《计算机网络自顶向下方法》读书笔记（一）

计算机网络自顶向下

计算机网络与因特网
排队时延和分组丢失

计算机网络与因特网

一、具体构成的描述：对于什么是因特网这个问题有两种回答方式：其一，我们能够描述因特网的具体构成，即因特网的基本硬件和软件组件；其二，我们能够根据分布式应用提供的联网基础设施来描述因特网。
1、端系统
（1）定义：端系统（主机）：处于因特网的边缘部分就是在因特网上的所有主机，即与因特网相连的设备，此处的“端”是“末端”之意。端系统可以是手表、平板电脑、电视等。
（2）具体功能：
<1>端系统通过通信链路和分组交换机连接在一起。（对于通信链路和分组交换机的详细介绍会在下一章）。通信链路有不同类型的物理媒体组成，这些物理媒体包括同轴电缆、铜线、光纤和无线电频谱。不同的链路能够以不同的速率传输数据，链路的传输速率以比特/秒度量。当一台端系统向另一台端系统发送数据时，发送端系统将数据分段，并为每段加上首部字节。由此形成的信息包用计算机网络的术语来说称为分组。

#mermaid-svg-VTaoKSaNOdzBQBUf .label{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);fill:#333;color:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .label text{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .node rect,#mermaid-svg-VTaoKSaNOdzBQBUf .node circle,#mermaid-svg-VTaoKSaNOdzBQBUf .node ellipse,#mermaid-svg-VTaoKSaNOdzBQBUf .node polygon,#mermaid-svg-VTaoKSaNOdzBQBUf .node path{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-VTaoKSaNOdzBQBUf .node .label{text-align:center;fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .node.clickable{cursor:pointer}#mermaid-svg-VTaoKSaNOdzBQBUf .arrowheadPath{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .edgePath .path{stroke:#333;stroke-width:1.5px}#mermaid-svg-VTaoKSaNOdzBQBUf .flowchart-link{stroke:#333;fill:none}#mermaid-svg-VTaoKSaNOdzBQBUf .edgeLabel{background-color:#e8e8e8;text-align:center}#mermaid-svg-VTaoKSaNOdzBQBUf .edgeLabel rect{opacity:0.9}#mermaid-svg-VTaoKSaNOdzBQBUf .edgeLabel span{color:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .cluster rect{fill:#ffffde;stroke:#aa3;stroke-width:1px}#mermaid-svg-VTaoKSaNOdzBQBUf .cluster text{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:12px;background:#ffffde;border:1px solid #aa3;border-radius:2px;pointer-events:none;z-index:100}#mermaid-svg-VTaoKSaNOdzBQBUf .actor{stroke:#ccf;fill:#ECECFF}#mermaid-svg-VTaoKSaNOdzBQBUf text.actor>tspan{fill:#000;stroke:none}#mermaid-svg-VTaoKSaNOdzBQBUf .actor-line{stroke:grey}#mermaid-svg-VTaoKSaNOdzBQBUf .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .messageLine1{stroke-width:1.5;stroke-dasharray:2, 2;stroke:#333}#mermaid-svg-VTaoKSaNOdzBQBUf #arrowhead path{fill:#333;stroke:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .sequenceNumber{fill:#fff}#mermaid-svg-VTaoKSaNOdzBQBUf #sequencenumber{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf #crosshead path{fill:#333;stroke:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .messageText{fill:#333;stroke:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .labelBox{stroke:#ccf;fill:#ECECFF}#mermaid-svg-VTaoKSaNOdzBQBUf .labelText,#mermaid-svg-VTaoKSaNOdzBQBUf .labelText>tspan{fill:#000;stroke:none}#mermaid-svg-VTaoKSaNOdzBQBUf .loopText,#mermaid-svg-VTaoKSaNOdzBQBUf .loopText>tspan{fill:#000;stroke:none}#mermaid-svg-VTaoKSaNOdzBQBUf .loopLine{stroke-width:2px;stroke-dasharray:2, 2;stroke:#ccf;fill:#ccf}#mermaid-svg-VTaoKSaNOdzBQBUf .note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-VTaoKSaNOdzBQBUf .noteText,#mermaid-svg-VTaoKSaNOdzBQBUf .noteText>tspan{fill:#000;stroke:none}#mermaid-svg-VTaoKSaNOdzBQBUf .activation0{fill:#f4f4f4;stroke:#666}#mermaid-svg-VTaoKSaNOdzBQBUf .activation1{fill:#f4f4f4;stroke:#666}#mermaid-svg-VTaoKSaNOdzBQBUf .activation2{fill:#f4f4f4;stroke:#666}#mermaid-svg-VTaoKSaNOdzBQBUf .mermaid-main-font{font-family:"trebuchet ms", verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .section{stroke:none;opacity:0.2}#mermaid-svg-VTaoKSaNOdzBQBUf .section0{fill:rgba(102,102,255,0.49)}#mermaid-svg-VTaoKSaNOdzBQBUf .section2{fill:#fff400}#mermaid-svg-VTaoKSaNOdzBQBUf .section1,#mermaid-svg-VTaoKSaNOdzBQBUf .section3{fill:#fff;opacity:0.2}#mermaid-svg-VTaoKSaNOdzBQBUf .sectionTitle0{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .sectionTitle1{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .sectionTitle2{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .sectionTitle3{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .sectionTitle{text-anchor:start;font-size:11px;text-height:14px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .grid .tick{stroke:#d3d3d3;opacity:0.8;shape-rendering:crispEdges}#mermaid-svg-VTaoKSaNOdzBQBUf .grid .tick text{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .grid path{stroke-width:0}#mermaid-svg-VTaoKSaNOdzBQBUf .today{fill:none;stroke:red;stroke-width:2px}#mermaid-svg-VTaoKSaNOdzBQBUf .task{stroke-width:2}#mermaid-svg-VTaoKSaNOdzBQBUf .taskText{text-anchor:middle;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .taskText:not([font-size]){font-size:11px}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutsideRight{fill:#000;text-anchor:start;font-size:11px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutsideLeft{fill:#000;text-anchor:end;font-size:11px}#mermaid-svg-VTaoKSaNOdzBQBUf .task.clickable{cursor:pointer}#mermaid-svg-VTaoKSaNOdzBQBUf .taskText.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutsideLeft.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutsideRight.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-VTaoKSaNOdzBQBUf .taskText0,#mermaid-svg-VTaoKSaNOdzBQBUf .taskText1,#mermaid-svg-VTaoKSaNOdzBQBUf .taskText2,#mermaid-svg-VTaoKSaNOdzBQBUf .taskText3{fill:#fff}#mermaid-svg-VTaoKSaNOdzBQBUf .task0,#mermaid-svg-VTaoKSaNOdzBQBUf .task1,#mermaid-svg-VTaoKSaNOdzBQBUf .task2,#mermaid-svg-VTaoKSaNOdzBQBUf .task3{fill:#8a90dd;stroke:#534fbc}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutside0,#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutside2{fill:#000}#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutside1,#mermaid-svg-VTaoKSaNOdzBQBUf .taskTextOutside3{fill:#000}#mermaid-svg-VTaoKSaNOdzBQBUf .active0,#mermaid-svg-VTaoKSaNOdzBQBUf .active1,#mermaid-svg-VTaoKSaNOdzBQBUf .active2,#mermaid-svg-VTaoKSaNOdzBQBUf .active3{fill:#bfc7ff;stroke:#534fbc}#mermaid-svg-VTaoKSaNOdzBQBUf .activeText0,#mermaid-svg-VTaoKSaNOdzBQBUf .activeText1,#mermaid-svg-VTaoKSaNOdzBQBUf .activeText2,#mermaid-svg-VTaoKSaNOdzBQBUf .activeText3{fill:#000 !important}#mermaid-svg-VTaoKSaNOdzBQBUf .done0,#mermaid-svg-VTaoKSaNOdzBQBUf .done1,#mermaid-svg-VTaoKSaNOdzBQBUf .done2,#mermaid-svg-VTaoKSaNOdzBQBUf .done3{stroke:grey;fill:#d3d3d3;stroke-width:2}#mermaid-svg-VTaoKSaNOdzBQBUf .doneText0,#mermaid-svg-VTaoKSaNOdzBQBUf .doneText1,#mermaid-svg-VTaoKSaNOdzBQBUf .doneText2,#mermaid-svg-VTaoKSaNOdzBQBUf .doneText3{fill:#000 !important}#mermaid-svg-VTaoKSaNOdzBQBUf .crit0,#mermaid-svg-VTaoKSaNOdzBQBUf .crit1,#mermaid-svg-VTaoKSaNOdzBQBUf .crit2,#mermaid-svg-VTaoKSaNOdzBQBUf .crit3{stroke:#f88;fill:red;stroke-width:2}#mermaid-svg-VTaoKSaNOdzBQBUf .activeCrit0,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCrit1,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCrit2,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCrit3{stroke:#f88;fill:#bfc7ff;stroke-width:2}#mermaid-svg-VTaoKSaNOdzBQBUf .doneCrit0,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCrit1,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCrit2,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCrit3{stroke:#f88;fill:#d3d3d3;stroke-width:2;cursor:pointer;shape-rendering:crispEdges}#mermaid-svg-VTaoKSaNOdzBQBUf .milestone{transform:rotate(45deg) scale(0.8, 0.8)}#mermaid-svg-VTaoKSaNOdzBQBUf .milestoneText{font-style:italic}#mermaid-svg-VTaoKSaNOdzBQBUf .doneCritText0,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCritText1,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCritText2,#mermaid-svg-VTaoKSaNOdzBQBUf .doneCritText3{fill:#000 !important}#mermaid-svg-VTaoKSaNOdzBQBUf .activeCritText0,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCritText1,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCritText2,#mermaid-svg-VTaoKSaNOdzBQBUf .activeCritText3{fill:#000 !important}#mermaid-svg-VTaoKSaNOdzBQBUf .titleText{text-anchor:middle;font-size:18px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf g.classGroup text{fill:#9370db;stroke:none;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:10px}#mermaid-svg-VTaoKSaNOdzBQBUf g.classGroup text .title{font-weight:bolder}#mermaid-svg-VTaoKSaNOdzBQBUf g.clickable{cursor:pointer}#mermaid-svg-VTaoKSaNOdzBQBUf g.classGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-VTaoKSaNOdzBQBUf g.classGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf .classLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.5}#mermaid-svg-VTaoKSaNOdzBQBUf .classLabel .label{fill:#9370db;font-size:10px}#mermaid-svg-VTaoKSaNOdzBQBUf .relation{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-VTaoKSaNOdzBQBUf .dashed-line{stroke-dasharray:3}#mermaid-svg-VTaoKSaNOdzBQBUf #compositionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #compositionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #aggregationStart{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #aggregationEnd{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #dependencyStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #dependencyEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #extensionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf #extensionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf .commit-id,#mermaid-svg-VTaoKSaNOdzBQBUf .commit-msg,#mermaid-svg-VTaoKSaNOdzBQBUf .branch-label{fill:lightgrey;color:lightgrey;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .pieTitleText{text-anchor:middle;font-size:25px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .slice{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf g.stateGroup text{fill:#9370db;stroke:none;font-size:10px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf g.stateGroup text{fill:#9370db;fill:#333;stroke:none;font-size:10px}#mermaid-svg-VTaoKSaNOdzBQBUf g.statediagram-cluster .cluster-label text{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf g.stateGroup .state-title{font-weight:bolder;fill:#000}#mermaid-svg-VTaoKSaNOdzBQBUf g.stateGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-VTaoKSaNOdzBQBUf g.stateGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-VTaoKSaNOdzBQBUf .transition{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-VTaoKSaNOdzBQBUf .stateGroup .composit{fill:white;border-bottom:1px}#mermaid-svg-VTaoKSaNOdzBQBUf .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px}#mermaid-svg-VTaoKSaNOdzBQBUf .state-note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-VTaoKSaNOdzBQBUf .state-note text{fill:black;stroke:none;font-size:10px}#mermaid-svg-VTaoKSaNOdzBQBUf .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-VTaoKSaNOdzBQBUf .edgeLabel text{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-VTaoKSaNOdzBQBUf .node circle.state-start{fill:black;stroke:black}#mermaid-svg-VTaoKSaNOdzBQBUf .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-VTaoKSaNOdzBQBUf #statediagram-barbEnd{fill:#9370db}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-state .divider{stroke:#9370db}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-VTaoKSaNOdzBQBUf .note-edge{stroke-dasharray:5}#mermaid-svg-VTaoKSaNOdzBQBUf .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-VTaoKSaNOdzBQBUf .error-icon{fill:#522}#mermaid-svg-VTaoKSaNOdzBQBUf .error-text{fill:#522;stroke:#522}#mermaid-svg-VTaoKSaNOdzBQBUf .edge-thickness-normal{stroke-width:2px}#mermaid-svg-VTaoKSaNOdzBQBUf .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-VTaoKSaNOdzBQBUf .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-VTaoKSaNOdzBQBUf .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-VTaoKSaNOdzBQBUf .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-VTaoKSaNOdzBQBUf .marker{fill:#333}#mermaid-svg-VTaoKSaNOdzBQBUf .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-VTaoKSaNOdzBQBUf {color: rgba(0, 0, 0, 0.75);font: ;}

数据分段 -- 加上首部字节

发送端系统

目的端系统

<2>端系统通过因特网服务提供商（ISP）接入因特网，每个ISP自身就是一个由多台分组交换机和多段通信链路组成的网络。各ISP为端系统提供了各种不同类型的网络接入。
<3>协议：端系统、分组交换机和其他因特网部件都要运行一系列协议。协议相关知识可见计算机网络开放的端口和协议
（3）服务描述
我们现在从为应用程序提供服务的基础设施的角度来描述因特网。因特网除了传统的web冲浪应用外，还有在线社交网络、视频会议等。因为这些应用程序涉及多个相互交换数据的端系统，所以它们被称为分布式应用程序。
2、接入网
(1)家庭接入:DSL、电缆、FTTH、拔号和卫星
当前，宽带住宅接入有两种最流行的类型:数字用户线和电缆。
<1>电话线接入：住户通常从提供本地电话接入的本地电话公司处获得DSL因特网接入。因此，当使用DSL时，用户的本地电话公司也是它的ISP。

#mermaid-svg-5dQMg5UItUlOnwbj .label{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);fill:#333;color:#333}#mermaid-svg-5dQMg5UItUlOnwbj .label text{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .node rect,#mermaid-svg-5dQMg5UItUlOnwbj .node circle,#mermaid-svg-5dQMg5UItUlOnwbj .node ellipse,#mermaid-svg-5dQMg5UItUlOnwbj .node polygon,#mermaid-svg-5dQMg5UItUlOnwbj .node path{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-5dQMg5UItUlOnwbj .node .label{text-align:center;fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .node.clickable{cursor:pointer}#mermaid-svg-5dQMg5UItUlOnwbj .arrowheadPath{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .edgePath .path{stroke:#333;stroke-width:1.5px}#mermaid-svg-5dQMg5UItUlOnwbj .flowchart-link{stroke:#333;fill:none}#mermaid-svg-5dQMg5UItUlOnwbj .edgeLabel{background-color:#e8e8e8;text-align:center}#mermaid-svg-5dQMg5UItUlOnwbj .edgeLabel rect{opacity:0.9}#mermaid-svg-5dQMg5UItUlOnwbj .edgeLabel span{color:#333}#mermaid-svg-5dQMg5UItUlOnwbj .cluster rect{fill:#ffffde;stroke:#aa3;stroke-width:1px}#mermaid-svg-5dQMg5UItUlOnwbj .cluster text{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:12px;background:#ffffde;border:1px solid #aa3;border-radius:2px;pointer-events:none;z-index:100}#mermaid-svg-5dQMg5UItUlOnwbj .actor{stroke:#ccf;fill:#ECECFF}#mermaid-svg-5dQMg5UItUlOnwbj text.actor>tspan{fill:#000;stroke:none}#mermaid-svg-5dQMg5UItUlOnwbj .actor-line{stroke:grey}#mermaid-svg-5dQMg5UItUlOnwbj .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333}#mermaid-svg-5dQMg5UItUlOnwbj .messageLine1{stroke-width:1.5;stroke-dasharray:2, 2;stroke:#333}#mermaid-svg-5dQMg5UItUlOnwbj #arrowhead path{fill:#333;stroke:#333}#mermaid-svg-5dQMg5UItUlOnwbj .sequenceNumber{fill:#fff}#mermaid-svg-5dQMg5UItUlOnwbj #sequencenumber{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj #crosshead path{fill:#333;stroke:#333}#mermaid-svg-5dQMg5UItUlOnwbj .messageText{fill:#333;stroke:#333}#mermaid-svg-5dQMg5UItUlOnwbj .labelBox{stroke:#ccf;fill:#ECECFF}#mermaid-svg-5dQMg5UItUlOnwbj .labelText,#mermaid-svg-5dQMg5UItUlOnwbj .labelText>tspan{fill:#000;stroke:none}#mermaid-svg-5dQMg5UItUlOnwbj .loopText,#mermaid-svg-5dQMg5UItUlOnwbj .loopText>tspan{fill:#000;stroke:none}#mermaid-svg-5dQMg5UItUlOnwbj .loopLine{stroke-width:2px;stroke-dasharray:2, 2;stroke:#ccf;fill:#ccf}#mermaid-svg-5dQMg5UItUlOnwbj .note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-5dQMg5UItUlOnwbj .noteText,#mermaid-svg-5dQMg5UItUlOnwbj .noteText>tspan{fill:#000;stroke:none}#mermaid-svg-5dQMg5UItUlOnwbj .activation0{fill:#f4f4f4;stroke:#666}#mermaid-svg-5dQMg5UItUlOnwbj .activation1{fill:#f4f4f4;stroke:#666}#mermaid-svg-5dQMg5UItUlOnwbj .activation2{fill:#f4f4f4;stroke:#666}#mermaid-svg-5dQMg5UItUlOnwbj .mermaid-main-font{font-family:"trebuchet ms", verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .section{stroke:none;opacity:0.2}#mermaid-svg-5dQMg5UItUlOnwbj .section0{fill:rgba(102,102,255,0.49)}#mermaid-svg-5dQMg5UItUlOnwbj .section2{fill:#fff400}#mermaid-svg-5dQMg5UItUlOnwbj .section1,#mermaid-svg-5dQMg5UItUlOnwbj .section3{fill:#fff;opacity:0.2}#mermaid-svg-5dQMg5UItUlOnwbj .sectionTitle0{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .sectionTitle1{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .sectionTitle2{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .sectionTitle3{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .sectionTitle{text-anchor:start;font-size:11px;text-height:14px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .grid .tick{stroke:#d3d3d3;opacity:0.8;shape-rendering:crispEdges}#mermaid-svg-5dQMg5UItUlOnwbj .grid .tick text{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .grid path{stroke-width:0}#mermaid-svg-5dQMg5UItUlOnwbj .today{fill:none;stroke:red;stroke-width:2px}#mermaid-svg-5dQMg5UItUlOnwbj .task{stroke-width:2}#mermaid-svg-5dQMg5UItUlOnwbj .taskText{text-anchor:middle;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .taskText:not([font-size]){font-size:11px}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutsideRight{fill:#000;text-anchor:start;font-size:11px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutsideLeft{fill:#000;text-anchor:end;font-size:11px}#mermaid-svg-5dQMg5UItUlOnwbj .task.clickable{cursor:pointer}#mermaid-svg-5dQMg5UItUlOnwbj .taskText.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutsideLeft.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutsideRight.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-5dQMg5UItUlOnwbj .taskText0,#mermaid-svg-5dQMg5UItUlOnwbj .taskText1,#mermaid-svg-5dQMg5UItUlOnwbj .taskText2,#mermaid-svg-5dQMg5UItUlOnwbj .taskText3{fill:#fff}#mermaid-svg-5dQMg5UItUlOnwbj .task0,#mermaid-svg-5dQMg5UItUlOnwbj .task1,#mermaid-svg-5dQMg5UItUlOnwbj .task2,#mermaid-svg-5dQMg5UItUlOnwbj .task3{fill:#8a90dd;stroke:#534fbc}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutside0,#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutside2{fill:#000}#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutside1,#mermaid-svg-5dQMg5UItUlOnwbj .taskTextOutside3{fill:#000}#mermaid-svg-5dQMg5UItUlOnwbj .active0,#mermaid-svg-5dQMg5UItUlOnwbj .active1,#mermaid-svg-5dQMg5UItUlOnwbj .active2,#mermaid-svg-5dQMg5UItUlOnwbj .active3{fill:#bfc7ff;stroke:#534fbc}#mermaid-svg-5dQMg5UItUlOnwbj .activeText0,#mermaid-svg-5dQMg5UItUlOnwbj .activeText1,#mermaid-svg-5dQMg5UItUlOnwbj .activeText2,#mermaid-svg-5dQMg5UItUlOnwbj .activeText3{fill:#000 !important}#mermaid-svg-5dQMg5UItUlOnwbj .done0,#mermaid-svg-5dQMg5UItUlOnwbj .done1,#mermaid-svg-5dQMg5UItUlOnwbj .done2,#mermaid-svg-5dQMg5UItUlOnwbj .done3{stroke:grey;fill:#d3d3d3;stroke-width:2}#mermaid-svg-5dQMg5UItUlOnwbj .doneText0,#mermaid-svg-5dQMg5UItUlOnwbj .doneText1,#mermaid-svg-5dQMg5UItUlOnwbj .doneText2,#mermaid-svg-5dQMg5UItUlOnwbj .doneText3{fill:#000 !important}#mermaid-svg-5dQMg5UItUlOnwbj .crit0,#mermaid-svg-5dQMg5UItUlOnwbj .crit1,#mermaid-svg-5dQMg5UItUlOnwbj .crit2,#mermaid-svg-5dQMg5UItUlOnwbj .crit3{stroke:#f88;fill:red;stroke-width:2}#mermaid-svg-5dQMg5UItUlOnwbj .activeCrit0,#mermaid-svg-5dQMg5UItUlOnwbj .activeCrit1,#mermaid-svg-5dQMg5UItUlOnwbj .activeCrit2,#mermaid-svg-5dQMg5UItUlOnwbj .activeCrit3{stroke:#f88;fill:#bfc7ff;stroke-width:2}#mermaid-svg-5dQMg5UItUlOnwbj .doneCrit0,#mermaid-svg-5dQMg5UItUlOnwbj .doneCrit1,#mermaid-svg-5dQMg5UItUlOnwbj .doneCrit2,#mermaid-svg-5dQMg5UItUlOnwbj .doneCrit3{stroke:#f88;fill:#d3d3d3;stroke-width:2;cursor:pointer;shape-rendering:crispEdges}#mermaid-svg-5dQMg5UItUlOnwbj .milestone{transform:rotate(45deg) scale(0.8, 0.8)}#mermaid-svg-5dQMg5UItUlOnwbj .milestoneText{font-style:italic}#mermaid-svg-5dQMg5UItUlOnwbj .doneCritText0,#mermaid-svg-5dQMg5UItUlOnwbj .doneCritText1,#mermaid-svg-5dQMg5UItUlOnwbj .doneCritText2,#mermaid-svg-5dQMg5UItUlOnwbj .doneCritText3{fill:#000 !important}#mermaid-svg-5dQMg5UItUlOnwbj .activeCritText0,#mermaid-svg-5dQMg5UItUlOnwbj .activeCritText1,#mermaid-svg-5dQMg5UItUlOnwbj .activeCritText2,#mermaid-svg-5dQMg5UItUlOnwbj .activeCritText3{fill:#000 !important}#mermaid-svg-5dQMg5UItUlOnwbj .titleText{text-anchor:middle;font-size:18px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj g.classGroup text{fill:#9370db;stroke:none;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:10px}#mermaid-svg-5dQMg5UItUlOnwbj g.classGroup text .title{font-weight:bolder}#mermaid-svg-5dQMg5UItUlOnwbj g.clickable{cursor:pointer}#mermaid-svg-5dQMg5UItUlOnwbj g.classGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-5dQMg5UItUlOnwbj g.classGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj .classLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.5}#mermaid-svg-5dQMg5UItUlOnwbj .classLabel .label{fill:#9370db;font-size:10px}#mermaid-svg-5dQMg5UItUlOnwbj .relation{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-5dQMg5UItUlOnwbj .dashed-line{stroke-dasharray:3}#mermaid-svg-5dQMg5UItUlOnwbj #compositionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #compositionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #aggregationStart{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #aggregationEnd{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #dependencyStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #dependencyEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #extensionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj #extensionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj .commit-id,#mermaid-svg-5dQMg5UItUlOnwbj .commit-msg,#mermaid-svg-5dQMg5UItUlOnwbj .branch-label{fill:lightgrey;color:lightgrey;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .pieTitleText{text-anchor:middle;font-size:25px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .slice{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj g.stateGroup text{fill:#9370db;stroke:none;font-size:10px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj g.stateGroup text{fill:#9370db;fill:#333;stroke:none;font-size:10px}#mermaid-svg-5dQMg5UItUlOnwbj g.statediagram-cluster .cluster-label text{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj g.stateGroup .state-title{font-weight:bolder;fill:#000}#mermaid-svg-5dQMg5UItUlOnwbj g.stateGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-5dQMg5UItUlOnwbj g.stateGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-5dQMg5UItUlOnwbj .transition{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-5dQMg5UItUlOnwbj .stateGroup .composit{fill:white;border-bottom:1px}#mermaid-svg-5dQMg5UItUlOnwbj .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px}#mermaid-svg-5dQMg5UItUlOnwbj .state-note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-5dQMg5UItUlOnwbj .state-note text{fill:black;stroke:none;font-size:10px}#mermaid-svg-5dQMg5UItUlOnwbj .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-5dQMg5UItUlOnwbj .edgeLabel text{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-5dQMg5UItUlOnwbj .node circle.state-start{fill:black;stroke:black}#mermaid-svg-5dQMg5UItUlOnwbj .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-5dQMg5UItUlOnwbj #statediagram-barbEnd{fill:#9370db}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-state .divider{stroke:#9370db}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-5dQMg5UItUlOnwbj .note-edge{stroke-dasharray:5}#mermaid-svg-5dQMg5UItUlOnwbj .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-5dQMg5UItUlOnwbj .error-icon{fill:#522}#mermaid-svg-5dQMg5UItUlOnwbj .error-text{fill:#522;stroke:#522}#mermaid-svg-5dQMg5UItUlOnwbj .edge-thickness-normal{stroke-width:2px}#mermaid-svg-5dQMg5UItUlOnwbj .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-5dQMg5UItUlOnwbj .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-5dQMg5UItUlOnwbj .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-5dQMg5UItUlOnwbj .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-5dQMg5UItUlOnwbj .marker{fill:#333}#mermaid-svg-5dQMg5UItUlOnwbj .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-5dQMg5UItUlOnwbj {color: rgba(0, 0, 0, 0.75);font: ;}

家庭电话

分配器

DSL调制解调器

家庭PC

中心局

因特网

电话网

如图所示，每个用户的DSL调制解调器使用现有的电话线与位于电话公司的本地中心局（CO）中的数字用户线接入复用器（DSLAM）交换数据。家庭的DSL调制解调器得到数字数据后将其转换为高频音，以通过电话线传输给本地中心局；来自许多家庭的模拟信号在DSLAM处被转换回数字形式.
这种电话线接入因为上行速率和下行速率不一致，所以这种接入不对称。
<2>电缆因特网接入：电缆因特网接入利用了有线公司现有的有线电视基础设施。住宅从提供有线电视的公司获得了电缆因特网接入。

#mermaid-svg-hFdiHz9QbhIzZE6j .label{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);fill:#333;color:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .label text{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .node rect,#mermaid-svg-hFdiHz9QbhIzZE6j .node circle,#mermaid-svg-hFdiHz9QbhIzZE6j .node ellipse,#mermaid-svg-hFdiHz9QbhIzZE6j .node polygon,#mermaid-svg-hFdiHz9QbhIzZE6j .node path{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-hFdiHz9QbhIzZE6j .node .label{text-align:center;fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .node.clickable{cursor:pointer}#mermaid-svg-hFdiHz9QbhIzZE6j .arrowheadPath{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .edgePath .path{stroke:#333;stroke-width:1.5px}#mermaid-svg-hFdiHz9QbhIzZE6j .flowchart-link{stroke:#333;fill:none}#mermaid-svg-hFdiHz9QbhIzZE6j .edgeLabel{background-color:#e8e8e8;text-align:center}#mermaid-svg-hFdiHz9QbhIzZE6j .edgeLabel rect{opacity:0.9}#mermaid-svg-hFdiHz9QbhIzZE6j .edgeLabel span{color:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .cluster rect{fill:#ffffde;stroke:#aa3;stroke-width:1px}#mermaid-svg-hFdiHz9QbhIzZE6j .cluster text{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:12px;background:#ffffde;border:1px solid #aa3;border-radius:2px;pointer-events:none;z-index:100}#mermaid-svg-hFdiHz9QbhIzZE6j .actor{stroke:#ccf;fill:#ECECFF}#mermaid-svg-hFdiHz9QbhIzZE6j text.actor>tspan{fill:#000;stroke:none}#mermaid-svg-hFdiHz9QbhIzZE6j .actor-line{stroke:grey}#mermaid-svg-hFdiHz9QbhIzZE6j .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .messageLine1{stroke-width:1.5;stroke-dasharray:2, 2;stroke:#333}#mermaid-svg-hFdiHz9QbhIzZE6j #arrowhead path{fill:#333;stroke:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .sequenceNumber{fill:#fff}#mermaid-svg-hFdiHz9QbhIzZE6j #sequencenumber{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j #crosshead path{fill:#333;stroke:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .messageText{fill:#333;stroke:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .labelBox{stroke:#ccf;fill:#ECECFF}#mermaid-svg-hFdiHz9QbhIzZE6j .labelText,#mermaid-svg-hFdiHz9QbhIzZE6j .labelText>tspan{fill:#000;stroke:none}#mermaid-svg-hFdiHz9QbhIzZE6j .loopText,#mermaid-svg-hFdiHz9QbhIzZE6j .loopText>tspan{fill:#000;stroke:none}#mermaid-svg-hFdiHz9QbhIzZE6j .loopLine{stroke-width:2px;stroke-dasharray:2, 2;stroke:#ccf;fill:#ccf}#mermaid-svg-hFdiHz9QbhIzZE6j .note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-hFdiHz9QbhIzZE6j .noteText,#mermaid-svg-hFdiHz9QbhIzZE6j .noteText>tspan{fill:#000;stroke:none}#mermaid-svg-hFdiHz9QbhIzZE6j .activation0{fill:#f4f4f4;stroke:#666}#mermaid-svg-hFdiHz9QbhIzZE6j .activation1{fill:#f4f4f4;stroke:#666}#mermaid-svg-hFdiHz9QbhIzZE6j .activation2{fill:#f4f4f4;stroke:#666}#mermaid-svg-hFdiHz9QbhIzZE6j .mermaid-main-font{font-family:"trebuchet ms", verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .section{stroke:none;opacity:0.2}#mermaid-svg-hFdiHz9QbhIzZE6j .section0{fill:rgba(102,102,255,0.49)}#mermaid-svg-hFdiHz9QbhIzZE6j .section2{fill:#fff400}#mermaid-svg-hFdiHz9QbhIzZE6j .section1,#mermaid-svg-hFdiHz9QbhIzZE6j .section3{fill:#fff;opacity:0.2}#mermaid-svg-hFdiHz9QbhIzZE6j .sectionTitle0{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .sectionTitle1{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .sectionTitle2{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .sectionTitle3{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .sectionTitle{text-anchor:start;font-size:11px;text-height:14px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .grid .tick{stroke:#d3d3d3;opacity:0.8;shape-rendering:crispEdges}#mermaid-svg-hFdiHz9QbhIzZE6j .grid .tick text{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .grid path{stroke-width:0}#mermaid-svg-hFdiHz9QbhIzZE6j .today{fill:none;stroke:red;stroke-width:2px}#mermaid-svg-hFdiHz9QbhIzZE6j .task{stroke-width:2}#mermaid-svg-hFdiHz9QbhIzZE6j .taskText{text-anchor:middle;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .taskText:not([font-size]){font-size:11px}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutsideRight{fill:#000;text-anchor:start;font-size:11px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutsideLeft{fill:#000;text-anchor:end;font-size:11px}#mermaid-svg-hFdiHz9QbhIzZE6j .task.clickable{cursor:pointer}#mermaid-svg-hFdiHz9QbhIzZE6j .taskText.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutsideLeft.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutsideRight.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-hFdiHz9QbhIzZE6j .taskText0,#mermaid-svg-hFdiHz9QbhIzZE6j .taskText1,#mermaid-svg-hFdiHz9QbhIzZE6j .taskText2,#mermaid-svg-hFdiHz9QbhIzZE6j .taskText3{fill:#fff}#mermaid-svg-hFdiHz9QbhIzZE6j .task0,#mermaid-svg-hFdiHz9QbhIzZE6j .task1,#mermaid-svg-hFdiHz9QbhIzZE6j .task2,#mermaid-svg-hFdiHz9QbhIzZE6j .task3{fill:#8a90dd;stroke:#534fbc}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutside0,#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutside2{fill:#000}#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutside1,#mermaid-svg-hFdiHz9QbhIzZE6j .taskTextOutside3{fill:#000}#mermaid-svg-hFdiHz9QbhIzZE6j .active0,#mermaid-svg-hFdiHz9QbhIzZE6j .active1,#mermaid-svg-hFdiHz9QbhIzZE6j .active2,#mermaid-svg-hFdiHz9QbhIzZE6j .active3{fill:#bfc7ff;stroke:#534fbc}#mermaid-svg-hFdiHz9QbhIzZE6j .activeText0,#mermaid-svg-hFdiHz9QbhIzZE6j .activeText1,#mermaid-svg-hFdiHz9QbhIzZE6j .activeText2,#mermaid-svg-hFdiHz9QbhIzZE6j .activeText3{fill:#000 !important}#mermaid-svg-hFdiHz9QbhIzZE6j .done0,#mermaid-svg-hFdiHz9QbhIzZE6j .done1,#mermaid-svg-hFdiHz9QbhIzZE6j .done2,#mermaid-svg-hFdiHz9QbhIzZE6j .done3{stroke:grey;fill:#d3d3d3;stroke-width:2}#mermaid-svg-hFdiHz9QbhIzZE6j .doneText0,#mermaid-svg-hFdiHz9QbhIzZE6j .doneText1,#mermaid-svg-hFdiHz9QbhIzZE6j .doneText2,#mermaid-svg-hFdiHz9QbhIzZE6j .doneText3{fill:#000 !important}#mermaid-svg-hFdiHz9QbhIzZE6j .crit0,#mermaid-svg-hFdiHz9QbhIzZE6j .crit1,#mermaid-svg-hFdiHz9QbhIzZE6j .crit2,#mermaid-svg-hFdiHz9QbhIzZE6j .crit3{stroke:#f88;fill:red;stroke-width:2}#mermaid-svg-hFdiHz9QbhIzZE6j .activeCrit0,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCrit1,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCrit2,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCrit3{stroke:#f88;fill:#bfc7ff;stroke-width:2}#mermaid-svg-hFdiHz9QbhIzZE6j .doneCrit0,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCrit1,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCrit2,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCrit3{stroke:#f88;fill:#d3d3d3;stroke-width:2;cursor:pointer;shape-rendering:crispEdges}#mermaid-svg-hFdiHz9QbhIzZE6j .milestone{transform:rotate(45deg) scale(0.8, 0.8)}#mermaid-svg-hFdiHz9QbhIzZE6j .milestoneText{font-style:italic}#mermaid-svg-hFdiHz9QbhIzZE6j .doneCritText0,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCritText1,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCritText2,#mermaid-svg-hFdiHz9QbhIzZE6j .doneCritText3{fill:#000 !important}#mermaid-svg-hFdiHz9QbhIzZE6j .activeCritText0,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCritText1,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCritText2,#mermaid-svg-hFdiHz9QbhIzZE6j .activeCritText3{fill:#000 !important}#mermaid-svg-hFdiHz9QbhIzZE6j .titleText{text-anchor:middle;font-size:18px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j g.classGroup text{fill:#9370db;stroke:none;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:10px}#mermaid-svg-hFdiHz9QbhIzZE6j g.classGroup text .title{font-weight:bolder}#mermaid-svg-hFdiHz9QbhIzZE6j g.clickable{cursor:pointer}#mermaid-svg-hFdiHz9QbhIzZE6j g.classGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-hFdiHz9QbhIzZE6j g.classGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j .classLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.5}#mermaid-svg-hFdiHz9QbhIzZE6j .classLabel .label{fill:#9370db;font-size:10px}#mermaid-svg-hFdiHz9QbhIzZE6j .relation{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-hFdiHz9QbhIzZE6j .dashed-line{stroke-dasharray:3}#mermaid-svg-hFdiHz9QbhIzZE6j #compositionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #compositionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #aggregationStart{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #aggregationEnd{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #dependencyStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #dependencyEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #extensionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j #extensionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j .commit-id,#mermaid-svg-hFdiHz9QbhIzZE6j .commit-msg,#mermaid-svg-hFdiHz9QbhIzZE6j .branch-label{fill:lightgrey;color:lightgrey;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .pieTitleText{text-anchor:middle;font-size:25px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .slice{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j g.stateGroup text{fill:#9370db;stroke:none;font-size:10px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j g.stateGroup text{fill:#9370db;fill:#333;stroke:none;font-size:10px}#mermaid-svg-hFdiHz9QbhIzZE6j g.statediagram-cluster .cluster-label text{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j g.stateGroup .state-title{font-weight:bolder;fill:#000}#mermaid-svg-hFdiHz9QbhIzZE6j g.stateGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-hFdiHz9QbhIzZE6j g.stateGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-hFdiHz9QbhIzZE6j .transition{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-hFdiHz9QbhIzZE6j .stateGroup .composit{fill:white;border-bottom:1px}#mermaid-svg-hFdiHz9QbhIzZE6j .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px}#mermaid-svg-hFdiHz9QbhIzZE6j .state-note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-hFdiHz9QbhIzZE6j .state-note text{fill:black;stroke:none;font-size:10px}#mermaid-svg-hFdiHz9QbhIzZE6j .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-hFdiHz9QbhIzZE6j .edgeLabel text{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-hFdiHz9QbhIzZE6j .node circle.state-start{fill:black;stroke:black}#mermaid-svg-hFdiHz9QbhIzZE6j .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-hFdiHz9QbhIzZE6j #statediagram-barbEnd{fill:#9370db}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-state .divider{stroke:#9370db}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-hFdiHz9QbhIzZE6j .note-edge{stroke-dasharray:5}#mermaid-svg-hFdiHz9QbhIzZE6j .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-hFdiHz9QbhIzZE6j .error-icon{fill:#522}#mermaid-svg-hFdiHz9QbhIzZE6j .error-text{fill:#522;stroke:#522}#mermaid-svg-hFdiHz9QbhIzZE6j .edge-thickness-normal{stroke-width:2px}#mermaid-svg-hFdiHz9QbhIzZE6j .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-hFdiHz9QbhIzZE6j .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-hFdiHz9QbhIzZE6j .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-hFdiHz9QbhIzZE6j .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-hFdiHz9QbhIzZE6j .marker{fill:#333}#mermaid-svg-hFdiHz9QbhIzZE6j .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-hFdiHz9QbhIzZE6j {color: rgba(0, 0, 0, 0.75);font: ;}

同轴电缆

数以万计的家庭

光纤节点

CMTS

因特网

电缆因特网接入需要特殊的调制解调器，这种调制解调器被称为电缆调制解调器。如同DSL调制解调器，电缆解调器通常是一个外部设备，通过一个以太网连接到家庭PC（以太网后面会介绍到）在电缆头端，电缆调制解调器端接系统与DSL网络的DSLAM具有类似的功能，即将来许多下行家庭中的电缆调制解调器发送的模拟信号转换回数字形式。
<3>电缆因特网接入特征：
1.接入通常是不对称的，下行信道分配的传输速率通常比上行信道高。
2.共享广播媒体。如果几个用户同时经下行信道下载0一个视频文件，每个用户接收文件的实际速率大大低于电缆总计的下行速率

排队时延和分组丢失

每台分组交换机有多条链路与之相连。对于每条相连的链路,该分组交换机具有一个输出缓存(output buffer,也称为输出队列(output queue)),它用于存储路由器准备发往那条链路的分组。该输出缓存在分组交换中起着重要的作用。除了存储转发时延以外,分组还要承受输出缓存的排队时延(queuing delay)。这些时延是变化的,变化的程度取决于网络的拥塞程度。因为缓存空间的大小是有限的,一个到达的分组可能发现该缓存已被其他等待传输的分组完全充满了。在此情况下,将出现分组丢失(丢包)(packet loss),到达的分组或已经排队的分组之一将被丢弃。