Department of Information Engineering1 What is a bit? –the basic unit in computer –represent a binary number : 0 and 1 a group of bits can represent any number –binarydecimal Binary addition

Department of Information Engineering2 Byte - a group of 8 bits –represent decimal number from 0 to 255 –binarydecimal – – – – – –

Department of Information Engineering3 Computer understands only number, not English –but numbers can be used to represent alphabets decimalText 0NUL 1Control-A A... 97a...

Department of Information Engineering4 One alphabet/number takes one byte to store A for apple (in decimal) (in binary)

Department of Information Engineering5 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

Department of Information Engineering6 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)

Department of Information Engineering7 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

Department of Information Engineering8 How to record music in digital form time Measure the signal at regular time interval e.g. CD makes 44,100 measurement/second or measure at every second Analog waveform Height is represented by a 16 bit number ( )

Department of Information Engineering9 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

Department of Information Engineering10 Can we listen to CD quality music from Internet using 56K modem? –Bit rate of CD (single channel) * 16 = 705.6K bits per second –56K modem is too slow !! –Cannot listen to CD on Internet Samples/sec 16 bits/sample

Department of Information Engineering11 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

Department of Information Engineering12 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 levels –total memory required = 600*400*3 = 720,000 bytes

Department of Information Engineering13 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

Department of Information Engineering14 Compressed video –fortunately, video contains a lot of redundancy –compression ratio: 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

Department of Information Engineering15 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

Department of Information Engineering16 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 K bps by design, because the volume of download traffic (to home) is much larger than upload

Department of Information Engineering17 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

Department of Information Engineering18 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

Department of Information Engineering19 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

Department of Information Engineering20 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)