Transcript Slide 1

IMS5401
Web-based Systems Development
Topic 2: Elements of the Web
(c) Digital representation (multimedia)
(d) Digital transmission (data comms)
www.monash.edu.au
Agenda
1.
2.
3.
4.
Digital representation and transmission
Topic 2 (c) Information representation
Topic 2 (d) Digital transmission
Some implications of digital representation for
web applications
5. Some implications of digital transmission for
web applications
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Elements of the Web
Connecting
computers
Digital
representation THE WEB
of documents
Display and
organisation of
documents
Linking
documents
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1. 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
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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 (bitmap 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
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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)
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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
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2. Topic 2(c) Information
representation: Brief history
• Coding systems for communication (1840 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)
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‘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 = 2530Mb
• (Remember, these are in bytes, so multiply by 8 to
convert to the number of bits)
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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
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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
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File formats for web documents
• Standards?! (discuss in later lecture)
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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
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3. Topic 2(d): Data transmission
Client
Server
Copy of
page from
web server
is displayed
by browser
Data Transmission
Web page
available for
transmission
to web client
User computer
Transmission
Media
Web server
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Transmission media
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Local area cabling - Ethernet
“Normal” telephone lines
ISDN (across telephone lines)
DSL (across telephone lines)
Cable
Satellite
Wireless
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Typical transmission speeds (in bits/sec)
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Normal telephone lines (POTS) - 56k
ISDN - 128k
DSL - 256k-1.5m
Satellite - 400k
Wireless - 1m - 5m
Cable - 500k - 2m
Ethernet 10m - 1000m
BUT NOTE: All speeds are VERY sensitive to
traffic conditions, etc and ‘real’ speeds are
usually much lower than theoretical speeds
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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?
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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
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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
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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
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4. Implications of multimedia for the web
• The web was built to connect documents and
pages, not data (contrast with traditional IS)
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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?
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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?
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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
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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?
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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
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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
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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?!
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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
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5. 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?
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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?
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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)?
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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?
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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?!
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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
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6. 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?
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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!)
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