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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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They are grouped together because each affects the timing of a bit. Since millions and billions of bits travel on a medium in one second, timing is extremely important.</p><p> <i>Dispersion</i> is when the signal broadens in time. It is caused by the type of media involved. If serious enough, one bit can start to interfere with the next bit and confuse it with the bits before and after it. The signal must not be allowed to spread out, since billions of bits per second may be sent on the network. Dispersion can be fixed by proper cable design, limiting cable lengths, and finding the proper impedance. In optical fibers, using laser light of a very specific wavelength can control dispersion. For wireless communications, dispersion can be minimized by the frequencies used to transmit.</p><p>All digital systems are clocked, meaning it is the clock pulses that cause everything to happen. Clock pulses cause the CPU to calculate, the data to be stored in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, timing <i>jitter</i> will occur. This means that bits will arrive a little earlier and later than expected. Jitter can be fixed by a series of complicated clock synchronizations, including hardware and software, or protocol synchronizations.</p><p> <i>Latency</i>, also known as delay, has two main causes. First, Einstein's theory of relativity states, "nothing can travel faster than the speed of light in a vacuum (3.0 x 10<sup>8</sup> meters/second)". Wireless networking signals travel at slightly less than the speed of light in vacuum. Networking signals on copper media travel in the range of 1.9x10<sup>8</sup> m/s to 2.4x10<sup>8</sup> m/s. Networking signals on optical fiber travel at approximately 2.0x10<sup>8</sup> m/s. So to travel a distance, a bit takes at least a small amount of time to get to where it is going. Second, if the bit goes through any devices, the transistors and electronics introduce more latency. The solution to the problem of latency is the careful use of internetworking devices, different encoding strategies, and various layer protocols.</p><p>Modern networks typically operate at speeds of 1 Mbps to 1000 Mbps (1Gbps -- one billion bits per second). If bits are broadened by dispersion, then 1s can be mistaken for 0s (zeros) and 0s (zeros) for 1s. If groups of bits get routed differently and no attention is paid to timing, the jitter can cause errors as the receiving computer tries to reassemble packets into a message. If groups of bits are late, the networking devices and other destination computers might get hopelessly lost and overwhelmed by a billion bits per second.</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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