Digital Data Transmission: Encoding Techniques and Modulation Schemes, Slides of Data Communication Systems and Computer Networks

Download Digital Data Transmission: Encoding Techniques and Modulation Schemes and more Slides Data Communication Systems and Computer Networks in PDF only on Docsity! Modulation Rate 5 bits = 5 psec - > NRZI ‘++ I bit = 1 signal element = I psec Manchester +#—_>—s «4 I bit = 1 signal element = I usec 0.5 psec docsity.com Scrambling • Use scrambling to replace sequences that would produce constant voltage • Filling sequence  Must produce enough transitions to sync  Must be recognized by receiver and replace with original  Same length as original • No dc component • No long sequences of zero level line signal • No reduction in data rate • Error detection capability docsity.com 4p) jaa) (a) a xe) Cc Co) ep) N 00 aa) docsity.com Bipolar-AMI HDB3 (odd number of Os Valid bipolar signal Bipolar violation B ¥ last substitution) Digital Data Analog Signal • Philosophy  Transmission of data through public telephone network  Long distance transmission  Transmission through guided media  Public telephone network support analog signals of voice frequency about 300 to 3400 Hz  Modems are used to convert digital signals to analog signal in voice frequency range  Higher frequency modems are used for microwave transmission docsity.com Encoding Techniques • Carrier Signal  A continuous oscillating signal usually a sine wave  The characteristics of the signal are modified to send data  Receiver decodes the signal and discards the carrier signal  Such a technique is called modulation  Modulation involve operation on frequency, amplitude and phase of the carrier signal • Encoding Schemes  Amplitude Shift Keying (ASK)  Frequency Shift Keying (FSK)  Phase Shift Keying (PSK) docsity.com Frequency Shift Keying • Properties  Two binary values are represented by two different frequencies centered on a carrier frequency  The carrier signal is given by Asin(2πfc t ) • For FSK S( t ) = A sin(2πf1 t ) binary 1 S( t ) = A sin(2π f2 t ) binary 0 where f1 and f2 are offset from carrier frequency Example If the carrier frequency is 1170 then f1= 1270 f2 = 1070  For full duplex transmission two different carrier frequency are used in opposite direction  A set of offset frequencies for each carrier frequency is used to represent data  e.g. 1170 and 1270,1070 in one direction where as 2125 and 2225,2025 in opposite direction could be used for full duplex transmission docsity.com FSK continued.. (b) Frequency-shift keying Digital states Analog waves docsity.com Frequency Shift Keying • BFSK (Binary Frequency Shift Keying)  Two binary values are represented  Two frequencies are used  Common form of FSK • MFSK (Multiple Frequency Shift Keying)  Multiple binary values are represented  Multiple frequencies are used S(t) = A cos(2πfi t ) fi = fc + (2i – 1- M)fd Where fc = The carrier frequency fd = The difference frequency M = Number of different signal elements = 2L L = Number of bits per signals element docsity.com Phase Shift Keying • The phase of the carrier is shifted for representing 1 or 0 • Binary PSK  Two phases represent two binary digits • Differential PSK  A wave in phase with the previous one represent binary 0  A wave out of phase (opposite phase) with the previous one represent binary 1 • For PSK (phase shift of 1800 S( t ) = A sin(2πfc t + π) binary 1 S( t ) = A sin(2π fc t ) binary 0 docsity.com Differential PSK docsity.com Quadrature PSK • More efficient use by each signal element representing more than one bit  e.g. shifts of /2 (90o)  Each element represents two bits  Can use 8 phase angles and have more than one amplitude  9600bps modem use 12 angles , four of which have two amplitudes • Offset QPSK (orthogonal QPSK)  Delay in Q stream docsity.com Examples of QPSF and OQPSK Waveforms bitnumber 14 2 3 4 5 6 7 8 9 10 bit value 1 -1 4 1 1 -1—~(-1 1 1 4 | Q | Q | Q | Q | Q input signal re ' I(t) 1 ar) 5 ! 7 9 ' i ee Qit) 2 4 6 8 ' 10 phase of -pl4 pi4 -3p/4 3pl4 pl4 output signal Q(t - T,) phase of -pl4 -pl4 pl4 3p/4 -3p/4 -3p/4 3pi4 p/4_— pid output signal docsity.com Performance of Digital to Analog Modulation Schemes • Bandwidth  ASK and PSK bandwidth directly related to bit rate  FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies  (See Stallings for math) • In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK docsity.com Quadrature Amplitude Modulation • QAM used on asymmetric digital subscriber line (ADSL) and some wireless • Combination of ASK and PSK • Logical extension of QPSK • Send two different signals simultaneously on same carrier frequency  Use two copies of carrier, one shifted 90°  Each carrier is ASK modulated  Two independent signals over same medium  Demodulate and combine for original binary output docsity.com