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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">4.4</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Basics of Encoding Networking Signals</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">4.4.2</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Modulation and encoding</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 key function of this target indicator is that both terms are used extensively in networking, both terms are similar, but both terms must be distinguished from each other. Modulation refers to using one signal to vary another. Thus in Amplitude modulation, the signal wave varies the amplitude of the carrier wave. In Frequency modulation, the signal wave varies the frequency of the carrier wave. In Phase modulation, the signal wave varies the relative phase of the carrier wave.</p> <p>Encoding is a somewhat broader term. In its most succinct definition, encoding is how binary one and binary zero are represented. We use the term encoding in the broadest sense, meaning how binary 1 and binary 0 (zero) are represented physically. This should be made tangible to the students. Data communications encodes binary 1s and 0s (zeros) as voltages onto copper (using various encoding schemes, such as NRZ, Manchester, 4B/5B), data communications encodes light into optical fibers (again, using various schemes like 4B/5B and 8B/10B), and data communications encodes EM waves into free space (using a wide variety of schemes). Again, encoding is how are the mathematical abstractions (binary ones and zeros) represented in something measurable in the physical world.</p> <p>Also the students should come to appreciate that messages have been historically encoded as voltages on copper wires for at least 150 years. Secondly, they should realize that many modern networks still use voltage pulses on copper wires to achieve data communications. Again, an oscilloscope demonstration is very helpful if at all possible.</p> <p>Also the students should come to appreciate that messages have been historically encoded as visible light pulses for thousands of years, albeit at rather low data rates. Secondly, students should realize that many modern data networks use pulsed LED and laser light on optical fibers and in free space to achieve data communications. A laser pen and an optical fiber patch cable are useful demonstration tools for this target indicator.</p> <p>Also the students should come to appreciate that messages have been historically encoded as electromagnetic waves for about 100 years. Finally, students should realize that many modern data networks use free-space (unbounded) electromagnetic waves to achieve data communication. Such networks are often called wireless networks, and they tend to use the Infrared, Microwave, and Radio Wave parts of the electromagnetic spectrum. An AM/FM radio and an oscilloscope are useful demonstration tools for this target indicator.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Encoding means converting 1s and 0s (zeros) into something real and physical, such as:</p><ul type="disc"><li>an electrical pulse on a wire</li><li>a light pulse on an optical fiber</li><li>a pulse of electromagnetic waves into space</li></ul><p>Two methods of accomplishing this are <i>TTL encoding</i> and <i>Manchester encoding</i>.</p><p>TTL (transistor-transistor logic) encoding is the simplest. It is characterized by a high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 [zero] V for binary 0 [zero]). In optical fibers, binary 1 might be a bright LED or laser light, and binary 0 (zero), dark or no light. In wireless networks, binary 1 might mean a carrier wave is present, and binary 0 (zero), no carrier at all.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47544/RIOID=knet-v214aRIO121141/knet/v214adataimage1/1.gif" width="12" height="12"></p><p>Manchester encoding is more complex, but is more immune to noise and is better at remaining synchronized. In Manchester encoding, the voltage on copper wire, the brightness of LED or laser light in optical fiber, or the power of an EM wave in wireless has the bits encoded as transitions. Observe that the Manchester encoding results in 1 being encoded as a low-to-high transition and 0 (zero) being encoded as a high-to-low transition. Because both 0s (zeros) and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver.</p><p>Closely related to encoding is modulation, which specifically means taking a wave and changing, or modulating it so that it carries information. To give you an idea of what modulation is, examine three forms of modifying, or modulating, a carrier wave to encode bits:</p><ul type="disc"><li> <i>AM (amplitude modulation)</i> - the amplitude, or height, of a carrier sine wave is varied to carry the message</li><li> <i>FM (frequency modulation)</i> - the frequency of the carrier wave is varied to carry the message </li><li> <i>PM (phase modulation)</i> - the phase, or beginning and ending points of a given cycle, of the wave is varied to carry the message</li></ul><p>Other more complex forms of modulation also exist. The Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47544/RIOID=knet-v214aRIO121141/knet/v214adataimage2/2.gif" width="12" height="12"> shows three ways binary data can be encoded onto a carrier wave by the process of modulation. Binary 11 (Note: read as one one, not eleven) can be communicated on a wave by either AM (wave on/wave off), FM (wave wiggles a lot for 1s, a little for 0s [zeros]), or PM (one type of phase change for 0s [zeros], another for 1s).</p><p>Messages can be encoded in a variety of ways:</p><ol><li>As voltages on copper; Manchester and NRZI encoding are popular on copper-based networks.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47544/RIOID=knet-v214aRIO121141/knet/v214adataimage3/3.gif" width="12" height="12"></li><li>As guided light; Manchester and 4B/5B encoding are popular on fiber based networks.</li><li>As radiated EM waves; a wide variety of encoding schemes (variations on AM, FM, and PM) are used on wireless networks.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47544/RIOID=knet-v214aRIO121141/knet/v214adataimage4/4.gif" width="12" height="12"></li></ol><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://www.rad.com/networks/1994/digi_enc/main.htm">Digital Encoding</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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