What is a bit? the basic unit in computer represent a binary number : 0 and 1

1. What is a bit? • the basic unit in computer • represent a binary number : 0 and 1 • a group of bits can represent any number • binary decimal • 0 0 • 1 1 • 10 2 • 11 3 • 100 4 Binary addition 1 + 1 ------- 1 0

2. Byte - a group of 8 bits • represent decimal number from 0 to 255 • binary decimal • 0000 0000 0 • 0000 0001 1 • 0000 0010 2 • . . . . . . • 1111 1110 254 • 1111 1111 255

3. Computer understands only number, not English • but numbers can be used to represent alphabets • decimal Text 0 NUL 1 Control-A . . . . . . 30 0 31 1 . . . . . . 65 A . . . . . . 97 a . . . . . .

4. One alphabet/number takes one byte to store A for apple 65 32 102 111 (in decimal) 01000001 00100000 01100110 01001111 (in binary)

5. How to store one page of text with 300 words? • If each word is 5 letters long • total number of alphabets in the text = 300 x 5 = 1500 • take up 1500 bytes of storage • or 1500 x 8 = 12,000 bits

6. What is a 56K modem? • 56K refers to the speed of transmission • this means the equipment can transmit 56,000 bits per second • another word for speed is called “bandwidth” • broadband means very fast communication • Nomenclature • K = 1,000 (kilo) • 56K = 56,000 • M = 1,000,000 (mega, million) • 2M = 2,000,000 • G = 1,000,000,000 (giga, billion) • T = 1,000,000,000,000 (tera)

7. How long does it take to transmit one page of text? • If the text is 300 words • equivalent to 12,000 bits • 56K modem can transmit 56,000 bits per second (bps) • time to transmit the page = 12,000/56,000 = 0.21 second

8. How to record music in digital form Height is represented by a 16 bit number (0 - 65535) Analog waveform time Measure the signal at regular time interval e.g. CD makes 44,100 measurement/second or measure at every 0.000022 second

9. Storage of a 60 minutes CD • for one channel • 3,600 * 44,100 * 2 (bytes) = 320 M bytes • for two channels (stereo) • 320 M bytes * 2 = 640 M bytes • storage capacity of a CD is around 700 M bytes • includes some other overheads

10. Can we listen to CD quality music from Internet using 56K modem? • Bit rate of CD (single channel) • 44100 * 16 = 705.6K bits per second • 56K modem is too slow !! • Cannot listen to CD on Internet 16 bits/sample Samples/sec

11. Fortunately, with compression technique, we can reduce the file size of CD • What is compression? • Reduce the size of the file by removing redundancy • What is MP3? • An audio compression technique • compression ration is about 12:1 • equivalent size of 60 min music in MP3 • 58.8K bps • a bit rate that 56K modem is just about to support

12. Image • digital image is represented by dots • the higher the density of the dots, the better the resolution • each dot is called a pixel (or picture element) • for a digital camera, a small image takes about 600x400 dots • each dots is represented by three primary colors • each color is represented by 8 bits (1 byte) for 0 - 255 levels • total memory required = 600*400*3 = 720,000 bytes

13. Video • video is a sequence of images • PAL transmits 50 images/second • bit rate of video? • 720,000 * 8 * 50 bits per second • 288,000,000 bits per second !! • Not feasible using 56k modem

14. Compressed video • fortunately, video contains a lot of redundancy • compression ratio: 100 - 1 • so that we only need to transmit 288,000,000/100 bps • or 2.88 M bps • therefore in order to watch video at home, we need broadband communication

15. Difference between MPEG1 and MPEG2 • (MPEG: motion picture engineering group) • MPEG1 (VCD) • early compression standard • not very high resolution • tape quality • bandwidth ~ 1.5 Mbps • MP3 means MPEG1 layer 3 (compression for audio) • MPEG2 (DVD) • 2nd generation standard • higher resolution, laser disk quality • bandwidth ~ 2-4 Mbps

16. How to provide broadband Internet access • Telephone network • currently you telephone line is slow because it is shared by many households in the building • if the line is not shared, much faster • the technology used by IMS (HK telecom) is called ASDL (asymmetric digital subscriber loop) • 1.5 Mbps - 9 Mbps download speed • asymmetric because upload speed is only around 64-128 K bps • by design, because the volume of download traffic (to home) is much larger than upload

17. Cable • optical fibre to your building, then copper wire to your home • 30 Mbps per channel (BUT SHARED !) • cable TV was designed for broadcasting • not point-to-point communication like telephone • pros and cons • if many households shared the bandwidth, then cable is slower than ADSL • but if there are not many households using the network simultaneously, cable is fast • but cable company can always provide more bandwidth by offering more channels

18. Wireless network • LMDS (local multipoint distribution services0 • attractive to Internet Service Providers (ISP) that do not have telephone/cable network • can be deployed quickly and inexpensively • operate at very high frequency 28GHz • 155 Mbps

19. Low-earth-orbit (LEO) satellites • LMDS can be blocked by tree • ADSL needs line of good quality • but satellites are expensive • Motorola’s Iridium project is not doing too well • StarTV may partner some ISPs to provide Internet service one day

20. High speed digital link to home • bypass the analog telephone network • use digital (leased) line all the way to home • e.g CUHK campus network • new housing projects in HK • ref: High Speed Data Races Home (Scientific American, October 1999)