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IT-212, How Computers Work

Internet Video, Audio, & WWW

Electrical and Computer EngineeringSpring 2002

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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

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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

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Streaming Audio Events (1)

User Clicks on Active Portion of Web PageWeb Server for Page Detects ClickServer 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

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Streaming Audio Events (2)

Different File May Be Downloaded Based on Speed of ConnectionSends File Using UDPPC Decompresses and Decodes Files, Fills BufferWhen Buffer Is Full, Data Sent to Sound CardAs Buffer Empties, Transmission Attempts to Keep Full, but If Buffer Becomes Empty Recorded music stops until refilled Live broadcast skips portions

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Streaming Video

Video Capture Card Digitizes Images at 30 Frames Per SecondFrames Are Compressed Drop Frames on Slower Connections

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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

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Streaming Video

Compressed Video Decompressed and Stored in Buffer (Temporary Storage) in System RAMData 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

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Web Browsers

Netscape NavigatorMicrosoft Internet ExploreSun StarOffice

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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

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Accessing Web Pages (2)

Enter URL or IP adx directly Universal Resource Locator

http://cpe.gmu.edu/~khintz If no file name specified, looks for index.htm

or index.html ftp://ftp.redhat.com

Web server, e.g., Apache, routes request to particular directory Only subdirectories of that directory are

accessible

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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

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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

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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

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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)

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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 PageAnimated Logo Associated With Browser (Netscape, IE) Stops When Web Page Construction Is Complete

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Example HTML

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HTML, Source (1)

<!doctype html public "-//w3c//dtd html 4.0 transitional//en">

<html><head> <meta http-equiv="Content-Type" content="text/html;

charset=iso-8859-1"> <meta name="Author" content="Kenneth J. Hintz"> <meta name="GENERATOR" content="Mozilla/4.75 [en]

(Win98; U) [Netscape]"> <title>inft-212 sample_html</title></head>

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HTML, Source (1)

<body><h1> INFT-212 Sample HTML </h1>

<p><br>&nbsp;<a href="http://cpe.gmu.edu">Hyperlink to CPE home

page</a>

<p><img SRC="CAMPUS1.JPG" height=96 width=128>

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HTML, Source (2)

<ul><li> sample numbered list</li><ol><li>numbered 1</li><li>numbered 2</li></ol><br>&nbsp;<li>sample unnumbered list</li><ul><li>unnumbered 1</li><li>unnumbered 2</li></ul></ul>

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HTML, Source (3)

<hr WIDTH="100%">

<br>Maintainer of page:&nbsp; <a href="mailto:

khintz@gmu.edu">khintz@gmu.edu</a><br>

<hr WIDTH="100%">

</body></html>

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Enhanced Servers

ColdFusion Superset of HTML language Allows queries into databases, e.g.,

MS-SQL7, postgreSQL Provide better web page development

environment

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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

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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

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Web Search Engines (3)

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