Transcript Data Transmission

IMS5401
Web-based Systems Development
Topic 2: Elements of the Web
(e) Data Communications
Copyright 2004 Monash University
Agenda
1. Key concepts
2. Data transmission at each end
3. Data transmission in the middle
4. Implications of bandwidth issues for web
developers
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Elements of the Web
Connecting
computers
Digital
representation
of documents
THE WEB
Linking
documents
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Display and
organisation of
documents
1. Key Concepts
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Basics
Data transmission
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Remember the basics of linking
computers (2a) and documents (2b)
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Internet = network of computers and
computer networks; every machine has an IP
address to identify it
A standard protocol called TCP/IP enables
data to be sent from any computer to any
other computer on the Internet
Web connects documents on web servers on
the Internet; every document has a URL to
identify it
HTTP allows users to request copies of pages
to be sent by a web server to a client
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The Data Transmission Questions
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If I want to get a web page from your server,
how long will it take for me to get it?
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…. And, underlying this, the key question…
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How will the length of time taken affect
people’s willingness to use the web for various
purposes?
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Data Transmission
Copy of
page from
web server
is displayed
by browser
Client
User computer
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Data Transmission
Web page
available for
transmission
to web client
Server
Transmission
Media
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Web server
2. Data transmission at each
end
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The process
At the client end
At the server end
Specialist elements
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The Data transmission process
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User enters URL (or clicks on link to URL)
Browser sends request for page
Data communications magic occurs and
request data flows from client to server
Request arrives at web server
Web server receives request, finds page,
makes a copy and sends it
More data communications magic occurs and
page data flows from server to client
Page arrives at user computer and browser
displays it
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Receiving at the client end ….
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Receive data from data input - assemble in
processor - display on monitor
How fast can my computer assemble and display
the data it is receiving? Performance issues buffering for streaming media
Browser control over how much of the page I see
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Receiving and sending from the
server end ….
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Process request - retrieve page from disk transmit via data output
Performance requirements - disk access
speed/number of concurrent requests
Response times - acceptable limits
Server performance and power (I/O, not
processing!) - specialised server machines
Specialised server services - ISPs
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Specialist aspects at the ends
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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
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Specialised server capabilities - multicasting
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3. Data transmission in the
middle
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Bandwidth
The transmission media
The media and their characteristics
Transmission speeds
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Bandwidth
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The term used to describe the capacity of a
data transmission medium
High bandwidth = fast data transmission
Low bandwidth = slow data transmission
Bandwidth is measured in bits per second
(bps)
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Types of Transmission Media
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Categorised by medium of transmission
• Cable-based vs radio transmission
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Categorised by bandwidth
• Narrow band vs broadband
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The Transmission Media
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Local area cabling - Ethernet
Telephone lines
ISDN
DSL
Cable
Satellite
Wireless
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Ethernet local area network
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Standard for connecting computers within a
building/local area
Direct connection from port on user machine
to network
Complex technology, but simple from the
user’s viewpoint
Varying transmission speeds possible, but all
give very high data transfer rates
Originally cable-based, but wireless Ethernet
is now possible too
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Telephone line (POTS)
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Oldest method
Data is converted from digital to analogue
form at the sender end, transmitted along the
telephone line like voice communication and
converted back to digital form at the receiver
end
Modem does conversion
Can achieve only relatively slow transmission
speeds
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ISDN
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Integrated Services Digital Network
First improvement to telephone lines
Uses telephone lines and works in similar
way, but has its own ‘channel’ in the phone
line
Uses adapter instead of modem
Faster than telephone lines
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DSL
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Digital Subscriber Line
Still uses telephone line
Relatively recent;many versions - ADSL, HDSL,
IDSL, SDSL, RADSL, etc.
ADSL= most common (Asymmetric DSL; different
bandwidth in each direction)
Uses specialised conversion equipment at both
ends (like a modem)
Restricted by distance from telephone exchange,
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Cable
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Special coaxial or fibre optic cable designed
for and dedicated to transmission of digital
data
Very fast
Uses specialised cable modem
Still not uniformly available, especially
outside big cities
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Wireless
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Radio transmission beamed from transmitter
to receiver
Needs transmission towers, antenna, etc
Needs correct alignment, line of sight
Can be affected by weather, etc
Recently being used for cable-less local area
transmission (airport)
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Satellite
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Radio transmission beamed from transmitter
to satellite and back down to receiver
Relatively recent
Needs expensive receiving dishes, etc
Needs correct alignment, line of sight
Can be affected by bad weather, atmospheric
conditions, etc
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Transmission Speeds (remember,
these are in bits per second)
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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
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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 even more limited than
others; raises significant issues for what is
realistically achievable
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Other issues for transmission
media and transmission speeds
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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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4. Implications for web system
developers
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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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Questions for web system
developers
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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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Network effects of data
communications limitations
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Data communications is a vital factor in
determining:
• Network availability, cost and performance
• Differences in these from one user to another
• The viability of different media on the network
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This has implications for:
• Network usability and usage
• Network extendability and development
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