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1、Computer Networks,计算机与控制学院 王鑫,参考文献,Andrew S. Tanenbaum, Computer Networks (Fourth Edition), 清华大学出版社 & Prentice Hall, 2004 James F. Kurose等著，陈鸣译. 计算机网络自顶向下方法与Internet特色（第三版）.机械工业出版社，2005 谢希仁.计算机网络（第四版）.电子工业出版社，2003,Contents,Introduction(7),1,The Physical Layer(5),2,The Data Link Layer(6),3,The MAC Su
2、b-layer(6),4,The Network Layer(6),5,The Transport Layer(4),6,The Application Layer(4),7,Network Security(4),8,1 Introduction,学习目的：了解计算机网络发展历史、计算机网络的分类、网络体系结构（OSI/RM、TCP/IP）、计算机网络相关的基本概念和交换技术等。 学习重点：熟练掌握计算机网络、网络协议、网络体系结构等三个重要概念。 学习难点：掌握三种交换技术之间的异同。,What is computer networks?,Computer Network: means a
3、 collection of autonomous computers（自主/自治计算机） interconnected. It is composed of hosts, protocols and communication-subnet. The difference between Computer Network and Distributed System Distributed System: A collection of autonomous (independent) computers is transparency（透明的）to its users. It is a s
4、oftware system built on top of a network. Thus, the key distinction between a network and a distributed system is the software (especially the operating system: NOS and Distributed OS), rather than the hardware.,The development of computer networks,First, Terminal-oriented network （1）Computing techn
5、ology and communication technology. （2）Circuit-exchange(电路交换): Connection establishment, Data transform, Connection release. Second, Packet-exchange network: 1969.11, ARPANET,The development of computer networks,(a) Circuit switching (b) Message switching (c) Packet switching,The development of comp
6、uter networks,Third, Network Architecture IBM：SNA(系统网络体系结构)，1974，the first architecture of network ISO：OSI/RM(开放系统互连基本参考模型,Reference model) TCP/IP：Internet Digital：DNA(Distributed network architecture, 分布式系统体系结构) Fourth, Wide Band(Broad band) network Broad Band and Integrated,1.1 Uses of Computer Ne
7、tworks,Business Applications Home Applications Mobile Users Social Issues,Business Applications,The goals of the computer networks: Resource sharing （hardware 、software and Data，资源共享） Communication medium（通信手段）,Home Applications,Access to remote information(访问远程信息) Person to person communication(个人间
8、通信) Interactive entertainment(交互式娱乐) Ecommerce(电子商务),1.2 Network Hardware,Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks,Broadly speaking(从广义上讲)，two types of transmission technology: （1）Broadcast links （2）Point-to-point links Broadcast
9、 network(广播式网络)：broadcasting and multicasting Point-to-point network(点到点网络)：packettransmission(分组转发) and routingalgorithms（路由选择）,Local Area Networks(局域网),Three characteristics: (1)Size(范围) (2)Transmission technology(传输技术) (3)Topology(拓扑结构),Metropolitan Area Networks(城域网),A metropolitan area network
10、based on cable TV,Wide Area Networks(广域网),Wireless Networks,Categories of wireless networks: System interconnection Wireless LANs Wireless WANs,（a）Bluetooth （b）Wireless LAN,Home Networks,Computers (desktop PC, PDA, shared peripherals Entertainment (TV, DVD, VCR, camera, stereo, MP3) Telecomm (teleph
11、one, cell phone, intercom, fax) Appliances (microwave, fridge, clock) Telemetry(遥感，遥测) (utility meter（电表）, burglar alarm（防盗自动警铃）, babycam （婴儿监护器）).,The first homework,P180：NO.42, NO.43,1.3 Network Software,Protocol Hierarchies Design Issues for the Layers Connection-Oriented and Connectionless Servi
12、ces Service Primitives The Relationship of Services to Protocols,Protocol Hierarchies(协议分层),To reduce their design complexity, most networks are organized as a stack of layers or levels. Advantages of Protocol Hierarchies:(分层的优点) (1) Independence of layers: These layers are relative independence.(各层
13、之间是相对独立的) (2) If any layer is changed, other layers are not affected, so far as not changing interface. (灵活性好) (3) Each layer can be realized in right technology.(结构上可以分割开) (4) It is easy to realize and maintain. (易于实现和维护) (5) Standardization (能促进标准化工作),Protocol Hierarchies(协议分层),Defects of Protocol
14、 Hierarchies:(分层的缺点) (1)Some functions may be realized repeatedly. (功能重复实现，资源浪费) (2) The number of layer is not certain.(层次数难以确定),Example,机票 (购买) 行李 (托运) 旅客 (出发) 飞机 (起飞) 飞行航线,机票 (投诉) 行李 (认领) 旅客 (到达) 飞机 (着陆) 飞行航线,飞行航线,Example,层次的观点： - 每层实现一种特定的服务 - 通过自己内部的功能 - 依赖自己的下层提供的服务,机票 (购买) 行李 (托运) 旅客 (出发) 飞机
15、(起飞) 飞行航线,机票 (投诉) 行李 (认领) 旅客 (到达) 飞机 (着陆) 飞行航线,飞行航线,Example,分层的空中旅行组织: 服务,从出发地到目的地的航线：导航服务,柜台-to-柜台：“旅客+行李” 票务服务,行李托运-to-行李认领：行李服务,登机入口-to-到达出口：旅客乘务服务,跑道-to-跑道：飞机“航运”服务,Protocol Hierarchies(协议分层),Protocol Hierarchies(协议分层),Protocol Hierarchies(协议分层),Protocol is an agreement between the communicating
16、 parties on how communication is to proceed. Three factors：Syntax（语法），Semantics（语义），Synchronous（同步） Network architecture is a set of layers and protocols The difference of Protocol、Service and SAP,Design Issues for the Layers,(1)A mechanism for identifying senders and receivers. (2)The rules for dat
17、a transfer: Simplex communication(单工通信) Half-duplex communication(半双工通信) Full-duplex communication(全双工通信) (3)Error control(差错控制) (4)Flow control(流量控制) (5)Disassembling(分割) and gathering（合并）messages (6)Multiplexing(多路复用) and demultiplexing(多路分用、解多路复用) (7)Routing(路由选择),Connection-Oriented and Connecti
18、onless Services,Service Primitives(服务原语),The Relationship of Services to Protocols,A service is a set of primitives (operations) that a layer provides to the layer above it. A protocol is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entit
19、ies within a layer. Protocol is horizontal, and service is vertical.,1.4 Reference Models,The OSI Reference Model The TCP/IP Reference Model A Comparison of OSI and TCP/IP A Critique of the OSI Model and Protocols A Critique of the TCP/IP Reference Model,The OSI Reference Model,The OSI model has sev
20、en layers. The partitioned principles: (1)A layer should be created where a different level of abstraction is needed. (2)Each layer should perform a well defined function. (3)The function of each layer should be chosen with an eye toward defining internationally standardized protocols. (4)The layer
21、boundaries should be chosen to minimize the information flow across the interfaces. (5)The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy(庞大的).,The OSI Ref
22、erence Model,The OSI Reference Model,The Physical Layer: To transmit bits, Such as mechanical characteristic, electrical characteristic, time characteristic and function characteristic. The Data Link Layer: To transform data frames without errors. The Network Layer: how to route from source to desti
23、nation.(transmit packet) The transport Layer: the most important layer. To accept data from the session layer, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. The Session layer: To establish sessions betwee
24、n different users. (a) A session service is token management.(令牌管理) (b) Another session service is synchronization.（同步） The Presentation layer: its function contains: (a) data compress and decompress (b) data encrypt and decrypt (c) data format The Application Layer: (a) To provide service for appli
25、cation procedure. (b) file transfer,Data Transmission in the OSI Model,The TCP/IP Reference Model,The TCP/IP Reference Model,The Host-to-Network Layer(主机至网络层): point out that the host has to connect to the network. The Internet Layer（互连层）: Packet routing(分组路由) and avoiding congestion（避免拥塞）. The Tran
26、sport Layer(传输层): end-to-end communication. (TCP and UDP) TCP: an reliable, connection-oriented protocol UDP: an unreliable, connectionless protocol The Application Layer(应用层),The TCP/IP Reference Model,A Comparison of the OSI and TCP/IP Reference Models,The OSI/RM defines three concepts: Services,
27、Interfaces and Protocols. But the TCP/IP doesnt. Connection-oriented and connectionless communication.,A Critique of the OSI Model and Protocols,Bad timing(糟糕的提出时机) Bad technology（糟糕的技术） Bad implementations（糟糕的实现） Bad politics（糟糕的策略）,A Critique of the TCP/IP,The TCP/IP does not clearly distinguish t
28、he concepts of service, interface and protocol. The TCP/IP is not at all general and is poorly suited to describing and protocol stack other than TCP/IP. The host-to-network layer is an interface, but not a layer. The TCP/IP does not distinguish the physical and data link layers. Although, the IP an
29、d TCP protocols were carefully thought out and well implemented, many of the other protocols were ad hoc(特别) and have defect.,Summary,1.5 Performance parameters of computer networks,1. Bandwidth(带宽): The range of signal frequency 2. Delay(时延)： 发送时延 数据块长度/信道带宽 传播时延 信道长度/电磁波在信道上的传播速率 电磁波在自由空间的传播速率是光速，即3.0105km/s, 在铜缆中的传播速率约为2.3105km/s, 在光纤中传播速率约为2.0105km/s. 处理时延：数据在交换结点为存储转发而进行一些必要的处理所花费的时间。 时延带宽积 传播时延 带宽 从公式可知：时延带宽积又可以称为以比特为单位的链路长度。 往返时延：表示从发送端发送数据开始，到发送端收到接收端的确认，总共经历的时延。,Thank You !,