计算机网络谢希仁试题

<p><font face="Verdana">计算机网络谢希仁试题</font><br/></p>
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<p class="MsoNormal" style="MARGIN-LEFT: 37.5pt; TEXT-INDENT: -37.5pt; mso-list: l0="l0"  level1="level1"  lfo1; tab-stops: list="list"  37.5pt"><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt; mso-bidi-font-family: 宋体"><span style="mso-list: Ignore">第一章<span style="FONT: 7pt="7pt"  'times New="New"  Roman'">&nbsp;&nbsp;&nbsp; </span></span></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">概述（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">P19</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">计算机网络的发展可划分为几个阶段？每个阶段各有何特点？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：计算机网络的发展可分为以下四个阶段。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）面向终端的计算机通信网：其特点是计算机是网络的中心和控制者，终端围绕中心计算机分布在各处，呈分层星型结构，各终端通过通信线路共享主机的硬件和软件资源，计算机的主要任务还是进行批处理，在</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">20</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">世纪</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">60</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">年代出现分时系统后，则具有交互式处理和成批处理能力。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）分组交换网：分组交换网由通信子网和资源子网组成，以通信子网为中心，不仅共享通信子网的资源，还可共享资源子网的硬件和软件资源。网络的共享采用排队方式，即由结点的分组交换机负责分组的存储转发和路由选择，给两个进行通信的用户段续（或动态）分配传输带宽，这样就可以大大提高通信线路的利用率，非常适合突发式的计算机数据。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">3</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）形成计算机网络体系结构：为了使不同体系结构的计算机网络都能互联，国际标准化组织</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">ISO</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">提出了一个能使各种计算机在世界范围内互联成网的标准框架</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">—</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">开放系统互连基本参考模型</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI.</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">。这样，只要遵循</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">标准，一个系统就可以和位于世界上任何地方的、也遵循同一标准的其他任何系统进行通信。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">4</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）高速计算机网络：其特点是采用高速网络技术，综合业务数字网的实现，多媒体和智能型网络的兴起。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、试简述分组交换的特点</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：分组交换实质上是在</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">“</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">存储</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">——</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">转发</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">”</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">基础上发展起来的。它兼有电路交换和报文交换的优点。分组交换在线路上采用动态复用技术传送按一定长度分割为许多小段的数据</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">——</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">分组。每个分组标识后，在一条物理线路上采用动态复用的技术，同时传送多个数据分组。把来自用户发端的数据暂存在交换机的存储器内，接着在网内转发。到达接收端，再去掉分组头将各数据字段按顺序重新装配成完整的报文。分组交换比电路交换的电路利用率高，比报文交换的传输时延小，交互性好。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>3</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、试从多个方面比较电路交换、报文交换和分组交换的主要优缺点。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）电路交换</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">电路交换就是计算机终端之间通信时，一方发起呼叫，独占一条物理线路。当交换机完成接续，对方收到发起端的信号，双方即可进行通信。在整个通信过程中双方一直占用该电路。它的特点是实时性强，时延小，交换设备成本较低。但同时也带来线路利用率低，电路接续时间长，通信效率低，不同类型终端用户之间不能通信等缺点。电路交换比较适用于信息量大、长报文，经常使用的固定用户之间的通信。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）报文交换</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">将用户的报文存储在交换机的存储器中。当所需要的输出电路空闲时，再将该报文发向接收交换机或终端，它以</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">“</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">存储</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">——</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">转发</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">”</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">方式在网内传输数据。报文交换的优点是中继电路利用率高，可以多个用户同时在一条线路上传送，可实现不同速率、不同规程的终端间互通。但它的缺点也是显而易见的。以报文为单位进行存储转发，网络传输时延大，且占用大量的交换机内存和外存，不能满足对实时性要求高的用户。报文交换适用于传输的报文较短、实时性要求较低的网络用户之间的通信，如公用电报网。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">3</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）分组交换</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">分组交换实质上是在</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">“</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">存储</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">——</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">转发</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">”</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">基础上发展起来的。它兼有电路交换和报文交换的优点。分组交换在线路上采用动态复用技术传送按一定长度分割为许多小段的数据</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">——</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">分组。每个分组标识后，在一条物理线路上采用动态复用的技术，同时传送多个数据分组。把来自用户发端的数据暂存在交换机的存储器内，接着在网内转发。到达接收端，再去掉分组头将各数据字段按顺序重新装配成完整的报文。分组交换比电路交换的电路利用率高，比报文交换的传输时延小，交互性好。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>5</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">试讨论在广播式网络中对网络层的处理方法。讨论是否需要这一层？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：广播式网络是属于共享广播信道，不存在路由选择问题，可以不要网络层，但从</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的观点，网络设备应连接到网络层的服务访问点，因此将服务访问点设置在高层协议与数据链路层中逻辑链路子层的交界面上，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">IEEE 802</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">标准就是这样处理的。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>6</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">试将</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">和</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的体系结构进行比较。讨论其异同之处。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">和</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的相同点是二者均采用层次结构，而且都是按功能分层。（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">和</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的不同点：</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 10.5pt; mso-bidi-font-family: 宋体">①</span><span lang="EN-US" style="COLOR: black; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">分七层，自下而上分为物理层、数据链路层、网络层、运输层、会话层、表示层和应用层，而</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">分四层：网络接口层、网间网层（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）、传输层（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）和应用层。严格讲，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">网间网协议只包括下三层，应用程序不算</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的一部分。</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 10.5pt; mso-bidi-font-family: 宋体">②</span><span lang="EN-US" style="COLOR: black; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">层次间存在严格的调用关系，两个（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">N</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）层实体的通信必须通过下一层（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">N-1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）层实体，不能越级，而</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP/IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">可以越过紧邻的下一层直接使用更低层次所提供的服务（这种层次关系常被称为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">“</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">等级</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">”</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">关系），因而减少了一些不必要的开销，提高了协议的效率。</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 10.5pt; mso-bidi-font-family: 宋体">③</span><span lang="EN-US" style="COLOR: black; mso-bidi-font-size: 10.5pt">OSI</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">只考虑用一种标准的公用数据网。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>7</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">计算机网络可从哪几个方面进行分类？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：从网络的交换功能进行分类：电路交换、报文交换、分组交换和混合交换；从网络的拓扑结构进行分类：集中式网络、分散式网络和分布式网络；从网络的作用范围进行分类：广域网</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">WAN</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、局域网</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">LAN</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、城域网</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">MAN</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">；从网络的使用范围进行分类：公用网和专用网。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>8</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、计算机网络中的主干网和本地接入网各有何特点？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：主干网络一般是分布式的，具有分布式网络的特点：其中任何一个结点都至少和其它两个结点直接相连；本地接入网一般是集中式的，具有集中式网络的特点：所有的信息流必须经过中央处理设备（交换结点），链路从中央交换结点向外辐射。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>9</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">计算机网络有哪几部分组成？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：一个计算机网络应当有三个主要的组成部分：（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）若干主机，它们向用户提供服务；（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）一个通信子网，它由一些专用的结点交换机和连接这些结点的通信链路所组成的；（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">3</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）一系列协议，这些协议为主机之间或主机和子网之间的通信而用的。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>10</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、试在下列条件下比较电路交换和分组交换。要传送的报文共</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">x</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">），从源站到目的站共经过</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">k</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">段链路，每段链路的传播时延为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">d</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">），数据率为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">C</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit/s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）。在电路交换时电路的建立时间为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）。在分组交换时分组长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">），且各结点的排队等待时间可忽略不计。问在怎样的条件下，分组交换的时延比电路交换的要小？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：对电路交换，当</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">t=s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">时，链路建立；</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <span lang="EN-US"><br/></span></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">当</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">t=s+x/C</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，发送完最后一</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">；</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">当</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">t=s+x/C+kd</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，所有的信息到达目的地。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">对分组交换，当</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">t=x/C</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">发送完最后一</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">；</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">为到达目的地，最后一个分组需经过</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">k-1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">个分组交换机的转发，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">每次转发的时间为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p/C</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">所以总的延迟</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">= x/C+(k-1)p/C+kd <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">所以当分组交换的时延小于电路交换</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>x/C+(k-1)p/C+kd</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">＜</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">s+x/C+kd</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">时，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>(k-1)p/C</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">＜</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">s <br/><br/>11</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、在上题的分组交换网中，设报文长度和分组长度分别为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">x</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">和（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p+h</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">），其中</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">为分组的数据部分的长度，而</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">h</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">为每个分组所带的控制信息固定长度，与</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的大小无关。通信的两端共经过</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">k</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">段链路。链路的数据率为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">b</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">bit/s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">），但传播时延和结点的排队时间均可忽略不计。若打算使总的时延为最小，问分组的数据部分长度</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">应取为多大？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：分组个</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">x/p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">传输的总比特数：</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">(p+h)x/p <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">源发送时延：</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">(p+h)x/pb <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">最后一个分组经过</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">k-1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">个分组交换机的转发，中间发送时延：</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">(k-1)(p+h)/b <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">总发送时延</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">D=</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">源发送时延</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">+</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">中间发送时延</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>D=(p+h)x/pb+(k-1)(p+h)/b <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">令其对</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">p</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">的导数等于</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">0</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，求极值</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>p=√hx/(k-1) <br/><br/>13</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、面向连接服务与无连接服务各自的特点是什么？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：面向连接服务在数据交换之前必须先建立连接，保留下层的有关资源，数据交换结束后，应终止这个连接，释放所保留的资源。而对无连接服务，两个实体之间不建立连接就可以通信，在数据传输时动态地分配下层资源，不需要事先进行预保留。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">面向连接服务的特点是，在服务进行之前必须建立数据链路</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">(</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">虚电路</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">)</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">然后在进行数据传输，传输完毕后，再释放连接。在数据传输时，好象一直占用了一条这样的电路。适合于在一定期间内要向同一目的地发送许多报文的情况。对传输数据安全，不容易丢失和失序。但由于虚电路的建立，维护和释放要耗费一定的资源和时间。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">无连接服务的特点，在服务工程中不需要先建立虚电路，链路资源在数据传输过程中动态进行分配。灵活方便，比较迅速；但不能防止报文的丢失、重复或失序。适合于传送少量零星的报文。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>14</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">协议与服务又何区别？有何关系？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、协议是控制对等实体之间通信的规则，是水平的。服务是下层通过层间接口向上层提供的功能，是垂直的。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/>2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、协议的实现保证了能够向上一层提供服务，要实现本层协议还需使用下层提供的服务。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/><br/>20</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、试计算以下两种情况的发送时延和传播时延：</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">数据长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">107bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，数据发送速率为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">100kbit/s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，传播距离为</span><?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:chmetcnv w:st="on" unitname="km" sourcevalue="1000" hasspace="False" negative="False" numbertype="1" tcsc="0"><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1000km</span></st1:chmetcnv><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，信号在媒体上的传播速率为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2×<st1:chmetcnv w:st="on" unitname="m" sourcevalue="108" hasspace="False" negative="False" numbertype="1" tcsc="0">108m</st1:chmetcnv>/s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> </span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">数据长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">103bit</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，数据发送速率为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1Gbit/s</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">，传输距离和信号在媒体上的传播速率同上。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）：发送延迟</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=107/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">100×1000</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=100s <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">传播延迟</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=1000×1000/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2×108</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=5×10-3s=5ms <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）：发送延迟</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=103/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">109</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=10-6s=1us <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">传播延迟</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=1000×1000/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">2×108</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=5×10-3s=5ms <br/><br/>22</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">、长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">100</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节的应用层数据交给运输层传送，需加上</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">20</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节的</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">TCP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">首部。再交给网络层传送，需加上</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">20</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节的</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">IP</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">首部。最后交给数据链路层的以太网传送，加上首部和尾部</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">18</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节。试求数据的传输效率。</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">若应用层数据长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1000</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节，数据的传输效率是多少？</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">答：数据长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">100</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节时</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">传输效率</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=100/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">100+20+20+18</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=63.3% <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">数据长度为</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1000</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">字节时，</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt"> <br/></span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">传输效率</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=1000/</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">（</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">1000+20+20+18</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-bidi-font-size: 10.5pt">）</span><span lang="EN-US" style="COLOR: black; FONT-FAMILY: ??; mso-bidi-font-size: 10.5pt">=94.5%<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></span></p>
<p><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">物理层（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">P66</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、物理层要解决哪些问题？物理层的主要特点是什么？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）物理层要解决的主要问题：</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 宋体; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">①</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">物理层要尽可能屏蔽掉物理设备、传输媒体和通信手段的不同，使上面的数据链路层感觉不到这些差异的存在，而专注于完成本层的协议与服务。　　</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 宋体; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">②</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">给其服务用户（数据链路层）在一条物理的传输媒体上传送和接收比特流（一般为串行按顺序传输的比特流）的能力。为此，物理层应解决物理连接的建立、维持和释放问题。</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 宋体; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">③</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">在两个相邻系统之间唯一地标识数据电路。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）物理层的主要特点：</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 宋体; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">①</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">由于在</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">OSI</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">之前，许多物理规程或协议已经制定出来了，而且在数据通信领域中，这些物理规程已被许多商品化的设备所采用。加之，物理层协议涉及的范围广泛，所以至今没有按</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">OSI</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的抽象模型制定一套新的物理层协议，而是沿用已存在的物理规程，将物理层确定为描述与传输媒体接口的机械、电气、功能和规程特性。</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 宋体; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">②</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">由于物理连接的方式很多，传输媒体的种类也很多，因此，具体的物理协议相当复杂。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>4</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、、物理层的接口有哪些方面的特性？各包含什么内容？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">(1)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">机械特牲</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">说明接口所用接线器的形状和尺寸、引线数目和排列、固定和锁定装置等等。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> (2)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">电气特性</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">说明在接口电缆的哪条线上出现的电压应为什么范围。即什么样的电压表示</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">或</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">0</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> (3)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">功能特性</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">说明某条线上出现的某一电平的电压表示何种意义。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">(4)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">规程特性</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">说明对于不同功能的各种可能事件的出现顺序。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>5</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、奈氏准则与香农公式在数据通信中的意义是什么？比特和波特有何区别？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：奈氏准则与香农公式的意义在于揭示了信道对数据传输率的限制，只是两者作用的范围不同。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">奈氏准则给出了每赫带宽的理想低通信道的最高码元的传输速率是每秒</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">个码元。香农公式则推导出了带宽受限且有高斯白噪声干扰的信道的极限信息传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">C=Wlog2</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1+S/N</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">），其中</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">W</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">为信道的带宽（以赫兹为单位），</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">S</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">为信道内所传信号的平均功率，</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">N</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">为信道内部的高斯噪声功率。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">比特和波特是两个完全不同的概念，比特是信息量的单位，波特是码元传输的速率单位。但信息的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">比特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">/</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">每秒</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">” </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">一般在数量上大于码元的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">波特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，且有一定的关系，若使</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">个码元携带</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">n</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">比特的信息量，则</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">M Baud</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的码元传输速率所对应的信息传输率为</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">M×n bit/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，但某些情况下，信息的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">比特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">/</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">每秒</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">” </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">在数量上小于码元的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">波特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，如采用内带时钟的曼切斯特编码，一半的信号变化用于时钟同步，另一半的信号变化用于信息二进制数据，码元的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">波特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">是信息的传输速率</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">比特</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">/</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">每秒</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">倍。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>7</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">常见的传输媒体有哪几种？各有何特点？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">(1)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">双绞线：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">抗电磁干扰</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> ●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">模拟传输和数字传输都可以使用双绞线</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>(2)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">同轴电缆：同轴电缆具有很好的抗干扰特性</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>(3)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">光纤：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">传输损耗小，中继距离长，对远距离传输特别经济；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">抗雷电和电磁干扰性能好；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">无串音干扰，保密性好，也不易被窃听或截取数据；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">体积小，重量轻。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>(4)</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">电磁波：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">微波波段频率很高，其频段范围也很宽，因此其通信信道的容量很大；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> ●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">微波传输质量较高；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">微波接力通信的可靠性较高；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">微波接力通信与相同容量和长度的电缆载波通信比较，建设投资少，见效快。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">当然，微波接力通信也存在如下的一些缺点：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">相邻站之间必须直视，不能有障碍物。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">微波的传播有时也会受到恶劣气候的影响；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">与电缆通信系统比较，微波通信的隐蔽性和保密性较差；</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">●</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">对大量的中继站的使用和维护要耗费一定的人力和物力。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>8</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、什么是曼彻斯特编码和差分曼彻斯特编码？其特点如何？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：曼彻斯特编码是将每一个码元再分成两个相等的间隔。码元</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">是在前一个间隔为高电平而后一个间隔为低电平。码元</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">0</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">则正好相反，从低电平变到高电平。这种编码的好处是可以保证在每一个码元的正中间出现一次电平的转换，这对接收端的提取位同步信号是非常有利的。缺点是它所占的频带宽度比原始的基带信号增加了一倍。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">差分曼彻斯特编码的规则是若码元为</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，则其前半个码元的电平与上一个码元的后半个码元的电平一样；但若码元为</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">0</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，则其前半个码元的电平与上一个码元的后半个码元的电平相反。不论码元是</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">10</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">或，在每个码元的正中间的时刻，一定要有一次电平的转换。差分曼彻斯特编码需要较复杂的技术，但可以获得较好的抗干扰性能。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>9</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">模拟传输系统与数字传输系统的主要特点是什么？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：模拟传输：只能传模拟信号，信号会失真。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">数字传输：可传模拟与数字信号，噪声不累计，误差小。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/><br/>10</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> EIA-232</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">和</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">RS-449</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">接口标准各用在什么场合？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：通常</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">EIA-232</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">用于标准电话线路（一个话路）的物理层接口，而</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">RS-449</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">则用于宽待电路（一般是租用电路）</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>11</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、基带信号和宽带信号的传输各有什么特点？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）基带信号是将数字信号</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">或</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">0</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">直接用两种不同的电压来表示，然后送到线路上去传输。（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）宽带信号则是将基带信号进行调制后形成的频分复用模拟信号。基带信号进行调制后，其频谱移到较高的频率处。由于每一路基带信号的频谱被搬移到不同的频段上，因此合在一起后并不会互相干扰。这样做可以在一条线路中同时传送许多路的数字信号，因而提高了线路的利用率。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>12</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">有</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">600MB</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">（兆字节）的数据，需要从南京传送到北京。一种方法是将数据写到磁盘上，然后托人乘火车将这些磁盘捎去。另一种方法是用计算机通过长途电话线路（设信息传送的速率是</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2.4Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）传送此数据。试比较这两种方法的优劣。若信息传送速率为</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，其结果又如何？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：假定连续传送且不出错。若用</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">2.4Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">速率，传</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">600MB</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">（</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">=600×1048576×8=5033164800 bit</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">）需要</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">24.3</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">天。若用</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">速率传送，则需时间</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">1.73</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">天。比托人乘火车捎去要慢，且更贵。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>13</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> 56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的调制解调器是否突破了香农的信道极限传输速率？这种调制解调器的使用条件是什么？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的调制解调器主要用于用户与</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的通信，这时从用户到</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">之间只需经过一次</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">A/D</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">转换，比两个用户之间使用的</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">调制解调器的量化噪声要小，所以信噪比进一步提高。虽然</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">调制解调器的速率基本已达到香农的信道极限传输速率，但是</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的调制解调器的使用条件不同，它提高了信噪比，它没有突破香农极限传输速率的公式。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的调制解调器的使用条件是</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">也使用这种调制解调器（这里是为了进行数字信号不同编码之间的转换，而不是数模转换），并且在</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">与电话交换机之间是数字信道。若</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">使用的只是</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">调制解调器，则用户端的</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">的调制解调器会自动降低到与</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">33.6Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">调制解调器相同的速率进行通信。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>14</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> </span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">在介绍双绞线时，我们说：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">在数字传输时，若传输速率为每秒几个兆比特，则传输距离可达几公里。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">但目前我们使用调制解调器与</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">相连时，数据的传输速率最高只能达到</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">56Kb/s</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">，与每秒几个兆比特相距甚远。这是为什么？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">“</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">在数字传输时，若传输速率为每秒几个兆比特，则传输距离可达几公里。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">”</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">这是指使用数字线路，其两端的设备并没有带宽的限制。当我们使用调制解调器与</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ISP</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">相连时，使用的是电话的用户线。这种用户线进入市话交换机处将带宽限制在</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">3400Hz</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">以下，与数字线路的带宽相差很大。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/>15</span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">、传播时延、发送时延和重发时延各自的物理意义是什么？</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">答：传播时延是指电磁波在信道中传输所需要的时间。它取决于电磁波在信道上的传输速率以及所传播的距离。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">发送时延是发送数据所需要的时间。它取决于数据块的长度和数据在信道上的发送速率。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br/></span><span style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: 宋体; mso-ascii-font-family: ??; mso-hansi-font-family: ??; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">重发时延是因为数据在传输中出了差错就要重新传送，因而增加了总的数据传输时间。</span><span lang="EN-US" style="FONT-SIZE: 10.5pt; COLOR: black; FONT-FAMILY: ??; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-bidi-font-family: 'Times New="New"  Roman'; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"> <br style="mso-special-character: line-break"/><br style="mso-special-character: line-break"/></span></p>

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