Presentation on theme: "Www.monash.edu.au IMS5401 Web-based Systems Development Topic 2: Elements of the Web (c) Digital representation (multimedia) (d) Digital transmission (data."— Presentation transcript:
IMS5401 Web-based Systems Development Topic 2: Elements of the Web (c) Digital representation (multimedia) (d) Digital transmission (data comms)
2 Agenda 1.Digital representation and transmission 2.Topic 2 (c) Information representation 3.Topic 2 (d) Digital transmission 4.Some implications of digital representation for web applications 5.Some implications of digital transmission for web applications
3 Elements of the Web THE WEB Connecting computers Digital representation of documents Display and organisation of documents Linking documents
Digital representation and transmission Computers can only receive, store, process and transmit binary digital data (ie the electro-magnetic equivalent of 1s and 0s) Managing information in different forms - numbers, symbols, text, graphics, sound, etc - is dependent on our ability to convert them into binary digital form Complex specialised software is needed to manage this conversion
5 Basic digital representation concepts Numbers - use binary mathematics Text and symbols - use a coding system in which numbers represent letters (ASCII code) Graphics - convert image to a grid of dots (bit- map or raster), or mathematically-defined shapes (vectors) Sound - convert sound wave frequency and amplitude to numbers If in doubt, see the guide in your resources
6 Basic data transmission concepts Data is transmitted in analogue form (electromagnetic waves); therefore needs conversion out of and into digital form at each end. Use modem (= MOdulate/DEModulate) Bandwidth = the term used to describe the rate at which a data transmission medium can carry data; measured in bits per second (bps) High bandwidth = fast data transmission (broadband) Low bandwidth = slow data transmission (narrow band)
7 Some key features of digital representation and transmission Simplicity Clumsiness of representation (for all except numbers) Disadvantages for storage and processing Convenience and ease of transmission Cost in terms of speed of transmission Need and capability for compression
Topic 2(c) Information representation: Brief history Coding systems for communication ( Morse) Digital codes (late 1800s - early 1960s) ASCII code for digital representation (1963) Computer graphics (1960s) Graphical user interfaces, icons, etc (early 1980s) Combining text, graphics, sound, animations, etc in a single document (early 1990s) Sound/video broadcasting on-line (early 1990s) Streaming media (mid-late 1990s)
9 ‘Typical’ file sizes A page of text = 3Kb A full screen graphic with ‘normal’ colours = 300Kb A full screen graphic with ‘high quality’ colours = 1Mb One minute of voice = 0.6Mb One minute of CD quality music = 5-10 Mb One second of high quality full screen video = Mb (Remember, these are in bytes, so multiply by 8 to convert to the number of bits)
10 Compression Enormous amounts of work have gone into developing compression techniques for digital media to reduce file sizes Wide variety of techniques for both: Lossless compression = no loss of quality; and Lossy compression = some loss of quality Very complex area; rapidly evolving; too messy to talk about here Provides a partial solution to the file size problem
11 File formats for web documents Multimedia before the web Why different file types? –Proprietary formats and ownership issues –Quality of reproduction issues –Efficiency issues File transmission on the Internet - MIME-types Multimedia on the web - MIME-types, browsers and plug-ins
12 File formats for web documents Standards?! (discuss in later lecture) Text - PDF Bitmapped graphics - GIF, JPEG, PNG(?) Vector graphics - SVG? Sound - WAV, AIF, WMA, MP3 Video - Quicktime, AVI, MPEG, WMV Animation - Shockwave Web software is (usually) designed to cope with these common standard formats, but others can cause problems
Topic 2(d): Data transmission Copy of page from web server is displayed by browser ClientServer Copy of page from web server is displayed by browser Web page available for transmission to web client Data Transmission Transmission Media Web serverUser computer
14 Transmission media Local area cabling - Ethernet “Normal” telephone lines ISDN (across telephone lines) DSL (across telephone lines) Cable Satellite Wireless
15 Typical transmission speeds (in bits/sec) Normal telephone lines (POTS) - 56k ISDN - 128k DSL - 256k-1.5m Satellite - 400k Wireless - 1m - 5m Cable - 500k - 2m Ethernet 10m m BUT NOTE: All speeds are VERY sensitive to traffic conditions, etc and ‘real’ speeds are usually much lower than theoretical speeds
16 A special note on wireless The wireless Internet became flavour of the month for a short while - Internet access by phone, PDA, etc The wireless world is extremely complex; driven by the market and by evolving standards; big players - Motorola, Nokia, Ericsson, etc Complex evolutionary path; many different ‘standards’, etc; WAP, WML, HDML, etc Early fad seems to have died; future? Note: transmission speed even more limited than others; what is realistically achievable?
17 Other issues for transmission media and transmission speeds Variation in traffic levels and its influence on speed Differences in up-load and download speeds Availability and accessibility Cost Installation effort and infrastructure Security
18 Specialist issues in sending and receiving At the server end: Process requests and retrieve page from disk; performance requirements - disk access speed/number of concurrent requests Response times/load - acceptable limits Server performance and power (I/O, not processing!) - specialised server machines and server services - ISPs At the client end: Receive page elements; assemble in processor; display on monitor Performance issues - file conversion; buffering for streaming media Browser control over page display
19 Specialist aspects at the ends Cache memory - Saving on getting the same thing twice Streaming audio and video - buffering Proxy servers - an extra server located between the client and the web server –Providing cache memory for many users –Filtering, protecting and controlling (ingoing and outgoing); using proxy servers as ‘firewalls’ –Monash proxy server as an example Specialised server capabilities - multicasting
Implications of multimedia for the web The web was built to connect documents and pages, not data (contrast with traditional IS) How important is it to have multimedia content? How does it affect usability of applications How easy is it to create (good) multimedia content? What are the appropriate knowledge and skills? Control of document ownership can become a problem when the copy is identical to the original What forms of alteration (compression) are acceptable?
21 How important is multimedia in a web application? What would the web be like if it could include only black and white text-based documents? How important is multimedia to particular applications (unwanted-useful-essential)? (eg how much would the porn industry use the web without multimedia?) How good does the multimedia have to be? To what extent is multimedia needed in “traditional” IS applications?
22 Multimedia documents and web usability The use of multiple media is seen as crucial for some applications; the success of the Web has been strongly based in its ability to use multimedia Problems in dealing with multimedia have also been the single greatest source of complaints and dissatisfaction about the web Some proposed uses of the web have never got going because of its weaknesses in dealing with multimedia
23 When does size matter? Download time depends on file size and speed of transmission To calculate the download time for a web page: work out the size of each of the page’s elements (text, graphics, etc); add them together; convert to bits; and Divide by the transmission speed (in bits per second) Answer = download time in seconds If our user is on a 56k modem, how long will they have to wait for multimedia?
24 What can be done about the storage requirements of digital media? Remove web page elements which need big files - no video, sound, complex animation Reduce actual size of web page elements - smaller pictures, shorter animation/video/ sound Trade-off quality against file size - fewer colours, lower quality sound, jerky animation/video Compress files
25 Issues for the Web developer (1) Identifying appropriate use of multimedia for an application Creating good multimedia content Creative concept skills? Detailed design/construction skills? Technological skills? Managing the trade-off between quality of content and usability of site
26 Issues for the Web developer (2) How important are the big web page elements to the site? How much do I have to change my page sizes to cater for my users’ data transmission rates? How far can I go in reducing design quality in order to achieve better page download speeds? Will users put up with slower download speeds to get a high quality page design? What matters most for my system and its users; how can I find out what my users think?!
27 Issues for the Web developer (3) Are there issues of copyright involved in the documents and their multimedia content? How is copyright protected (locally and internationally)? Whose problem is it (mine as creator, the web site administrator as distributor or the user)? Extremely complex and legally vague area at present
Implications of data communications for the web If I want to get a web page from your server, how long will it take for me to get it? …. And, underlying this, the key question… How will the length of time taken affect people’s willingness to use the web for various purposes?
29 The communications network dream and reality The Internet communications network dream: Universal connectivity from a wide range of devices Universal availability at low cost Broadband transmission rates The Internet communications network reality: Limited connectivity mainly from computers Limited availability at relatively high cost Broadband transmission for only a few How much (and how quickly) will this change?
30 How widely available are broadband links? Limiting factors infrastructure cost support How does this affect: You? Your local communities? (work/social) The wider community? Who are the winners and losers (the digital divide)?
31 Implications for applications How patient will users be in carrying out a particular task on the web, if the transmission speed is slow? How much will users be prepared to pay for faster web access? How much and how fast will data transmission speeds improve? How much can you improve speed for users by changing the design of your web site? What effect will these changes have on the design quality of your web site?
32 The evidence on user behaviour Wait times (Miller, 1968) : For perceived instantaneous response: < 0.1 sec For no interruption to user’s train of thought:<1.0 sec For keeping user’s attention: <10 secs Usability studies User reactions Consistency/predictability of wait time Expectation of wait time Can users be trained/persuaded to wait?!
33 Implications for web system developers Web users are extremely impatient; therefore bandwidth is the single biggest problem for web site developers Slow page loading speeds = unhappy users = failed systems There is not much can be done (in the short term) about data transmission speeds; therefore need to focus on how to reduce the amount of data which has to be transmitted Hence, the importance of digital representation and its effects on page size
Summary The web enables very advanced multimedia content … but at a significant cost in download times Enabling multimedia is one thing … but how easy is it to create good multimedia appropriate to an application? For which applications will users want/need multimedia content? How long will users have to wait, and will they be prepared to do so?
35 What does this mean for information professionals? Understanding what multimedia can do for an application Being able to create it?! Understanding how people respond to slow download times Designing within those limits Accepting the limitations of what can be done (with technology OR with people !)