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<html> <head> <meta content="en-us" http-equiv="Content-Language"> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> window.scrollTo(0,0); function newPage(pageID) { alert("COMMING SOON!!\nThis feature is currently in development."); } function launchMoreInfo(oid) { dash = oid.indexOf('-'); sub1 = oid.substring(0,dash); sub2 = oid.substring(dash+1); // this is an URL base on the // system that generated the static // download of the curriculum. httpref = "http://nomar.sdf.cisco.com/servlet/org.cli.delivery.rendering.servlet.MIServlet/Static=true,LMSID=DTUI,Engine=dynamic,Theme=cnamstheme,Style=ccna,Language=en,Version=2,RootID=knet-v214aCV47051/CHAPID=null/RLOID=null/RIOID=null/"; newref = httpref + sub1 +"/"+ sub2 +"/moreinfoframeset.html"; //alert("new newref :\n" + newref); window.open(newref,'MoreInfo','toolbar=no,status=yes,menubar=no,location=no,scrollbars=yes,height=442,width=756,left=20,top=20,resizable=yes') } function launchContentResource(contentID) { top.currentSupport = 0; 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The dominant process as data makes its way through the cloud is routing (a Layer 3 process). However, repeating, switching, DNS lookups, and various other processes may occur before a packet finds its way to its destination.</p> <p>Graphic one shows the OSI layers for one end node on the left, then the OSI layers for the networking device in the middle, and then the OSI layers for the other end node on the right. Note that for the Layer 7 networking device in the middle, the data packet is de-encapsulated up to the Layer 7 level and then re-encapsulated. The data packet is processed at all layers.</p> <p>This TI relates to CCNA Certification Exam Objective #1.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>The graphic generally shows that certain devices operate at all seven layers. Some devices (for example, your PC) are Layer 1-7 devices. In other words, they perform processes that can be associated with every layer of the OSI model. Encapsulation and decapsulation are two examples of this. A device called a gateway (essentially a computer which converts information from one protocol to another) is also a Layer 7 device. An example of a gateway would be a computer on a LAN that allows the network to connect to an IBM mainframe computer or to a network-wide facsimile (fax) system. In both of these examples, the data would have to go all the way up the OSI model stack to be converted into a data format the receiving device, either the mainframe or the fax unit, could use.</p><p>Finally, clouds may contain several kinds of media, NICs, switches, bridges, routers, gateways and other networking devices. Because the cloud is not really a single device, but a collection of devices that operate at all levels of the OSI model, it is classified as a Layer 1-7 device.</p><table cellpadding="0" cellspacing="4" width="63%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td valign="middle" width="8%"><img height="23" width="23" src="../../../../../images/ccna/common/icon2.gif"></td><td valign="middle" width="92%"><span class="cstitle">Web Links</span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212736,00.htm">Packets</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.cs.mun.ca/~donald/bsc/node13.html">Data Encapsulation</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp99/i02_fnd/sld006.htm">Encapsulation Slide</a></span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/encapsulation.html">Encapsulation of PDUs</a></span></td> </tr> </table></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
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