Download Presentation
## 1/45

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**Digital Signaling Versus Analog Signaling**• Digital signaling • Digital or analog data is encoded into a digital signal • Encoding may be chosen to conserve bandwidth or to minimize error • Analog Signaling • Digital or analog data modulates analog carrier signal • The frequency of the carrier fc is chosen to be compatible with the transmission medium used • Modulation: the amplitude, frequency or phase of the carrier signal is varied in accordance with the modulating data signal • by using different carrier frequencies, multiple data signals (users) can share the same transmission medium 3/45**Digital Signaling**• Digital data, digital signal • Simplest encoding scheme: assign one voltage level to binary one and another voltage level to binary zero • More complex encoding schemes: are used to improve performance (reduce transmission bandwidth and minimize errors). • Examples are NRZ-L, NRZI, Manchester, etc. • Analog data, Digital signal • Analog data, such as voice and video • Often digitized to be able to use digital transmission facility • Example: Pulse Code Modulation (PCM), which involves sampling the analog data periodically and quantizing the samples 4/45**Analog Signaling**• Digital data, Analog Signal • A modem converts digital data to an analog signal so that it can be transmitted over an analog line • The digital data modulates the amplitude, frequency, or phase of a carrier analog signal • Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) • Analog data, Analog Signal • Analog data, such as voice and video modulate the amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band • Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM) 5/45**Digital Data, Digital Signal**• Digital signal • discrete, discontinuous voltage pulses • each pulse is a signal element • binary data encoded into signal elements 6/45**Periodic signals**• Data element: a single binary 1 or 0 • Signal element: a voltage pulse of constant amplitude • Unipolar:All signal elements have the same sign • Polar:One logic state represented by positive voltage the other by negative voltage • Data rate:Rate of data (R) transmission in bits per second • Duration or length of a bit:Time taken for transmitter to emit the bit (Tb=1/R) • Modulation rate:Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digital encoding used. 7/45**Interpreting Signals**• Need to know • timing of bits: when they start and end • signal levels: high or low • factors affecting signal interpretation • Data rate: increase data rate increases Bit Error Rate (BER) • Signal to Noise Ratio (SNR): increase SNR decrease BER • Bandwidth: increase bandwidth increase data rate • encoding scheme: mapping from data bits to signal elements 8/45**Comparison of Encoding Schemes**• signal spectrum • Lack of high frequencies reduces required bandwidth, • lack of dc component allows ac coupling via transformer, providing isolation, • should concentrate power in the middle of the bandwidth • Clocking • synchronizing transmitter and receiver with a sync mechanism based on suitable encoding • error detection • useful if can be built in to signal encoding • signal interference and noise immunity • cost and complexity: increases when increases data rate 9/45**Encoding Schemes**Positive level (+5V) Negative level (-5V) Positive level (+5V)No line signal (0V)Negative level (-5V) 10/45**Encoding Schemes**11/45**NonReturn to Zero-Level (NRZ-L)**• Two different voltages for 0 and 1 bits • Voltage constant during bit interval • no transition, i.e. no return to zero voltage • more often, negative voltage for binary one and positive voltage for binary zero 12/45**NonReturn to Zero INVERTED (NRZI)**• Nonreturn to zero inverted on ones • Constant voltage pulse for duration of bit • Data encoded as presence or absence of signal transition at beginning of bit time • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0 • Example of differential encoding since have • data represented by changes rather than levels • more reliable detection of transition rather than level 13/45**Advantages and disadvantages of NRZ-L, NRZI**• Advantages • easy to engineer • good use of bandwidth • Disadvantages • dc component • lack of synchronization capability • Unattractive for signal transmission applications 14/45**Multilevel BinaryBipolar Alternate Mark Inversion (AMI)**• Use more than two levels (three levels, positive, negative and no line signal) • Bipolar-AMI • zero represented by no line signal • one represented by positive or negative pulse • one pulses alternate in polarity • no loss of sync if a long string of ones • long runs of zeros still a problem • no net dc component • lower bandwidth • easy error detection 15/45**Multilevel BinaryPseudoternary**• Binary one represented by absence of line signal • Binary zero represented by alternating positive and negative pulses • No advantage or disadvantage over bipolar-AMI • Each used in some applications 16/45**Multilevel Binary Issues**• Advantages: • No loss of synchronization if a long string of 1’s occurs, each introduce a transition, and the receiver can resynchronize on that transition • No net dc component, as the 1 signal alternate in voltage from negative to positive • Less bandwidth than NRZ • Pulse alternating provides a simple mean for error detection • Disadvantages • receiver distinguishes between three levels: +A, -A, 0 • a 3 level system could represent log23 = 1.58 bits • requires approx. 3dB more signal power for same probability of bit error 17/45**Theoretical Bit Error Rate (BER) For Various Encoding**Schemes 18/45**Manchester Encoding**• has transition in middle of each bit period • low to high represents binary one • transition serves as clock and data • high to low represents binary zero • used by IEEE 802.3 (Ethernet) LAN standard 19/45**Differential Manchester Encoding**• midbit transition is clocking only • transition at start of bit period representing binary 0 • no transition at start of bit period representing binary 1 • used by IEEE 802.5 token ring LAN 20/45**Advantages and disadvantages of Manchester Encoding**• Disadvantages • at least one transition per bit time and possibly two • maximum modulation rate is twice NRZ • requires more bandwidth • Advantages • synchronization on mid bit transition (self clocking codes) • has no dc component • has error detection capability (the absence of an expected transition can be used to detect errors) 21/45**Modulation Rate versus Data Rate**• Data rate (expressed in bps) • Data rate or bit rate R=1/Tb=1/1μs=1Mbps • Modulation Rate (expressed in baud) is the rate at which signal elements are generated • Maximum modulation ratefor Manchester is D=1/(0.5Tb)=2/1μs=2Mbaud 22/45**Scrambling**• Use scrambling to replace sequences that would produce constant voltage • These filling sequences must • produce enough transitions to maintain synchronization • be recognized by receiver & replaced with original • be same length as original • Design goals • have no dc component • have no long sequences of zero level line signal • have no reduction in data rate • give error detection capability 23/45**B8ZS and HDB3**24/45**Bipolar with 8-Zero Substitution (B8ZS)**• To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: • If 8 zeros occurs and the last voltage pulse was positive, then the 8 zeros are encoded as 000+–0–+ • If zeros occurs and the last voltage pulse was negative, then the 8 zeros are encoded as 000–+0+– 25/45**High Density Bipolar-3 zeros (HDB3)**• The scheme replaces strings with 4 zeros by sequences containing one or two pulses • In each case, the fourth zero is replaced with a code violation (V) • successive violations are of alternate polarity 26/45**Digital Data, Analog Signal**• Main use is public telephone system • has freq range of 300Hz to 3400Hz • use modem (modulator-demodulator) • The digital data modulates the amplitude A, frequency fc , or phase θof a carrier signal • Modulation techniques • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK) 27/45**Modulation Techniques**Amplitude Shift Keying (ASK) Binary Frequency Shift Keying (BFSK) Binary Phase Shift Keying (BPSK) 28/45**Amplitude Shift Keying (ASK)**• In ASK, the two binary values are represented by to different amplitudes of the carrier frequency • The resulting modulated signal for one bit time is • Susceptible to noise • Inefficient modulation technique • used for • up to 1200bps on voice grade lines • very high speeds over optical fiber 29/45**Binary Frequency Shift Keying (BFSK)**• The most common form of FSK is Binary FSK (BFSK) • Two binary values represented by two different frequencies ( f1 and f2 ) • less susceptible to noise than ASK • used for • up to 1200bps on voice grade lines • high frequency radio (3 to 30MHz) • even higher frequency on LANs using coaxial cable 30/45**Full-Duplex BFSK Transmission on a Voice-Grade line**f1 f2 f 3 f4 • Voice grade lines will pass voice frequencies in the range 300 to 3400Hz • Full duplex means that signals are transmitted in both directions at the same time 31/45**Multiple FSK (MFSK)**• More than two frequencies (M frequencies) are used • More bandwidth efficient compared to BFSK • More susceptible to noise compared to BFSK • MFSK signal: 32/45**Multiple FSK (MFSK)**• MFSK signal: • Period of signal element • Minimum frequency separation • MFSK signal bandwidth: 33/45**Example**• With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit data combinations: • This scheme can support a data rate of: 34/45**Example**• The following figure shows an example of MFSK with M=4. An input bit stream of 20 bits is encoded 2bits at a time, with each of the possible 2-bit combinations transmitted as a different frequency. 35/45**Phase Shift Keying (PSK)**• Phase of carrier signal is shifted to represent data • Binary PSK (BPSK): two phases represent two binary digits 36/45**Differential PSK (DPSK)**• In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal • Binary 0:signal burst with the same phase as the previous one • Binary 1:signal burst of opposite phase to the preceding one 37/45**Four-level PSK: Quadrature PSK (QPSK)**• More efficient use of bandwidth if each signal element represents more than one bit • eg. shifts of /2 (90o) • each signal element represents two bits • split input data stream in two & modulate onto the phase of the carrier • can use 8 phase angles & more than one amplitude • 9600bps modem uses 12 phase angles, four of which have two amplitudes: this gives a total of 16 different signal elements 38/45**Performance of ASK, FSK, MFSK, PSK and MPSK**• Bandwidth Efficiency • ASK/PSK: • MPSK: • MFSK: • Bit Error Rate (BER) • bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK (see Fig. 5.4) • for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance 41/45**Performance of MFSK and MPSK**• MFSK: increasing M decreases BER and decreases bandwidth Efficiency • MPSK: Increasing M increases BER and increases bandwidth efficiency 42/45**Quadrature Amplitude Modulation (QAM)**• QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards • 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 by 90° • each carrier is ASK modulated 43/45**QAM modulator**44/45**QAM Variants**• Two level ASK (two different amplitude levels) • each of two streams in one of two states • four state system • essentially QPSK • Four level ASK (four different amplitude levels) • combined stream in one of 16 states • Have 64 and 256 state systems • Improved data rate for given bandwidth • but increased potential error rate 45/45