第一章三网：电信网络、有线电视网络和计算机网络 the Telecommunication networks, Cable television networks and Computer networks网络最主要的功能：连通性和共享性 Connectivity and Sharing网络：由若干结点和连接这些节点的链路组成。网络中的结点可以是计算机、集线器、交换机、路由器等组成。composed by the nodes and the links of these nodes. Can be a computer, hubs, switches, routers, etc. 互联网：网络和网络可以通过路由器连起来，构成一个覆盖范围更大的网络 : network and the network can be connected through a router, constitute a greater network coverage 因特网：世界上最大的互连网。网络把计算机连接起来，因特网把网络连接起来。the worlds largest Interconnected Network。Network connect the computers, the Internet connect interconnected networks. 主机：连接在因特网上的计算机。Computers which connected on the Internet. 计网的组成：按工作方式分为两部分：边缘部分由所有连接在因特网上的主机组成。这部分是用户直接使用的，用来进行通信）和资源共享。核心部分由大量网络和连接这些网络的路由器组成。这部分是为边缘部分提供服务的According to the working style: Edges - consists of all the Host connected to the Internet. This part is the user use,directly used to communicate and resource sharing. Core part - made up of a large number of network and connect the network router. This section is to provide service for the edges provide connectivity and exchange 按逻辑结构分为以下两大子网：资源子网负责全网的数据处理，即向网络用户提供各种网络资源与网络服务，其主要包括主机和终端，主机通过高速通信线路与通信子网的通信控制处理机相连接，而终端是用户访问网络的界面。 通信子网由通信控制处理机、通信线路与其他通信设备组成，其主要完成网络数据传输、转发等通信处理任务。通信控制处理机在网络拓扑结构中被称为网络节点。通信线路为通信处理机之间以及通信处理机与主机之间提供通信信道。According to the logical structure: Resource subnet - to be responsible for the entire network data processing, that is, to the network users with a variety of network resources and network services, mainly including the Host and the terminal, the Host through high-speed communication lines connected to the communication subnet communication control processor, and the end user is access to the network interface. Communication subnet - by communication control processor, communication lines, and other communications equipment, its main network data transmission, such as forwarding communication processing task. Communication control processor is referred to as nodes in the network topology. Communication lines between communication processor and communication processor and provide a communication channel between the hosts. 三种计算机分组交换方式比较a)电路交换（Circuit Switching）整个报文的比特流连续地从源点直达终点，好像在一个管道中传输；在通话的全部时间内，通话的两个用户始终占用端到端的通信资源，所以资源利用率低b)报文交换（Message Switching）整个报文传送到相邻节点，全部存储下来后查找转发表，转发到下一个节点；通信线路利用率高、出错后纠错效率高等优点，被计算机与通信界广泛采用。存储转发是该技术的核心。c)分组（包）交换（Packet Switching）单个分组（只是整个报文的一部分）传送到相邻节点，存储下来后查找转发表，转发到下一个节点；分组交换的优点高效 动态分配传输带宽，对通信链路是逐段占用。 灵活 以分组为传送单位和查找路由。迅速 不必先建立连接就能向其他主机发送分组。可靠 保证可靠性的网络协议；分布式的路由选择协议使网络有很好的生存性。 High efficiency Dynamic allocation of transmission bandwidth, the communication link is a period of time.FlexibilityLookup routing and the packets as the transmission unit .Rapidit can quickly send packets to other hosts without establishing a connection at first.ReliabilityGuarantee the reliability of the network protocol, and the distributed routing protocol make the network have a sound survivability.计算机网络是利用通信线路(连网设备)将地理上分散的、具有独立功能的许多计算机系统连接起来，按照某种协议进行数据通信，以实现资源共享的信息系统。最简单计算机网络两个结点（计算机）一条链路Computer network is the information system that uses the communication lines (networking equipment) to connect the geographically dispersed computer systems and realizes the information resources sharing according to some protocol for data communication.计网的分类：1.按不同作用范围划分1.classified by scope (distance)个人区域网 PAN局域网 LAN城域网 MAN广域网 WAN因特网 Internet2. 按不同使用者划分 Classified by various users公用网（Public Networks）专用网（Private Networks）3. 按信息交流对象划分(TCP/IP协议为基础) Classified by information exchange objects (based on TCP/IP protocol )Internet（互联网、网际网或英特网）Intranet （内联网）Extranet (内联外延网)拓扑结构一个网络中各个节点之间互连的几何构形，即指各个节点之间互相连接方式。基本的网络拓扑结构有星型、环型、总线型等三种。任何一种网络系统都规定了它们各自的网络拓扑结构。The three basic topology of the network are Star, Bus and Ring type.分组时延的组成与原理1. 处理时延: Processing delay:检查比特差错决定输出链路Check bit errorDecide output link2.排队时延Queuing delay等待输出链路传输的时间取决于路由器拥塞的等级Waiting for output link to transmitDepending on the level of congestion in router3. 发送（传输）时延: Send delayR= 发送速率 (bps)L= 数据帧长度 (比特)发送比特进入链路的时间= L/R4. 传播时延: Propagation delayd = 信道长度s = 在信道中传播速度 (2x108 m/sec)传播时延 = d/s时延与网络利用率的关系根据排队论的理论，当某信道的利用率增大时，该信道引起的时延也就迅速增加。若令 D0 表示网络空闲时的时延，D 表示网络当前的时延，则在适当的假定条件下，可以用下面的简单公式表示 D 和 D0之间的关系：协议的三个基本要素Three basic elements of a protocol语法(Syntax)：怎么讲？语法包括数据格式、编码及信号电平等。data format, encoding and signal levelo 语义(Semantics)：讲什么？语义包括用于协调同步和差错处理的控制信息。control information for coordinating synchronization and error handling.o 同步(Synchronous/Time)：何时讲？同步包括速度匹配和事件实现排序。speed matching and event implementation sorting.计算机网络的体系结构a)OSI参考模型共有7层，由低层至高层分别为物理层 physical layer数据链路层 data link layer网络层 network layer传输层 transport layer会话层 session layer表示层 presentation layer应用层 application layerb)TCP/IP包括4层：应用层Application layer- 对应OSI上三层传输层Transport layer 网络层 Network layer 网络接口层Netwo