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UNIT 5. Introduction to Computer Networks

UNIT 5. Introduction to Computer Networks. Feng Lin Sichuan Univ. Outline. Unit 5.1 Basic of Internet Unit 5.2 More on Computer Networks Unit 5.3 Network Architecture. About me. Dr. Feng Lin Course Email: linfengscu@gmail.com Class Hours: 6 Lecture hours & 6 Lab hours

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UNIT 5. Introduction to Computer Networks

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  1. UNIT 5. Introduction to Computer Networks Feng Lin Sichuan Univ.

  2. Outline • Unit 5.1 Basic of Internet • Unit 5.2 More on Computer Networks • Unit 5.3 Network Architecture

  3. About me • Dr. Feng Lin • Course Email: linfengscu@gmail.com • Class Hours: 6 Lecture hours & 6 Lab hours • My Office: RM 325, Building B, Jichujiaoxue Hall, Wangjiang Campus. (望江校区基础教学楼B座325） • Office Hours: 9:00 AM – 11:30 AM, Fri.

  4. Unit 5.1 Basic of Internet

  5. Agenda • First Sight of Communication • Everything is Connected to Everything • Applications of Internet • Web • Email • Classification of Computer Networks • Transmission Media • LAN,MAN,WAN • Client/Sever vs Peer to Peer

  6. First Sight of Communication • The human need to communicate has motivated mankind’s creativity: • Cave dwellers drew pictures on walls. • Smoke signals, drum rhythms passed messages. • Postal service: a system for physically transporting documents and other small packages • Alexander Graham Bell: invented the telephone. • Data Communication: Data communications are the exchange of data between two devices via some form of transmission medium

  7. Sending device— initiates instruction to transmit data, instructions, or information Communications device — connects the sending device to the communications channel Communications channel— media on which data, instructions, or information travel Communications device — connects the communications channel to the receiving device Receiving device— accepts transmission of data, instructions, or information What is needed for successful communications?

  8. Everything is Connected to Everything • Computer Network: A collection of computers, display terminals, printers, and other devices linked either by physical or wireless means. • Seeds of Networking • 1966: ARPA (Advanced Research Projects Agency) State Defense Department’s research organization. • Focused major development effort on computer networking. • ARPA’s Goal: To promote research in advanced future technologies by funding university and industry research proposals. • Result: Thousands of databases became available to the public.

  9. Everything is Connected to Everything • Computer Networking • Internet: A world-wide network connecting millions of computer networks for the purpose of exchanging data and communications using special rules of communication. • internet: (lower case i) Any network connecting two or more computer networks. • Going online: Connecting to a collection of interconnected computers on a network. • Do banking. • Pay bills. • Buy groceries. • Book vacation travel. • Send messages. • Participate in discussions. • Do research. • Play games.

  10. Everything is Connected to Everything Web E-Mail InstantMessaging Chat Rooms Newsgroups Internet Telephony WebFolders Fax Machine or Computer Fax/Modem FTP VideoConferencing

  11. Internet Basics • The Internet Backbone • Major internet communication links. • Network/Internet service provider (NSP/ISP) • Construct and maintain the Internet backbone, such as AT&T • Network access point (NAP) – point on the Internet where several connections converge

  12. Nation-wide Internet Service Providers in China • There are four Nation-wide Internet Service Providers in China • CERNET: China Education and Research Network • CSTNET: China Science and Technology Network • CHINANET: China Public Computer Internet • GBNNET: Golden Bridge Information Network • To be permitted to connect with Internet backbone directly

  13. Internet Servers • Many computers are on the Internet • Yours – called a client computer • Internet server computers – computers that provide information and services on the Internet

  14. Internet Servers

  15. Internet Servers Web server: provides information and services to Web surfers

  16. Internet Servers Mail server: provides e-mail services and accounts

  17. Internet Servers FTP (file transfer protocol) server: maintains downloadable files

  18. Internet Servers IRC (Internet Relay Chat ) server: supports discussion groups and chat rooms

  19. Data Transmission in Internet • When you transmit data, it is broken up into small pieces called packets • A packet is a parcel of data that is sent across a network • Has the address of its sender • Has the address of the destination • Has some data • When they reach the destination, they are put back together into original form • The computers must follow proper communication protocols to transmit packets

  20. Communications Protocols network protocols: • machines rather than humans • all communication activity in Internet governed by protocols human protocols: • “what’s the time?” • “I have a question” • introductions … specific msgs sent … specific actions taken when msgs received, or other events protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

  21. Communications Protocols • TCP/IP (Transport Control Protocol/Internet Protocol传输控制协议/Internet协议)– primary protocol for transmitting information over the Internet • HTTP (Hypertext transfer protocol超文本传输协议 )– supports the movement of information over the Web • FTP (File transfer protocol，文件传输协议)– the communications protocol you use to transfer information from one computer to another

  22. Internet Address and Router • IP Address （IP地址）: An identifier for a computer or device on a TCP/IP network. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 204.194.30.130 • Router （路由器）: Device which checks the IP address of the data at every intersection on the back bone, that carries the IP address of its destination .

  23. Domain Name • People find it difficult to remember long strings of numbers, so host computers also have names called domain names • Domain name – “fully qualified domain name” (FQDN), usually typed in all lowercase • Key component of URLs and e-mail addresses • www. msu.edu/biologycatalog.htm - www.msu.edu is the domain name

  24. Domain Names 域名 • Typed in lowercase • Ends with three letter extension which is its top-level domain • Top-level domain

  25. Domain Name Space 域名空间 地理顶级域名 类别顶级域名

  26. Domain Name System • Each name corresponds to an IP address • Domain name system – Huge database that houses the names and IP addresses • Domain name servers – Computers that house the database

  27. Finding IP Address with “nslookup”

  28. Applications of Internet • Bulletin Board System (BBS) • Conferences Online • IRC (Internet Relay Chat) • Internet telephony • Web • Email

  29. Bulletin Board System (BBS) • Provides a messenger service. • Used as a repository for software and documents that may be copied. • Provides a place for people to converse. • This type system is quickly being replaced by the World Wide Web. • BBS’s are still in existence. • bbs.scu.edu.cn

  30. Conferences Online • Computer conference (electronic conference): An electronic meeting that can take place at the convenience of the participants. • Conference participants can type in questions or comments so that other members can see them and respond. • WELL (Whole Earth ‘Lectronic Link) • Has thousands of users accessing more than 100 public conferences (topics). • Usenet (Users’ Network) • Hundreds of conferences organized by topic.

  31. IRC (Internet Relay Chat) • Developed in 1988 by Jarkko Oikarinen in Finland. • The original was a text-based Internet version. • “Real-time” online chat facilities: chat rooms. • Communication is accomplished via typing text over a “channel” • Participants can occupy one of the many public chat rooms where everyone can read and respond, or go into “private rooms” or “celebrity” chat rooms.

  32. Internet telephony • The term Internet telephony specifically refers to the provisioning of communications services over the public Internet, rather than via the public switched telephone network (PSTN). . • The successful case: Skype, google talk

  33. World Wide Web (WWW or the Web) • What is the Web 万维网? • Billions of documents, called Web pages 网页, available to anyone connected to the Internet A Web site is a collection of related Web pages You can share information by creating Web pages or posting photos on a photo sharingcommunity A podcast is recorded audio stored on a Web sitethat can bedownloaded A Web page contains text, graphics, audio, video, and links to other Web pages A blog consists of time-stamped articles ina journal format

  34. Difference between the Web and Internet • In the words of Tim Berners-Lee, • On the Net, you find computers; on the Web, you find the documents, sounds, videos, and information • On the Net, the connections are cables between computers; on the Web, connections are hypertext links • The internet is a communications systems; and the web is an interlinked collection of information that flows over the Net

  35. How does Web work?—a very first sight

  36. Elements of Web • Web Browser: the programs to allow you view the web • Web Server: the programs to deliver web content to browser • Web Page: the web document file usually written in HTML or comparable markup language • HTTP (Hypertext Transfer Protocol) : the protocol that allows Web browsers to communicate with Web servers

  37. Web Page • A document file especially created for Web viewing • usually written in HTML or comparable markup language • The main distinction of web page is to provide hypertext • Hypertext will navigate to other web pages via links • Hypertext - Refers to any word or phrase in an electronic document, which can be used as a pointer to a related text passage. • Hypertext link - Any text or image that has been designated as a means of accessing related material. • Websites are composed of a series of Web pages.

  38. In hypertext, one follows the links to related information. Columbus sailed west under the auspices of Queen Isabella of Spain. She commissioned... Queen Isabella’s husband was King Ferdinand... A biography of King Ferdinand, he reigned from...

  39. HTML • HTML (Hypertext Markup Language)– is a set of specifications for creating documents • HTML document – plain text, or ASCII, document with embedded HTML tags • HTML tags specify how the document should appear when displayed • World Wide Web Consortium (W3C) – organization that reviews and revises HTML standard • HTML 3.2 in early 1997, 4.0 in late 1997, 4.01, in 1999

  40. HTML tags, such as <HR> and <B>, are enclosed in documents • Treated as instructions to the browser • <HR> creates a horizontal rule • <B>Caterpillars</B> love sugar • <B>,</B> make the word Caterpillars bold • Is not case-sensitive (<b> is same thing as <B>)

  41. Formatting tags – change the appearance of text • Link tags – specify where and how to display links • Media tags – specify how to display media elements • Operational tags – specify the basic setup for a Web page

  42. Server may have other software, and, for efficiency, devotes one port for HTTP requests, one port for FTP, and one port for e-mail • Logical port – created by communications software and used by client software to make a connection to a specific server • Port 80 – HTTP • Port 25 – SMTP e-mail • Port 21 – FTP

  43. HTTP • HTTP (Hypertext Transfer Protocol) works in conjunction with TCP/IP to get Web resources to your desktop • Has commands or methods that help your web browser communicate with server • Transports your request to server and back • HTTP status code – indicates whether or not the browser’s request could be fulfilled

  44. MicrosoftInternetExplorer Netscape Chrome Mozilla Firefox Safari Web browser • What is a Web browser 浏览器?

  45. Web Browser • Request a web page from a web server by typing URL • Display the received web page to users • The server sends the data for the web page which contains the information to be viewed and a set of instructions that tells your browser how to display it. • Microsoft Internet Explorer • Netscape Navigator • Firefox

  46. URL • The uniform resource locator (URL，统一资源定位符) is the unique identifier of a web page. • The location window displays the URL of the current page.

  47. name of the resource, specified by a path communication protocol identifier host name More Detail about URL • URL is the address of a Web resource • http://www.icarnegie.com/courses.html might be interpreted this way: Using hypertext transfer protocol(http), retrieve the file on www.icarnegie.com named /courses.html.

  48. URLs • The format is: protocol://host.domain[:port number]/path/filename • Protocol will be used to retrieve the information. Some of the options are • file: find a file on your computer • http: hypertext transfer protocol - find an html file on a WWW server • ftp: file transfer protocol - find a file on the Internet • telnet: connection to a telnet service (a terminal emulator) • mailto: send a mail message, use SMTP

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