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</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">4.3</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Basics of Signals and Noise in Communications Systems</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">4.3.7</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Noise</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 make the idea of electrical noise plausible to students.</p> <p>If you have access to a dual-trace oscilloscope and function generator, you can run the cable near some notorious noise source (electric motors, fluorescent lighting, power cables) and see what noise the signal acquires.</p> <p>If you rub a nail across a file near an AM radio, you can "hear" electromagnetic interference.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Noise is unwanted random addition to a signal. No electrical signal is without noise, however, it is important to keep the signal-to-noise (S/N) ratio as high as possible. The S/N ratio is an engineering calculation and measurement that involves dividing the signal strength by the noise strength. It gives a measure of how easy it will be to decipher the desired, intended signal from the unwanted, but unavoidable, noise. In other words, each bit receives additional unwanted signals from various sources. Too much noise can corrupt a bit, turning a binary 1 into a binary 0 (zero), or a 0 (zero) into a 1, thereby destroying the message. Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage1/1.gif" width="12" height="12"> shows five sources of noise that can affect a bit on a wire.</p><p> <b>NEXT-A and NEXT-B</b> <br>When electrical noise on the cable originates from signals on other wires in the cable, this is known as <i>cross talk. NEXT</i> stands for <i>near-end cross talk.</i> When two wires are near each other and untwisted, energy from one wire can wind up in an adjacent wire and vice versa. This can cause noise at both ends of a terminated cable. There are actually many forms of cross talk that must be considered when building networks. </p><p>NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and the use of quality twisted pair cables.</p><p>NEXT-A is Near End cross talk at computer A and NEXT-B is Near End cross talk at computer B.</p><p> <b>Thermal Noise</b> <br>Thermal noise, due to the random motion of electrons, is unavoidable but usually relatively small compared to the signals.</p><p> <b>AC Power/Reference Ground Noise</b> <br><i>AC Power and reference ground noises</i> are crucial problems in networking. AC line noise creates problems in homes, schools, and offices. Electricity is carried to appliances and machines by wires concealed in walls, floors, and ceilings. Consequently, inside these buildings AC power line noise is all around us. If not properly prevented, power line noise can cause problems for a network. </p><p>Ideally, the signal reference ground should be completely isolated from the electrical ground. Isolation would keep AC power leakage and voltage spikes off the signal reference ground. However, the chassis of a computing device serves as both the signal reference ground, and as the AC power line ground. Since there is a link between the signal reference ground and the power ground, problems with the power ground can lead to interference with the data system. Such interference can be difficult to detect and trace. Usually, it stems from the fact that electrical contractors and installers do not care about the length of the neutral and ground wires that lead to each electrical outlet. Unfortunately, when these wires are long, they can act as an antenna for electrical noise. It is this noise that interferes with the digital signals (bits) a computer must be able to recognize and process.</p><p>AC line noise coming from a nearby video monitor or hard disk drive can be enough to create errors in a computer system. By changing shape and voltage levels, the line noise is interfering with the desired signals. It is preventing the logic gates of the computer from detecting the leading and trailing edges of the square waves. This problem can be further compounded when a computer has a poor ground connection.</p><p> <b>EMI/RFI</b> <br>External sources of electrical impulses that can attack the quality of electrical signals on the cable include lighting, electrical motors, and radio systems. These types of interference are referred to as <i>electromagnetic interference (EMI),</i> and <i>radio frequency interference (RFI).</i> </p><p>Each wire in a cable can act like an antenna. When this happens, the wire actually absorbs electrical signals from other wires in the cable, and from electrical sources outside the cable. If the resulting electrical noise reaches a high enough level, it can become difficult for NICs to discriminate the noise from the data signal. This is particularly a problem because most LANs use frequencies in the 1-100 megahertz (MHz) frequency region, which happens to be where FM Radio signals, TV signals, and lots of appliances have their operating frequencies as well.</p><p>Take a look at how electrical noise, regardless of the source, impacts digital signals. Imagine that you want to send data, represented by the binary number 1011001001101, over the network. Your computer converts the binary number to a digital signal. Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage2/2.gif" width="12" height="12"> shows what the digital signal for 1011001001101 looks like. The digital signal travels through the networking media to the destination. The destination happens to be near an electrical outlet that is fed by both long neutral and long ground wires. These wires act as possible antennas for electrical noise. Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage3/3.gif" width="12" height="12"> shows what electrical noise looks like.</p><p>Because the destination computer chassis is used for both the earth ground and the signal reference ground, the noise generated interferes with the digital signal that the computer receives. Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage4/4.gif" width="12" height="12"> shows what happens to the signal when it is combined with this electrical noise. Instead of reading the signal as 1011001001101, the computer reads the signal as 1011000101101, making the data unreliable (corrupted).<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage5/5.gif" width="12" height="12"></p><p>Unlike copper wire, optical and wireless systems experience some of these forms of noise but are immune to others. For example, optical fiber is immune to NEXT and AC power/reference ground noise, while wireless systems are particularly prone to EMI/RFI. The focus here has been on noise in copper-based wiring systems. The problem of NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and the use of quality twisted pair cables.</p><p>There is nothing that can be done about thermal noise, other than to give the signals large enough amplitude so that it does not matter. In order to avoid the problem of AC/reference ground as described above, it is important to work closely with your electrical contractor and power company. This will enable you to get the best and shortest electrical ground. One way to do this is to investigate the cost of installing a single power transformer, dedicated to your LAN installation area. If this option can be afforded, the attachment of other devices to the power circuit can be controlled. Restricting how and where devices, such as motors or high-current electrical heaters, are attached can eliminate much of the electrical noise generated by them.</p><p>When working with your electrical contractor, you should ask that separate power distribution panels, known as breaker boxes, be installed for each office area. Since the neutral wires and ground wires from each outlet come together in the breaker box, taking this step will increase your chances of shortening the length of the signal ground. While installing individual power distribution panels for every cluster of computers can increase the up-front cost of the power wiring, it reduces the length of the ground wires, and limits several kinds of signal-burying electrical noise.</p><p>There are a number of ways to limit EMI and RFI. One way is to increase the size of the conductor wires. Another way is to improve the type of insulating material used. However, such changes increase the size and cost of the cable faster than they improve its quality. Therefore, it is more typical for network designers to specify a cable of good quality, and to provide specifications for the maximum recommended cable length between nodes.</p><p>Two techniques that cable designers have used successfully in dealing with EMI and RFI are <i>shielding</i> and <i>cancellation.</i> In cable that employs shielding, a metal braid or foil surrounds each wire pair or group of wire pairs. This shielding acts as a barrier to any interfering signals. However, as with increasing the size of the conductors, using braid or foil covering increases the diameter of the cable and the cost as well. Therefore, cancellation is the more commonly used technique to protect the wire from undesirable interference.</p><p>When electrical current flows through a wire, it creates a small, circular magnetic field around the wire.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47545/RIOID=knet-v214aRIO121136/knet/v214adataimage6/6.gif" width="12" height="12"> The direction of these magnetic lines of force is determined by the direction in which the current flows along the wire. If two wires are part of the same electrical circuit, electrons flow from the negative voltage source to the destination along one wire. Then the electrons flow from the destination to the positive voltage source along the other wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields will cancel each other out. They also will cancel out any outside magnetic fields as well. Twisting the wires can enhance this cancellation effect. By using cancellation in combination with the twisting of wires, cable designers can provide an effective method of providing self-shielding for wire pairs within the network media.</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://epics.aps.anl.gov/techpub/lsnotes/ls232/ls232.html">Tests on Conducted Electrical Noise</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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