计算机网络概述
计算及网络在信息时代的作用
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21世纪
数字化
网络化
信息化
三网
电信网络
计算机网络
有线电视网络
因特网
其他网络
因特网概述
网络分类
网络：许多计算机连接在一起；互联网：internet许多网络连接在一起；因特网：Internet 全球最大的一个互联网
因特网发展的三个阶段
上世纪60年代到80年代中期 单个网络ARPANET向互联网发展的过程上世纪80年代中期到90年代初 三级结构的因特网多层次ISP结构的因特网
多层次ISP结构的互联网
中国互联网
ISP、企业网以及网民
因特网的标准化工作
http://www.isoc.org http://www.IAB.org
1.3互联网的组成
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因特网
边缘部分
核心部分
客户服务器方式
对等方式
电路交换
报文交换
分组交换
因特网=边缘部分+核心部分=资源子网+通信子网
因特网边缘部分
主机之间的通讯方式
客户服务器方式（Client/Server方式–C/S方式）对等（Peer-to-Peer方式–P2P）方式
因特网的核心部分
数据交换方式
电路交换（Circuit Switching）报文交换（Message Switching）分组交换（Packet Switching）
电路交换方式（Circuit Switching）
面向连接
步骤
建立连接：申请占用通信资源通话：一直占用通信资源释放连接：释放通信资源
电路交换适合于数据量很大的实时性传输：核心路由器之间可以使用电路交换
分组交换方式（Packet Switching）
步骤
在发送端将数据分成多组。将每一段数据头部加上首部信息，行成多组报文。发送接收端分别接收到各组报文，组合变成发送端发送的数据
分组交换总结
优点：高效，灵活，迅速，可靠缺点：时延，开销
报文交换
报文一般比分组长的多报文交换的时延较长
三种交换方式的比较
计算机网络的类别
计算机网络最简单的定义：一些互相连接的、自治的计算机集合
作用范围：广域网WAN、城域网MAN、局域网LAN、个人区域网PAN使用者：公用网Public Network、专用网Private Network拓扑结构：总线型、环型、星型、树型、网状交换方式：电路交换网、报文交换网、分组交换网工作方式：资源子网、通信子网、接入网
计算机网络性能
性能指标
速率：连接在计算机网络上的主机在数字信道上传送数据位数的速率，也称为data rate或bit rate。单位是b/s, kb/s, Mb/s, Gb/s.宽带：数据通信领域中，数字信道所能传送的最高数据率，单位是 b/s, kb/s, Mb/s, Gb/s.吞吐量：即在单位时间内通过某个网络的数据量，单位b/s, Mb/s, 等.时延：发送时延、传播时延、处理时延、排队时延时延宽带积：时延带宽积=传播时延*宽带往返时间：从发送方发送数据开始，到发送方收到接收方确认。利用率：信道利用率、网络利用率。
非性能指标
费用质量标准化可靠性可拓展性可升级管理与维护
计算机网络的体系结构
基本概念
ISO：国际便准化组织OSI/RM：互联网法律上的国际标准TCP/IP Suite：因特网事实上的国际标准Network Protocols：数据交换遵守的规则、标准或约定网络体系结构：计算机网络各层及其协议的集合
开放系统信息交换设计的几个概念
实体（entity）：交换信息的硬件或软件进程协议（protocol）：控制两个对等实体通信的规则服务（service）：下层向上层提供服务，上层需要使用下层提供的服务来实现本层的功能服务访问点（SAP）：相邻两层实体间交换信息的地方
五层协议对应的数据单元
物理层===========0110101010100000100100数据链路层========数据帧网络层============IP数据报（IP分组）运输层============运输层报文应用层============应用层（传输数据单元PDU）
