《网络通信_ch1_概述》由会员分享，可在线阅读，更多相关《网络通信_ch1_概述（47页珍藏版）》请在金锄头文库上搜索。
1、Bilingual Course Network Communications网络通信(For Master Students in the Department of Electronic Engineering)Chapter 1 IntroductionInstructor: Dr. Tianshuang QiuSchool of the Electronic and Information EngineeringFall of 20101Chapter 1 Introductiono第1章是对第二部分到第四部分内容的总体介绍，向读 者展示一张关于网络通信的概况或“总图”。o从本质上看，
2、本书主要研究的主题有4个：n传输链路上的数据通信；n广域网；n局域网；n协议与TCP/IP协议体系。o第1章对这些内容做了简要说明（基本不包含第五部分 ）。o第2章简要介绍第五部分的内容21-1 IntroductionoCommunication SystemsnSource（信源）: 信息的发生地；把各种可能的消息变成电信号o信源编码：对信源进行压缩、加密等操作，称为信源编码nDestination（信宿）:信息的接收者，将信号再转变为消息nTransmitter（发送设备）: 将信源输出的信号变换成适合信道 传输的形式，并送往信道，又称为信道编码o例：Modem从PC机上获得一个数据流，
3、将其转换成模拟信号， 发送nChannel（信道）:信号传输的通道，是发送与接收设备之间的桥 梁o常用的信道有Twisted pair, Cable, Optical fiber, Space, Air etc. nNoise（噪声）:传输过程中必然引入的非信息信号，如热噪声、 脉冲噪声、干扰与衰落等nReceiver（接收设备）:把接收到的信号恢复成原始信号，即解 调、译码、解密等3Typical Communication Systems4Simplified Communications Model-Diagram5Simplified Data Communications Model
4、6Communication ModesoSimplex Transmissionn消息只能单方向传输oHalf-duplex Transmissionn双方均能发消息，但是不能同时收或同时发oFull-duplex Transmissionn双方均可同时收发消息7The Measurement of Informationo信息/消息/信号n信息（Information）：抽象，可以度量，定量分析n消息（Message）：具体，一般不定量分析n信号（Signal）：是信息（消息）的载体o信息量：信息多少的度量n事件发生的可能性越小，使人感到惊奇与意外，其信息量 ，即消息中的信息量与事件发生的
5、概率紧密相关o事件出现的概率信息量o事件出现的概率信息量oP=1信息量=0；P=0信息量=n信息量的大小与消息的种类和事件的重要程度无关8信息的概念o信息（Information）的概念n信息一词在我国历史悠久，南唐时期诗人李中有诗云 ：“梦断美人沉信息，目穿长路依楼台”。在这里，信息泛 指音讯和消息。n英文中Inforrmation一词的词根为infor，是通知的意 思，即被告知的事实或知识。n以上关于“信息”的说明，均不是作为科学名词或技术 术语出现的。n查词海，遗憾的是，未发现“信息”词目。但有文 字称“信息论是利用数学方法，研究信息的计量、传送、 变换和存储的一门科学”。9年号庙 号名
6、字即 位 时 间即 位 年 龄在 位 年 数死 时 年 龄世系注备升元烈租李bian93750756徐州人，父李荣 ，吴国重臣徐温 收以为养子，故 改名徐知诰吴国大承相徐温死，其养子徐知诰执掌吴政，受封为齐王，吴天祚三年 （937年），废吴帝自立，改元升元，国号大齐，升元三年复姓李，改名 bian，改国号为唐，史称南唐，都金陵，升元七年因服丹药，中毒而死保大、中 兴、交泰 显德、建 隆元宗李jing943281946烈祖长子以皇太子即位建隆、乾 德、开宝后主李煜961251542立jing第六子因自太子以上李jing五子皆早死，故煜得立为太子，中主李jing死，煜 以皇太子嗣位，975年，宋将
7、曹彬攻破金陵，煜投降，迁至汴京，宋太祖 封以为违命侯，后为宋太祖毒死10信息量的定义o信息量o若 ，则信息量的单位为bito若 ，则信息量的单位为nato若 ，则信息量的单位为哈特莱o 为消息出现的概率 n例：字母e的概率为P(e)=0.105，则其信息量为I(e)=3.24bitn例：字母q的概率为P(q)=0.001，则其信息量为I(q)=9.97bit11信息量的计算o离散信源n所产生的消息是一种有限个状态的随机序列o连续信源n所产生的消息的数目是无限的12Communication Tasks (Table 1.1)oTransmission system utilizationoIn
8、terfacingoSignal generation oSynchronizationoExchange managementoError detection and correctionoFlow controloAddressingoRoutingoRecovery oMessage formattingoSecurityoNetwork management13Transmission system utilizationoTransmission system utilization refers to the need to make efficient use of transm
9、ission facilities that are typically shared among a number of communicating devices. Various techniques (referred to as multiplexing) are used to allocate the total capacity of a transmission medium among a number of users. Congestion control techniques may be required to assure that the system is n
10、ot overwhelmed by excessive demand for transmission services.14InterfaceoTo communicate, a device must interface with the transmission system. oAll the forms of communication discussed in this book depend on the use of electromagnetic signals propagated over a transmission medium. 15Signal generatio
11、noThus, once an interface is established, signal generation is required for communication. The properties of the signal, such as form and intensity, must be such that they are (1) capable of being propagated through the transmission system, and (2) interpretable as data at the receiver.16Synchroniza
12、tionoNot only must the signals be generated to conform to the requirements of the transmission system and receiver, but there must be some form of synchronization between transmitter and receiver.17Error control transmitted signals are distorted to some extent before reaching their destination. Erro
13、r detection and correction are required in circumstances where errors cannot be tolerated. This is usually the case with data processing system.oFlow control is required to assume that the source does not overwhelm the destination by sending data faster than they can be processed and absorbed.18Exch
14、ange managementoIf data are to be exchanged in both directions over a period of time, the two parties must cooperate. For example, for two parties to engage in a telephone conversation, one party must dial the number of the other, causing signals to be generated that results in the ringing of the ca
15、lled phone. The called party complete a connection by lifting the receiver. oFor data processing device, more will be needed than simple establishing a connection; certain conventions must be decided upon. These conventions may include whether both devices may transmit simultaneously or must take tu
16、rns, the amount of data to be sent at one time, the format of the data, and what to do if certain contingencies (意 外事故) such as an error arise.19Addressing supposed) sender.22Network Management oFinally, a data communications facility is a complex system that cannot create or run itself. Network management capabilities are needed to configure the system, monitor its status, react to failures
