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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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Again, build upon the generic frame format presented in Chapter 6, the Token Ring frame format, and the FDDI frame format, to reinforce what types of information are contained in frames. Students need not memorize the frame format, but hopefully by now they can describe what typically is present in frames. The frame format provides a lot of insight into the operation of a given networking technology.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>The Ethernet and IEEE 802.3 frame fields are described in the following summaries:</p><ul type="disc"><li> <i>Preamble</i> - The alternating pattern of 1s and 0s (zeros) tells receiving stations that a frame is Ethernet or IEEE 802.3. The Ethernet frame includes an additional byte that is the equivalent of the Start of Frame (SOF) field specified in the IEEE 802.3 frame.</li><li> <i>Start-of-frame (SOF)</i> - The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame-reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet.</li><li> <i>Destination and source addresses</i> - The first three bytes of the addresses are specified by the IEEE on a vendor-dependent basis. The last three bytes are specified by the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single-node) address. The destination address can be unicast, multicast (group), or broadcast (all nodes).</li><li> <i>Type (Ethernet)</i> - The type specifies the upper layer protocol to receive the data after Ethernet processing is completed.</li><li> <i>Length (IEEE 802.3)</i> - The length indicates the number of bytes of data that follows this field.</li><li> <i>Data (Ethernet)</i>- After physical layer and link layer processing is complete, the data contained in the frame is sent to an upper layer protocol, which is identified in the type field. Although Ethernet version 2 does not specify any padding, in contrast to IEEE 802.3, Ethernet expects at least 64 bytes of data.</li><li> <i>Data (IEEE 802.3)</i> - After physical layer and link layer processing is complete, the data is sent to an upper layer protocol, which must be defined within the data portion of the frame. If data in the frame is insufficient to fill the frame to its minimum 64 byte size, padding bytes are inserted to ensure at least a 64 byte frame.</li><li> <i>Frame check sequence (FCS)</i> - This sequence contains a four byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames.</li></ul></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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