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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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Students are reminded of the encapsulation process, which generates a packet to be transferred over the network.</p> <p>This TI relates to CCNA Certification Exam Objective #5.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>In order for reliable communications to take place over a network, data to be sent must be put in manageable, traceable packages. This is done through the process of encapsulation as covered in chapter 2. Encapsulation begins in the application, presentation, and session layers, where data is prepared for transmission by the creation of a common data format.</p><p>The transport layer breaks up the data into manageable size units called segments. It also assigns sequence numbers to the segments to make sure the receiving host puts the data back together in the proper order. The network layer then encapsulates the segment creating a packet. It adds a destination and source network address (usually IP) to the packet.</p><p>The data link layer further encapsulates the packet and creates a frame. It adds the source and destination local (MAC) address to the frame. The data link layer then transmits the binary bits of the frame over the physical layer media.</p><p>When the data is transmitted on just a local area network, we talk about the data units as frames, because the MAC address is all that is necessary to get from source to destination host. If we need to send the data to another host over an Intranet or the Internet, these data units are referred to as packets. This is because the network address in the packet contains the final destination address of the host the data (packet) is being sent to.</p><p>The bottom three layers (network, data link, physical) of the OSI model are the primary movers of data across an Intranet or Internet. The main exception to this is a device called a gateway. It is a device designed to convert the data from one format, created by the application, presentation, and session layers to another. The gateway accomplishes this task by operating at all seven layers of the OSI model. This will be explained in more detail later in the chapter.</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.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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