1. 1 IT-212, How Computers Work Internet Video, Audio, & WWW Electrical and Computer Engineering Spring 2002

2. 2 Internet Video and Audio Internet Initially Text-only Medium Unable to handle large volume of data necessary to provide music and video (insufficient bandwidth) First network 56 kb/s First Video Was Short Clips Entire file be downloaded to hard drive prior to initiation

3. 3 Internet Video and Audio Continuous Play (Streaming) Action can start as soon as initial data is received  e.g., Real Player, MP3 audio Uses User Database Protocol (UDP)  Email uses Transmission Control Protocol (TCP) Error checks each packet, retransmits on error, takes time  UDP skips retransmission

4. 4 Streaming Audio Events (1) User Clicks on Active Portion of Web Page Web Server for Page Detects Click Server Sends Metafile Back to Browser Location of sound file Instructions on how to play sound file  e.g., which browser plug-in software should be used  Browser specific Typically developed by third parties Available for download and installation via web

5. 5 Streaming Audio Events (2) Different File May Be Downloaded Based on Speed of Connection Sends File Using UDP PC Decompresses and Decodes Files, Fills Buffer When Buffer Is Full, Data Sent to Sound Card As Buffer Empties, Transmission Attempts to Keep Full, but If Buffer Becomes Empty Recorded music stops until refilled Live broadcast skips portions

6. 6 Streaming Video Video Capture Card Digitizes Images at 30 Frames Per Second Frames Are Compressed Drop Frames on Slower Connections

7. 7 Streaming Video Compressed Video Is Broken Into One of Two Types of Packets (Protocols) by Server Internet Provider (IP) multicast packets  Single signal sent to multicast server which retransmits to multiple PCs  Reduced bandwidth  Best in multiple recipient mode UDP  Higher bandwidth  Less apt to require gaps or pauses in audio

8. 8 Streaming Video Compressed Video Decompressed and Stored in Buffer (Temporary Storage) in System RAM Data Is Segmented Into Video and Audio and Sent to Respective Cards Frames can be skipped Software attempts to detect erroneous video data by comparing examining future frames  Important because compression methods use 1 frame as key: error here continues on subsequent frames

9. 9 Web Browsers Netscape Navigator Microsoft Internet Explore Sun StarOffice

10. 10 Accessing Web Pages (1) Implicit Hyperlink Typically highlighted word or phrase Cursor changes shape when activated Displayed text can be unrelated to actual web page location

11. 11 Accessing Web Pages (2) Enter URL or IP adx directly Universal Resource Locator  http://cpe.gmu.edu/~khintz http://cpe.gmu.edu/~khintz If no file name specified, looks for index.htm or index.html  ftp://ftp.redhat.com ftp://ftp.redhat.com Web server, e.g., Apache, routes request to particular directory  Only subdirectories of that directory are accessible

12. 12 Getting Web Page (1) Access Internet Using LAN, or Internet Service Provider (ISP) Request goes to nearest Domain Name Server (DNS) Convert domain name to Internet Protocol (IP) Address  e.g., cpe.gmu.edu  129.174.140.5

13. 13 Getting Web Page (2) Browser Passes IP Address to Router Figures how to get to destination Uses reports of recent traffic to select best path Site Server at Intended Location Sends Confirmation of Connection Back to Originator Browser message changes to show connecting Message may be routed instead to proxy server  Mirror copy of “parent” server  Routed if path to parent is too congested

14. 14 Getting Web Page (3) Requested Web Page Is Defined by HTML That Controls Content and Format of Data Can include hypertext references to other files or other parts of same file HTML File Is Send Back to Browser May take different back back than path original request came over

15. 15 Getting Web Page (4) If Embedded Files References Are on Another Server, Target Site Issues Instruction to Other Locations to Transmit Graphics files Sound files Video files Browser Cache Local copy of web file reduces internet traffic Can automatically check if latest version or manually force reload from web site (shift-reload)

16. 16 Getting Web Page (5) Non-streaming Files Need to Be Received in Entirety Before Being Initiated wave, MIDI, AVI Browser Uses HTML Commands to Determine Where Items Will Go on Web Page Animated Logo Associated With Browser (Netscape, IE) Stops When Web Page Construction Is Complete

17. 17 Example HTML

18. 18 HTML, Source (1) inft-212 sample_html

19. 19 HTML, Source (1) INFT-212 Sample HTML Hyperlink to CPE home page

20. 20 HTML, Source (2) sample numbered list numbered 1 numbered 2 sample unnumbered list unnumbered 1 unnumbered 2

21. 21 HTML, Source (3) Maintainer of page: khintz@gmu.edu

22. 22 Enhanced Servers ColdFusion Superset of HTML language Allows queries into databases, e.g., MS- SQL7, postgreSQL Provide better web page development environment

23. 23 Web Search Engines (1) Typical Search Engines AltaVista Yahoo Excite Use Web crawlers (aka spiders) Automatically Search through hypertext When document is located, key data is transmitted back to crawler  Location of site (URL) noted  Embedded text

24. 24 Web Search Engines (2) Indexing Software Attempts to Classify Information Content of All Sites Frequency of certain words Words in Titles, headers Human reviewers User Enters Keywords Which to Search Some Search Engines allow sophisticated searches  e.g., Test AND Generation AND Software Web pages (with excerpted text) are displayed in list

25. 25 Web Search Engines (3) Documents Can Include Metatext to Guide (and Potentially Deceive) Search Engines Some Search Engines (e.g. Google) Perform Own Search and Then Pass Search Onto Other Search Engines