1. Department of Information Engineering 1 Objectives A basic understanding of the following –What is Internet? –What is client-server computing? –What is LAN? What is Ethernet? –What is Internet protocol? –How to write your web page? –What is broadband network? –What is MPEG1, MPEG2, and MP3? –What is e-commerce?

2. Department of Information Engineering 2 Internet –What is LAN? –What is Internet? –What is TCP/IP? –What is domain name? –Application - how does email work?

3. Department of Information Engineering 3 What is Local Area Network (LAN)? Problem –What is the simplest way to connect the following computers together?

4. Department of Information Engineering 4 What is Local Area Network (LAN)? Mesh –too many links ! Bus –single cable linking all computers

5. Department of Information Engineering 5 LAN Network address –to send data to a destination, we need an address –each computer is connected to the LAN by a network card –each card has a unique network address the data we sent has two parts –header: contains the source and destination address (like an envelop) –body: contains the information (like a letter) together, it is called a packet

6. Department of Information Engineering 6 Contention problem only one computer can use the cable at a time e.g. a -> c if more than one computer uses the cable to send data, collision ! e.g. a -> c, b -> d a b c d e a b c d e

7. Department of Information Engineering 7 Contention problem –How to solve the problem that more than one computer want to transmit at the same time classic problem in network –many solutions –example polling divide into time slots (time division) divide into frequency slots (frequency division) CDMA (most complex) ALOHA-based (Ethernet, GSM)

8. Department of Information Engineering 8 ALOHA-based protocol A classroom game –imagine you are in a classroom and you want to raise a question Rules –you must close your eyes –only one student can speak at a time –if more than one student speak, the teacher cannot hear the question Question –how to raise questions in an orderly fashion without any coordinator?

9. Department of Information Engineering 9 ALOHA-based protocol solution: –you listen before you raise question !! –if someone is speaking, do not raise question!! –thereby avoid collision –the first person who raises the question speak first –in our computer LAN, the first computer who sends data get the channel the computer then sends the destination address of the receiving computer, followed by the data

10. Department of Information Engineering 10 ALOHA-based protocol But what if two students raise question at the same time? Solution –each student rolls a dice and wait for a random period of time –if student A gets a 1, and student B gets a 3, then the student A will wait for 1 second, and B will wait for 3 seconds –after one second, A will speak out first –after three seconds, B will try to speak out, but B will listen first; if A is still talking, B will wait until A has finished

11. Department of Information Engineering 11 ALOHA-based protocol In computer network, this means each computer listens to the cable, if the cable is idle (no signal), sends immediately; otherwise, waits if more than one computer send data simultaneously, we have a collision in this case, each computer will wait for a random period of time, the one who sends first will grab the channel

12. Department of Information Engineering 12 Ethernet This type of networking protocol has been in use for nearly thirty years !! It is called Ethernet and currently over 90% of the LANs in used today are based on it however, with the abundance of processing power, the topology of Ethernet has changed from a shared bus to a star topology communication is via a switch S

13. Department of Information Engineering 13 GSM It is also the principle used in mobile network like GSM –each mobile phone transmits signal using a frequency slot –but the number of frequency slots is limited, so it is possible that two mobile phones may compete for the same frequency slot at the same time –if there is a collision, each mobile phone will wait for a random period of time, the one who wakes up first will get the frequency slot

14. Department of Information Engineering 14 What is Internet? Problem of interconnecting LAN –by late 70, many different types of LAN existed –What is the problem in connecting the LANs? a b c d e 1 2 3 4 5 Network A Network 1

15. Department of Information Engineering 15 What is Internet? Addressing problem –to send data to a destination, you need an address –different technology uses different naming convention for the address –LANs cannot communicate with each other needs a standard way to address computers

16. Department of Information Engineering 16 IP address –the universal address is called IP address –IP stands for Internet Protocol –it is a 12-digit number (e.g. 137.189.123.108) we therefore have two set of addresses –physical address address used by the LAN must use this address for delivery –Internet address just a unique number

17. Department of Information Engineering 17 Post office scenario Limitation of a LAN –LAN can deliver address that is supported by its own LAN technology –the physical address e.g. the Ethernet address in the card –cannot send address to another LAN that is other kind of technology (e.g. token ring) –need a middleman to forward the packet –router

18. Department of Information Engineering 18 a d 2 1 d Data 147.189.123.108 d router 147.189.123.108 1 1 Data 147.189.123.108 IP address

19. Department of Information Engineering 19 What is Internet? Internet communicates in exactly the same way as our postal service the packet we sent (data + IP address) is called datagram –(as we sometimes call air mail letters as aerogramme)

20. Department of Information Engineering 20 What is Internet? Problem with datagram (and letters) –like letters, if we send a sequence of datagrams, the following may happen the datagram is lost, needs to re-send again the datagram is not lost, we have to eliminate the duplicate packet the datagrams arrived out of sequence

21. Department of Information Engineering 21 What is TCP (Transmission control protocol) ? Use of TCP –IP only deals with the addressing problem, cannot deal with error such as lost, duplicate, and out-of-sequence datagram –to deal with error, we need to add a sequence number to each of the datagram we sent, and check whether the sequence is correctly received at the other side –the software to do this is called TCP –TCP is used to provide reliable communication on Internet

22. Department of Information Engineering 22 Data 147.189.123.108 Data 147.189.123.108 012234 TCP sequence number checked a d 2 1 d 147.189.123.108 d router 1 1 IP

23. Department of Information Engineering 23 Summary communication on Internet is based on TCP/IP it is operates in exactly the same way as our postal service this is really all you need to know about TCP/IP

24. Department of Information Engineering 24 What is domain name? Is 147.189.123.108 easy to remember? NO !! –A 12 digits long telephone number is hard for everyone to remember Solution? –remember a name, not a number –let the directory on Internet (DNS) translate the computer name to IP number for you, transparently

25. Department of Information Engineering 25 What is domain name? Your computer 147.189.123.108. Internet Directory (DNS) www.xyz.com 147.189.123.109 www.zzz.com 147.190.231.68... 1. www.xyz.com? 2. 147.189.123.109 3. Datagram to 147.189.123.08

26. Department of Information Engineering 26 Application Email Example: mchang @ ie.cuhk.edu.hk Name of the computer that collects mails to my dept Name of my mail box