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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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This classification of devices is important for understanding data flow on a network and also for troubleshooting networks. Data flow through Layer 1 devices and components involves no de-encapsulation or encapsulation. It merely involves transmission and transformation of the data at the bit level. No addresses are added or modified. No overhead information for delivery purposes is added. From a troubleshooting point of view, it is important for the student to rule out Layer 1 problems.</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 1 networking device in the middle, the data packet is not de-encapsulated. The data packet is processed at Layer 1 only.</p> <p>This TI relates to CCNA Certification Exam Objective #1.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121117/knet/v214adataimage1/1.gif" width="12" height="12"> illustrates that certain devices only operate at Layer 1. The packet flow through Layer 1 devices is simple. Physical media are considered Layer 1 components.<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121117/knet/v214adataimage2/2.gif" width="12" height="12">.</p><p>If the Layer 1 devices are passive (for example, plugs, connectors, jacks, patch panels, physical media), then the bits simply travel through the passive devices, ideally with minimal distortion or signal loss.</p><p>If the Layer 1 devices are active (for example, repeaters <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121117/knet/v214adataimage3/3.gif" width="12" height="12"> or hubs <img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121117/knet/v214adataimage4/4.gif" width="12" height="12">), then the bits are actually regenerated and retimed. Transceivers, also active devices,<img border="0" src="../../../../../CHAPID=knet-v214aCH47503/RLOID=knet-v214aRLO47536/RIOID=knet-v214aRIO121117/knet/v214adataimage5/5.gif" width="12" height="12"> act as adapters (AUI port to RJ-45), or as media converters (RJ-45 electrical to ST Optical). In all cases the transceivers act as a Layer 1 devices.</p><p>No Layer 1 device examines any of the headers or data of an encapsulated packet. Layer 1 devices are concerned only with bits, whether they are in the form of voltage, light, or microwaves.</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_gci212060,00.html">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://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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