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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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eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47506"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47667" + "/RIOID=" + "knet-v214aRIO121608"; returnRef = "../../../../../"+ params + "/"+ ch_sub1 +"/"+ ch_sub2 +"/pageframeset.html"; top.prevPage = returnRef; } function launchFile(filepath) { newWindow = window.open(filepath,"","top=70,left=120,width="+(screen.width*.50)+",height="+(screen.height*.50)+",resizable=yes,scrollbars=yes,toolbar=yes,menubar=yes,status=yes"); /* if (is.ie && filepath.lastIndexOf(".pdf") != -1) { pageTimer = setInterval("winLoadCheck()", 1000); } */ } function winLoadCheck() { if (newWindow.document.readyState == 'complete') { clearInterval(pageTimer); newWindow.location.reload(); } } // BrowserCheck Object // provides most commonly needed browser checking variables // 19990326 // Copyright (C) 1999 Dan Steinman // Distributed under the terms of the GNU Library General Public License // Available at http://www.dansteinman.com/dynapi/ function BrowserCheck() { var b = navigator.appName if (b=="Netscape") this.b = "ns" else if (b=="Microsoft Internet Explorer") this.b = "ie" else this.b = b this.v = parseInt(navigator.appVersion) this.ns = (this.b=="ns" && this.v>=4) this.ns4 = (this.b=="ns" && this.v==4) this.ns5 = (this.b=="ns" && this.v==5) this.ie = (this.b=="ie" && this.v>=4) this.ie4 = (navigator.userAgent.indexOf('MSIE 4')>0) this.ie5 = (navigator.appVersion.indexOf('MSIE 5.0')>0) this.ie55 = (navigator.appVersion.indexOf('MSIE 5.5')>0) if (this.ie5) this.v = 5 this.min = (this.ns||this.ie) } // automatically create the "is" object is = new BrowserCheck()</script> </head> <body onLoad="window.focus();" alink="#000000" vlink="#000000" link="#000000" bgcolor="#E7E7E7" background="../../../../../images/ccna/common/bg.gif"> <table width="100%" cellspacing="0" cellpadding="2" border="0"><tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="rlohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="rlohdr">6.5</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Media Access Control (MAC)</td> </tr> </table> </td> </tr> <tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="riohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="riohdr">6.5.4</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Nondeterministic MAC protocols</td> </tr> </table> </td> </tr> <tr> <td colspan="4"> <table width="90%" cellspacing="0" cellpadding="0" border="0"> <tr> <td class="smtext"> <p> </p><table cellpadding="0" cellspacing="0" width="100%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" cellspacing="0" cellpadding="2" width="100%" border="0"> <tr> <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td> </tr> <tr> <td valign="top"> </td><td valign="top"><span class="cstext"> <p>The purpose of this target indicator is to introduce one of the two basic categories of MAC algorithms. Opportunistic approaches to media access control rely on random number backoff algorithms to allocate slot times and opportunities to transmit. While seemingly chaotic, they are actually extremely efficient at allocating access to the medium. The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) algorithm is introduced. Students should memorize the meaning of the acronym. Students should be encouraged to put the algorithm in their own words. Students should be required to flowchart the algorithm in their own words.</p> <p>The efficiency of CSMA/CD may seem counter-intuitive to the students, even chaotic. One classroom activity is to have four students kinesthetically act out the algorithm. Teach them the following rules. Each student has six sentences to transmit, and the maximum frame size is thee sentences. Each student is to wait for silence. Each student, if hearing silence, may start to talk. If there is no collision when each student has started talking, they may finish up to three sentences (the maximum frame size). If there is a collision, each student yells "collision!" and backs off a "random" number of seconds. Whoever has counted out the least number of seconds will listen for silence, and if everyone is following the algorithm, start to transmit. Eventually everyone should get a chance to get their sentence through.</p> <p>This TI relates to CCNA Certification Exam Objectives #3 and #60.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Nondeterministic MAC protocols use a <i>first-come</i>,<i> first-served (FCFS)</i> approach. In the late 1970s, the University of Hawaii developed and used a radio communication system (ALOHA) that connected the various Hawaiian Islands. <img border="0" src="../../../../../CHAPID=knet-v214aCH47506/RLOID=knet-v214aRLO47667/RIOID=knet-v214aRIO121608/knet/v214adataimage1/1.gif" width="12" height="12"> The protocol they used allowed anyone to transmit at will. This led to radio wave collisions that could be detected by listeners during transmissions. However, what started as ALOHA, eventually became a modern MAC protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).</p><p>CSMA/CD is a simple system. Everyone on the system listens for quiet, at which time it is OK to transmit. However, if two people talk at the same time, a collision occurs, and neither person can transmit. Everyone else on the system also hears the collision, waits for silence, and then tries to transmit. <img border="0" src="../../../../../CHAPID=knet-v214aCH47506/RLOID=knet-v214aRLO47667/RIOID=knet-v214aRIO121608/knet/v214adataimage2/2.gif" width="12" height="12"></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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