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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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At first this method may seem like chaos to the students, but hopefully they remember its presentation from Chapter 6. CSMA/CD is actually a very efficient solution to the issue of having multiple hosts share the same medium. Again, having the students act out the algorithm [summarized in the flowchart in graphic 9] is recommended.</p> <p>This TI relates to CCNA Certification Exam Objectives #52.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Ethernet is a shared-media broadcast technology -- summarized in the Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121632/knet/v214adataimage1/1.gif" width="12" height="12"> -<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121632/knet/v214adataimage4/4.gif" width="12" height="12">. The access method CSMA/CD used in Ethernet performs three functions:</p><ol><li>Transmitting and receiving frames</li><li>Decoding frames and checking them for valid addresses before passing them to the upper layers of the OSI model</li><li>Detecting errors within frames or on the network</li></ol><p>In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode. This means when a device wants to send data, it must first check to see whether the networking media is busy. The device must check if there are any signals on the networking media. After the device determines the networking media is not busy, the device will begin to transmit its data. While transmitting its data in the form of signals, the device also listens. It does this to ensure no other stations are transmitting data to the networking media at the same time. After it completes transmitting its data, the device will return to listening mode.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121632/knet/v214adataimage5/5.gif" width="12" height="12"></p><p>Networking devices are able to tell when a collision has occurred because the amplitude of the signal on the networking media will increase. When a collision occurs, each device that is transmitting will continue to transmit data for a short time. This is done to ensure that all devices see the collision. Once all devices on the network have seen that a collision has occurred, those devices that were attempting to transmit when the collision was detected will invoke an algorithm. After those devices have backed off for a certain period of time (different for each device), any device can attempt to gain access to the networking media once again. When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data. The Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121632/knet/v214adataimage6/6.gif" width="12" height="12"> summarizes the CSMA/CD process.</p><p>Ethernet is a broadcast transmission medium. This means that all devices on a network can see all data that passes along the networking media. However, not all the devices on the network will process the data. Only the device whose MAC address and IP address matches the destination MAC address and destination IP address carried by the data will copy the data.</p><p>Once a device has verified the destination MAC and IP addresses carried by the data, it then checks the data packet for errors. If the device detects errors, the data packet is discarded. The destination device will not notify the source device of whether the packet arrived successfully or not. Ethernet is a connectionless network architecture and is referred to as a best-effort delivery system.</p></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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