1. Copyright 2004 Monash University IMS5401 Web-based Systems Development Topic 2: Elements of the Web (e) Data Communications

2. Copyright 2004 Monash University 2 Agenda 1. Key concepts 2. Data transmission at each end 3. Data transmission in the middle 4. Implications of bandwidth issues for web developers

3. Copyright 2004 Monash University 3 Elements of the Web THE WEB Connecting computers Digital representation of documents Display and organisation of documents Linking documents

4. Copyright 2004 Monash University 4 1. Key Concepts Basics Data transmission

5. Copyright 2004 Monash University 5 Remember the basics of linking computers (2a) and documents (2b) 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

6. Copyright 2004 Monash University 6 The Data Transmission Questions 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?

7. Copyright 2004 Monash University 7 Data Transmission Copy of page from web server is displayed by browser Web page available for transmission to web client Client Server Data Transmission Transmission Media Web serverUser computer

8. Copyright 2004 Monash University 8 2. Data transmission at each end The process At the client end At the server end Specialist elements

9. Copyright 2004 Monash University 9 The Data transmission process 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

10. Copyright 2004 Monash University 10 Receiving at the client end …. 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

11. Copyright 2004 Monash University 11 Receiving and sending from the server end …. 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

12. Copyright 2004 Monash University 12 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

13. Copyright 2004 Monash University 13 3. Data transmission in the middle Bandwidth The transmission media The media and their characteristics Transmission speeds

14. Copyright 2004 Monash University 14 Bandwidth 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)

15. Copyright 2004 Monash University 15 Types of Transmission Media Categorised by medium of transmission Cable-based vs radio transmission Categorised by bandwidth Narrow band vs broadband

16. Copyright 2004 Monash University 16 The Transmission Media Local area cabling - Ethernet Telephone lines ISDN DSL Cable Satellite Wireless

17. Copyright 2004 Monash University 17 Ethernet local area network 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

18. Copyright 2004 Monash University 18 Telephone line (POTS) 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

19. Copyright 2004 Monash University 19 ISDN 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

20. Copyright 2004 Monash University 20 DSL 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,

21. Copyright 2004 Monash University 21 Cable 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

22. Copyright 2004 Monash University 22 Wireless 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)

23. Copyright 2004 Monash University 23 Satellite 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

24. Copyright 2004 Monash University 24 Transmission Speeds (remember, these are in bits per second) 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

25. Copyright 2004 Monash University 25 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 even more limited than others; raises significant issues for what is realistically achievable

26. Copyright 2004 Monash University 26 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

27. Copyright 2004 Monash University 27 4. 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

28. Copyright 2004 Monash University 28 Questions for web system developers 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?

29. Copyright 2004 Monash University 29 Network effects of data communications limitations 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 This has implications for: Network usability and usage Network extendability and development