Transcript W3-1Rev
W3-1:Learning Outcomes Transmission Time/Propagation time What is coding (Morse Code, ASCII, EBCDIC) Explain Digital and Analogue definitions Advantages of digital vs. analogue Explain frequency, band, bandwidth, baud rate, bit rate and codec theory Explain “Why we need a modem”? Propagation Delay The time taken by the signal to travel from the source to the destination One way = 22,280 miles (GEO satellite distance about 36000 kms) Prop. Time=Distance/speed of signal transfer Satellite Transponders Transponders in the satellite receive the incoming signal and transmit them back to earth Attenuation A decrease in magnitude of power of the signal. (weakening of the signal) What is coding? A character is a symbol that has a common, constant meaning. A character could be a “A” or “B” or “1” or “8” Morse Code US-ASCII (7 bit code = 128 valid characters and 8 bit code = 256 valid characters),e.g. A=1000001 (7 bit ASCII code) EBCDIC (IBM’s standard Mainframe code, 8 bits, giving 256 valid character combinations) More info on link : https://publib.boulder.ibm.com/infocenter/co mphelp/v8v101/index.jsp?topic=%2Fcom.ibm .xlf101a.doc%2Fxlflr%2Fasciit.htm Digital A digital signal is discontinuous (gaps) and varies instantaneously, e.g. zero or 1. Analogue Analogue data is in a continuous form. It varies smoothly. Electronically, an analogue signal is a voltage that is continuously going up and down Analogue signals preserve all the information with no loss. It is, however, difficult and expensive to get a computer to work with analogue data, as well as re create origninal data once affected by random noise www.webopedia.com Analogue Advantages of Digital transmission vs. Analogue Less error because of only two distinct values (0, 1) more efficient transmission (switching, multiplexing) higher maximum transmission rate more secure (easier to encrypt) integration (of voice, data and video) easier with digital transmission Frequency The frequency is the number of cycles (waves) in an analogue signal that occur per second. It is measured in hertz (Hz) - 1HZ is one cycle or wave per second. Band A band is a range of frequencies. For example, the FM radio band covers the range of frequencies from 80MHz to 108 MHz. Bandwidth The bandwidth is the width of the band. The difference between the highest and lowest frequencies in a band In digital networks, bandwidth is used to refer to the amount of data that can be sent per second (bit rate or bandwidth). Bandwidth cont.. For example, each FM station has a bandwidth of 25kHz, so an FM station broadcasting on 99MHz (99,000kHz) can broadcast a signal that varies between 98,987.5kHz (LSB) and 99,012.5kHz (USB) Humans can hear 20Hz - 14KHz (varies upto 20 KHz) Most voice grade telephone circuits have a bandwidth of 300HZ to 4 KHz for voice transmission Baud rate The baud rate is the number of times that a signal changes in one second. It is measured in bauds. For example, 1,200 baud means that the voltage in an electrical cable changes 1,200 times every second. A baud rate of 2,400 baud means that the voltage changes 2,400 times every second. o how fast a modem can transmit data. Technically, the baud is the number of voltage or frequency changes that can be made in one second. When a modem is working at 300 baud, this means that the basic carrier frequency has 300 cycles per second. Bit rate The bit rate is the number of bits of data sent every second. It is measured in bits per second (bps) A bit rate of 33,600 BPS means that 33,600 bits are sent every second. A bit rate of 9.6kbps (9,600bps) means that 9,600 bits are sent every second. Bit rate = baud rate * number of bits per symbol/interval/voltage level for example… QAM has 4 bits per symbol/interval : so…bit rate = baud rate * 4 Codec (Coder/Decoder) To convert analogue voice into digital form for transmission over a digital link (need them at both ends). Needed for switching over telephone networks Modem (Modulator/Demodulator) To change computer digital signal to analogue for transmission over telephone line (local loop) which is analogue. Modulation is the technique that modifies the form of an electric signal so the signal can carry information on a communication link. e.g. change digital to analogue signal Modems... Modems can adjust the capacity (by changing bits per signal element) depending on quality of the line Read Chapter 3 (96-106) relevant parts Questions How does analogue data differ from digital signal? Why do we need a Modem? What is coding, how many characters 7-bit ASCII provides? Describe the advantages of digital over analogue. Explain the following terms: Propagation delay, Attenuation, Digital, Analogue, Frequency, Explain the following terms: Band, Bandwidth, Baud rate, Bit rate, Codec, Modulation