br><b>Sent:</b> Friday, 18 November 2011 3:26 PM<br><b>To:</b> Year 12 Software Development Teachers' Mailing List<br><b>Subject:</b> Re: [Year 12 SofDev] SD 2011 exam - MC Q<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal style='margin-bottom:12.0pt'>Hi Kevork. I guess it all depends on how much detail we can safely stuff into teenagers' heads in about 200 classes. <br><br>We naturally have to simplify things for them, like when a 4 year old asks "Why is the sky blue?"<br><br>:-)<o:p></o:p></p><div><p class=MsoNormal>On 18 November 2011 15:04, Kevork Krozian <<a href="mailto:kevork@edulists.com.au">kevork@edulists.com.au</a>> wrote:<o:p></o:p></p><p class=MsoNormal>Hi Friends,<br><br> I shudder when I see answers to questions that are half true.<br>Example:<br><br>Question 14<br><br> Which statement best describes the purpose of the Open Systems<br>Interconnections (OSI) physical layer?<br><br> A. to route a packet of data through a network to a remote location<br> B. to move individual data bits across a shared medium to another<br>point in the network<br> C. to ensure that packets of data are assembled and presented to an<br>application in the correct order<br> D. to ensure that packets of data received from another point in the<br>network are free of any errors caused by the medium connecting the two<br>points<br><br>We agree A ( Network layer ) and C ( Transport layer ) are not the answer.<br>However both B and D can seem correct.<br><br>The problem for me with B is that there are no ACTUAL bits moving. You<br>cannot no matter how hard you look notice a 0 or 1 on any medium moving<br>data. All you will see are distortions that represent these values.<br>Therefore individual bits are not moving. However, if you take the answer<br>to mean "representations" of bits are moving then it can be right except<br>it doesn't just happen on a shared medium eg. switches.<br><br>As for answer D, the Physical layer "can ensure that packets of data<br>received from another point in the network are free of any errors caused<br>by the medium connecting the two points"<br>Here is how.<br><br>Chapter 8 Semester 1 Cisco CCNA<br><br>Code Groups<br><br>Encoding techniques use bit patterns called symbols. The Physical layer<br>may use a set of encoded symbols - called code groups - to represent<br>encoded data or control information. A code group is a consecutive<br>sequence of code bits that are interpreted and mapped as data bit<br>patterns. For example, code bits 10101 could represent the data bits 0011.<br><br>Code groups are often used as an intermediary encoding technique for<br>higher speed LAN technologies. This step occurs at the Physical layer<br>prior to the generation of signals of voltages, light pulses, or radio<br>frequencies. By transmitting symbols, the error detection capabilities and<br>timing synchronization between transmitting and receiving devices are<br>enhanced. These are important considerations in supporting high speed<br>transmission over the media.<br><br>Although using code groups introduces overhead in the form of extra bits<br>to transmit, they improve the robustness of a communications link. This is<br>particularly true for higher speed data transmission.<br><br>Advantages using code groups include:<br>Reducing bit level error<br>Limiting the effective energy transmitted into the media<br>Helping to distinguish data bits from control bits<br>Better media error detection<br><br>So the physical layer is involved with better error detection and<br>reduction of bit level errors..<br><br>Therefore, which is the correct answer ?<br><br><br>Kevork<br>_______________________________________________<br><a href="http://www.edulists.com.au" target="_blank">http://www.edulists.com.au</a> - FAQ, Subscribe, Unsubscribe<br>IT Software Development Mailing List kindly supported by<br><a href="http://www.vcaa.vic.edu.au" target="_blank">http://www.vcaa.vic.edu.au</a> - Victorian Curriculum and Assessment Authority and<br><a href="http://www.vcaa.vic.edu.au/vce/studies/infotech/softwaredevel3-4.html" target="_blank">http://www.vcaa.vic.edu.au/vce/studies/infotech/softwaredevel3-4.html</a><br><a href="http://www.vitta.org.au" target="_blank">http://www.vitta.org.au</a> - VITTA Victorian Information Technology Teachers Association Inc<o:p></o:p></p></div><p class=MsoNormal><br><br clear=all><br>-- <br>Mark Kelly<br>Manager of ICT, Reporting, IT Learning Area<br>McKinnon Secondary College<br>McKinnon Rd McKinnon 3204, Victoria, Australia<br>Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253<br><a href="mailto:kel@mckinnonsc.vic.edu.au" target="_blank">kel@mckinnonsc.vic.edu.au</a><br>VCE IT Lecture Notes: <a href="http://vceit.com" target="_blank">http://vceit.com</a><br>Moderator: <a href="http://www.edulists.com.au/" target="_blank">IT Applications Edulist</a><br><br>Want a good time? Call 0112358. Ask for Mr Fibonacci.<o:p></o:p></p></div></body></html>