数据传输和网络拓扑

翔羽

<p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span lang="EN-US"><font face="Times New Roman">1</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、扩展频谱方式</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　在扩展频谱方式展频谱方式中，数据基带信号的频谱被扩展至几倍</span><span lang="EN-US"><font face="Times New Roman">-</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">几十倍后再被搬移至射频发射出去。这一作法虽然牺牲了频带带宽，却提高了通信系统的抗干扰能力和安全性。由于单位频带内的功率降低，对其它电子设备的干扰也减小了。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　采用扩展频谱方式的无线局域网一般选择所谓</span><span lang="EN-US"><font face="Times New Roman">ISM</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">频段，这里</span><span lang="EN-US"><font face="Times New Roman">ISM</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">分别取于</span><span lang="EN-US"><font face="Times New Roman">Industrial</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、</span><span lang="EN-US"><font face="Times New Roman">Scientific</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">及</span><span lang="EN-US"><font face="Times New Roman">Medical</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">的第一个字母。许多工业、科研和医疗设备辐射的能量集中于该频段，例如美国</span><span lang="EN-US"><font face="Times New Roman">ISM</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">频段由</span><span lang="EN-US"><font face="Times New Roman">902MHz-928MHz</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">，</span><span lang="EN-US"><font face="Times New Roman">2.4GHz-2.48GHz</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">，</span><span lang="EN-US"><font face="Times New Roman">5.725GHz-5.850GHz</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">三个频段组成。如果发射功率及带宽辐射满足美国联邦通信委员会（</span><span lang="EN-US"><font face="Times New Roman">FCC</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">）的要求，则无须向</span><span lang="EN-US"><font face="Times New Roman">FCC</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">提出专门的申请即可使用</span><span lang="EN-US"><font face="Times New Roman">ISM</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">频段。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　</span><span lang="EN-US"><font face="Times New Roman">2</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、窄带调制方式</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　在窄带调制方式中，数据基带信号的频谱不做任何扩展即被直接搬移到射频发射出去。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　与扩展频谱方式相比，窄带调制方式占用频带少，频带利用率高。采用窄带调制方式奈尴呔钟蛲</span><span style="FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体;">?话阊∮米ㄓ闷刀危?枰????椅尴叩绻芾聿棵诺男砜煞娇墒褂谩５比唬?部裳∮肐</span><span lang="EN-US"><font face="Times New Roman">SM</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">频段，这样可免去向无线电管理委员会申请。但带来的问题是，当临近的仪器设备或通信设备也在使用这一频段时，会严重影响通信质量，通信的可靠性无法得到保障。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　</span><span lang="EN-US"><font face="Times New Roman">3</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、红外线方式</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　基于红外线的传输技术最近几年有了很大发展。目前广泛使用的家电遥控器几乎都是采用红外线传输技术。做为无线局域网的传输方式，红外线的最大优点是这种传输方式不受无线电干扰，且红外线的使用不受国家无线电管理委员会的限制。然而，红外线对非透明物体的透过性极差，这导致传输距离受限。</span><font face="Times New Roman"> </font></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';"></span>&nbsp;</p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">网络拓扑</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　无线局域网的扩扑结构可归结为两类：无中心或对等式（</span><span lang="EN-US"><font face="Times New Roman">eer to Peer</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">）拓扑和有中心（</span><span lang="EN-US"><font face="Times New Roman">HUB-Based</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">）拓扑。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　</span><span lang="EN-US"><font face="Times New Roman">1</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、无中心拓扑</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　无中心拓扑的网络要求网中任意两个站点均可直接通信。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　采用这种拓扑结构的网络一般是用公用广播信道，各站点都可竞争公用信道，而信道接入控制（</span><span lang="EN-US"><font face="Times New Roman">MAC</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">）协议大多采用</span><span lang="EN-US"><font face="Times New Roman">CSMA</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">（载波监测多址接入）类型的多址接入协议。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　这种结构的优点是网络抗毁性好、建网容易、且费用较低。但当网中用户数（站点数）过多时，信道竞争成为限制网络性能的要害。并且为了满足任意两个站点可直接通信，网络中站点布局受环境限制较大。因此这种拓扑结构适用于用户相对减少的工作群网络规模。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　</span><span lang="EN-US"><font face="Times New Roman">2</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">、有中心拓扑</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　在中心拓扑结构中，要求一个无线站点充当中心站，所有站点对网络的访问均由其控制。　　这样，当网络业务量增大时网络吞吐性能及网络时延性能的而恶化并不剧烈。由于每个站点只需在中心站覆盖范围之内就可与其它站点通信，故网络中点站布局受环境限制亦小。此外，中心站为接入有线主干网提供了一个逻辑接入点。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　有中心网络拓扑结构的弱点是抗毁性差，中心点的故障容易导致整个网络瘫痪，并且中心站点的引入增加了网络成本。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　在实际应用中，无线网往往与有线主干网络结合起来使用。这时，中心站点充当无线网与有线主干网的转接器。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　三、网络接口</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　这涉及无线网中站点从哪一层接入网络系统。一般来讲，网络接口可以选择在</span><span lang="EN-US"><font face="Times New Roman">OSI</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">参考模型的物理层或数据链路层。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　所谓物理层接口指使用无线信道替代通常的有线信道，而物理层以上各层不变。这样做的最大优点是上层的网络操作系统及相应的驱动程序可不做任何修改。这种接口放式在使用时一般做为有线网的集线器和无线转发器以实现有线局域网间互连或扩大有线局域网的覆盖面积。</span></p><p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt;"><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">　　另一种接口方法是从数据链路层接入网络。这种接口方法并不沿用有线局域网的</span><span lang="EN-US"><font face="Times New Roman">MCA</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">协议，而采用更适合无线传输环境的</span><span lang="EN-US"><font face="Times New Roman">MAC</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">协议。在实现时，</span><span lang="EN-US"><font face="Times New Roman">MAC</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">层及其及其以下层对上层是透明的</span><span lang="EN-US"><font face="Times New Roman">,</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">配置相应的驱动程序来完成域上层的接口</span><span lang="EN-US"><font face="Times New Roman">,</font></span><span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman';">这样可保证现有的有线局域网操作系统或应用软件可在无线局域网上正常运转。</span></p><span style="FONT-SIZE: 10.5pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-bidi-font-size: 12.0pt; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;">　　目前，大部分无线局域网厂商都采用数据链路层接口方法</span>