ComputerNetworks

Chapter 6

天津大学软件学院

计算机导论

计算机网络 ?

• Transportation service: move objects– horse, train, truck, airplane ...

• Communication network: move information– bird, fire, telegraph, telephone, – Internet … 计算机网络

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•计算机网络是自主计算机的互连集合。•这些计算机各自是独立的，地位是平等的，他们通过有线或无线的传输介质连接起来，在计算机之间使用统一的通信协议。•不同的计算机网络可以采用网络设备实现互连，构成更大范围的互联网络。•在计算机网络上达到信息的高速传送以及硬件、软件和信息资源的共享。

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OSI MODEL( 开放系统互联模型）

SEVEN LAYERS

The Open Systems Interconnection model is a theoretical model that shows how any two different

systems can communicate with each other.

计算机导论

位

帧

信息包

会话层协议数据单元

传输层协议数据单元

表示层协议数据单元

应用层协议数据单元

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Flow of data in the OSI model

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CATEGORIES OF NETWORKS

LAN （局域网）

MAN （城域网）

WAN （广域网）

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LOCAL AREA NETWORK (LAN)

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METROPOLITAN AREA NETWORK (MAN)

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WIDE AREA NETWORK (WAN)

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计算机与外界局域网的连接是通过主机箱内插入一块网络接口板（或者是在笔记本电脑中插入一块 PCMCIA 卡）。网络接口板又称为通信适配器或网络适配器（ adapter ）或网络接口卡 NIC （ Network Interface Card ）

PCMCIA ： PC 机内存卡国际联合会

双绞线 4 对 8 条 线，八根线的布线规则是 1236线有用， 4578 线闲置。 双绞线 RJ45 水晶头的制作线序 ： T568B 标准描述的线序从左到右依次为： 1- 白橙、 2- 橙、 3-白绿、 4- 蓝、 5- 白蓝、 6- 绿、 7- 白棕、 8- 棕。

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CONNECTING DEVICES

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REPEATERS （中继器） A repeater is an electronic device that regenerates data. It

extends the physical length of a network. As a signal is transmitted, it may lose strength, and a weak signal may be interpreted erroneously by a receiver. A repeater can regenerate the signal and send it to the rest of the network.

Repeaters operate at the first layer of the OSI model.

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BRIDGES （网桥） A bridge is a traffic controller. It can divide a long bus into

smaller segments so that each segment is independent trafficwise. The bridge uses a table to decide if the frame needs to be forwarded to another segment. With a bridge, two or more pairs of stations can communicate at the same time.

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ROUTERS （路由器）

Routers are devices that connect LANs, MANs, and WANs. A router operates at the third layer of the OSI model. Whereas a bridge filters a frame based on the physical (data-link layer) address of the frame, a router routes a packet based on the logical address (network layer) of the packet.

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THE INTERNET AND TCP/IP TCP/IP and OSI model

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NETWORK LAYER At the network layer, TCP/IP supports the Internet

Protocol (IP). IP is an unreliable protocol and a best-effort delivery service. The term best-effort means that IP provides no error checking or tracking.

The data unit at the IP layer is called an IP datagram, an independent packet that travels from the source to the destination.

Protocol ：协议Datagram ：数据报，自带寻址信息的 , 独立地从数据源行走到终点的数据包

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Addressing TCP/IP requires that every computer connected to the Inter

net be identified by a unique international address. This address is sometimes referred to as the Internet address, or IP address.

Each Internet address consists of 4 bytes (32 bits). To make the 32-bit form shorter and easier to read, Internet addresses are usually written in decimal form with decimal points separating the bytes: dotted-decimal notation.

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域名系统• 主机名由计算机名加域名构成

– nc.tju.edu.cn• nc: 计算机名• tju.edu.cn 域名 由两个或多个部分组成，说明计算

机所属的组织或组织的一个子集– 域名的最后一部分（ top-level domain ）顶级域名

声明所属国家或组织类型.cn 中国.au 澳大利亚.ca 加拿大.uk 英国

• 域名系统（ domain name system DNS)

把域名翻译成数字 IP 地址

Host Name ： nc.tju.edu.cn

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1 字节网络地址

2 字节网络地址

3 字节网络地址

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TRANSPORT LAYER At the transport layer, TCP/IP defines two protocols: Tra

nsmission Control Protocol (TCP) and User Datagram Protocol (UDP).

The TCP provides full transport layer services to applications. TCP is a reliable transport protocol.

APPLICATION LAYER TCP/IP 中的应用层等同于 OSI 中会话层、表示层和应用层的组合。

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File Transfer Protocol (FTP ，文件传输协议 ) The standard protocol on the Internet for transferring a file

from one machine to another is the File Transfer Protocol (FTP). FTP was designed to respond to traditional problems related to file transfer, one problem is the different coding systems in use; one machine may use ASCII, and the other may use Unicode. Another problem is the different file formats in use. FTP was designed to resolve these problems.

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Simple Mail Transfer Protocol (SMTP, 简单邮件传输协议 ) By far the most popular application on the Internet today is

electronic mail (email). The protocol that supports email on the Internet is Simple Mail Transfer Protocol (SMTP). To send and receive email, the user must install both client and server SMTP software on a computer. SMTP is often used with another protocol such as Post Office Protocol (POP 邮局协议 ).

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Address SMIP uses a unique addressing system that consists of two parts: a local part and a domain name separated by an @ sign. The local part defines the name of a special file, called the user mailbox, where all the mail received for a user is stored for retrieval by the user agent. The domain name defines the computer (often symbolically) that serves as the SMTP server.

本地部分 域名

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Hypertext Transfer Protocol (HTTP ，超文本传输协议 )

Hypertext Transfer Protocol (HTTP) is a client-server program that is used to access and transfer documents in the World Wide Web.

Uniform Resource locator (URL 统一资源定位器 ) HTTP uses a special kind of addressing called the Uniform Resource Locator (URL), which is a standard for specifying any kind of information on the Internet. The URL defines four things: method, host computer, port, and path.

For example: http://www.w3.org/hypertext/project.html

method: http, host: www.w3.org ， pathname and name of file: hypertext/project.html 。

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作业• 6-18 ， 6-19• 6-30——6-32• 6-43——6-48

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A computer network is a combination of devices connected by transmission media. The Open Systems Interconnection (OSI) model is a theoretical model that shows how any two different systems can communicate with each other. The seven layers of the OSI model are the physical layer, data-link layer, Network layer, transport layer, session layer, presentation layer, and application layer. A local area network (LAN) allows resource sharing (hardware, software, and data) between computers. A LAN can de configured in a bus, ring, or star topology. A metropolitan area network (MAN) uses the services provided by a common carrier.

SUMMARY

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A wide area network (WAN) is the connection of individual computers or LANs over a large area. WANs are installed and run by common carriers. A repeater is a connecting device that regenerates data and extends the physical length of a network. A bridge is a connecting device that filters traffic. A router is a connecting device that routes packets. A gateway allows two networks, each with a completely different protocol suite, to communicate. An internetwork is two or more LANs, MANs, or WANs. Transmission Control Protocol/Internet Protocol (TCP/IP) is the set of protocols used by the Internet, a worldwide internetwork of computers.

SUMMARY (continued)

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The Internet Protocol (IP) is TCP/IP’s unreliable protocol at the internet layer. An IP address identifies each computer connected to the Internet. User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) are protocols at the transport layer. File Transfer Protocol (FTP) is a TCP/IP client-server application for coping files from one host to another. The protocol that supports electronic mail (email) on the Internet is Simple Mail Transfer Protocol (SMTP).

SUMMARY (continued)

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TELNET is a client-server application that allows a user to log on to a remote machine, giving the user access to the remote system. Hypertext Transfer Protocol (HTTP) is a client-server program for accessing and transferring documents on the World Wide Web (WWW), a collection of multimedia documents. The Uniform Resource Locator (URL) is a standard identifier for specifying information on the Internet. A browser is needed to access a page on the WWW. A document on the Internet can be classified as static, dynamic, or active.

SUMMARY (continued)