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Signal Encoding Lesson 05 NETS2150/2850 http://www.ug.cs.usyd.edu.au/~nets2150/ School of IT, The University of Sydney 1 Lecture Outline Encoding schemes for digital data to transmit in digital transmission systems – NRZ schemes – Manchester schemes in LANs – AMI schemes • With scrambling for WANs use Encoding schemes for digital data to transmit in analog transmission systems – ASK Scheme – FSK Scheme – PSK Scheme 2 Various Encoding Techniques Encoding is the conversion of streams of bits into a signal (digital or analog). Categories of Encoding techniques: – Digital data, digital signal Digital transmission – Analogue data, digital signal – Digital data, analog signal Analog transmission – Analogue data, analog signal 3 Digital Data, Digital Signal (Digital to Digital) Digital signal – Discrete, discontinuous voltage pulses – Each pulse is a signal element – Binary data encoded into signal elements 4 Interpreting Signals Need to know – Timing of bits - when they start and end – Signal levels Factors affecting interpretation of signals – SNR – Data rate – Bandwidth 5 Comparison of Encoding Schemes Error detection – Can be built into signal encoding Cost and complexity – Higher signal rate (& thus data rate) lead to higher costs Clocking – Synchronizing transmitter and receiver Signal spectrum – Bandwidth requirement – Presence of dc component 6 Digital-to-Digital Encoding Schemes 3 Broad Categories: Unipolar, Polar, and Bipolar -Nonreturn to Zero-Level (NRZ-L) Magnetic Recording -Nonreturn to Zero Inverted (NRZI) -Manchester LAN -Differential Manchester -Bipolar -AMI -B8ZS WAN -HDB3 7 Nonreturn to Zero-Level (NRZ-L) Polar Encoding Two different voltages for 0 and 1 bits Voltage constant during bit interval – no transition i.e. no return to zero voltage Negative voltage for one value and positive for the other 8 Nonreturn to Zero Inverted (NRZI) Polar Transition (low to high or high to low) denotes a binary 1 No transition denotes binary 0 This is an example of differential encoding 9 NRZ 10 Differential Encoding Polar Better encoding technique Data represented by changes rather than levels More reliable detection of bit in noisy channels rather than level 11 NRZ pros and cons Pros – Easy to engineer – Make good use of bandwidth Cons – Lack of synchronisation capability – Presence of a dc component Used for digital magnetic recording Not often used for signal transmission 12 Biphase Schemes Polar- signal elements have opposite voltage level (-ve and +ve) Overcomes the limitations on NRZ codes Two biphase techniques are commonly used: – Manchester – Differential Manchester Heavily used in LAN applications 13 Biphase Scheme1: Manchester Transition in middle of each bit interval Low to high represents one High to low represents zero Used by IEEE 802.3 (Ethernet LAN) 14 Biphase Scheme 2: Differential Manchester Midbit transition is clocking only Transition at start of a bit interval represents zero No transition at start of a bit interval represents one Note: this is a differential encoding scheme Used by IEEE 802.5 (Token Ring LAN) 15 16 Biphase Pros and Cons Cons – At least one transition per bit time and possibly two – Maximum baud rate is twice NRZ – Requires more bandwidth Pros – Synchronization on mid bit transition (self clocking) – Error detection • Absence of expected transition – No dc component 17 Multilevel Binary (Bipolar) Use more than two voltage levels Bipolar-AMI (Alternate Mark Inversion) – 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 (zeros still a problem) – Lower bandwidth – Easy error detection 18 Bipolar-AMI Encoding 19 Trade Off for Multilevel Binary Not as efficient as NRZ – Receiver must distinguish between three levels (+A, -A, 0) 20 Scrambling Technique Used to replace sequences that would produce constant voltage Produce “filling” sequence that: – Must produce enough transitions to sync – Must be recognized by receiver and replace with original – Same length as original Avoid long sequences of zero level line signal No reduction in data rate Error detection capability Two commonly used techniques are: B8ZS, and HDB3 Used for long distance transmission (WAN) 21 Bipolar With 8 Zeros Substitution (B8ZS) Based on bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive, encode as 000+-0-+ If octet of all zeros and last voltage pulse preceding was negative, encode as 000-+0+- Causes two violations of AMI code - intentional – Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros HDB3 – similar but based on 4 zeros 22 B8ZS 23 HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses 24 HDB3 Substitution Rules # of Bipolar Pulses (ones) since Last Substitution Polarity of Odd Even Preceding Pulse - 000- +00+ + 000+ -00- 25 B8ZS and HDB3 Change of polarity 26 Recap of Digital Signal Encoding Formats 0 1 NRZL High level Low level NRZI No transition at start of transition interval Bipolar-AMI No line signal +ve line signal Manchester Transition from high to low Transition from low to high in the middle of interval in the middle of interval Diff Manchester Tran at start of interval No transition at start of (always a Transition in the interval middle of interval) HDB3 Same as bipolar-AMI, except that any string of four zeros is replaced by a string with one code violation B8ZS Same as bipolar-AMI, except that any string of eight zeros are replaced by a string of two code violations 27 Digital Data, Analog Signal Some transmission media only transmit analog signals. Public telephone system – 300Hz to 3400Hz (voice frequency range) – Use modem (modulator-demodulator) 28 Digital to Analog modulation techniques: Modulation involves operation on signal characteristics: frequency, phase, amplitude. Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK) 29 Modulation Techniques (digital data, analog signal) 30 Amplitude Shift Keying Values represented by different amplitudes of carrier Usually, one amplitude is zero – i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient Up to 1200bps on voice grade lines Used over optical fiber 31 ASK 32 Relationship between baud rate and bandwidth in ASK 33 Example 1 Find the minimum bandwidth for an ASK signal transmitting at 2000 bps. The transmission mode is half-duplex. Solution In ASK, baud rate and bit rate are the same. The baud rate is therefore 2000. An ASK signal requires a minimum bandwidth equal to its baud rate. Therefore, the minimum bandwidth is 2000 Hz. 34 Example 2 Given a bandwidth of 5000 Hz for an ASK signal, what are the baud rate and bit rate? Solution In ASK the baud rate is the same as the bandwidth, which means the baud rate is 5000. But because the baud rate and the bit rate are also the same for ASK, the bit rate is 5000 bps. 35 Example 3 Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), draw the full-duplex ASK diagram of the system. Find the carriers and the bandwidths in each direction. Assume there is no gap between the bands in the two directions. Solution For full-duplex ASK, the bandwidth for each direction is BW = 10000 / 2 = 5000 Hz The carrier frequencies can be chosen at the middle of each band (see Fig. 5.5). fc (forward) = 1000 + 5000/2 = 3500 Hz fc (backward) = 11000 – 5000/2 = 8500 Hz 36 Solution to Example 3 37 Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using co-ax 38 FSK 39 Multiple FSK More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit 40 FSK on Voice Grade Line 41 Phase Shift Keying Phase of carrier signal is shifted to represent data Binary PSK – Two phases represent two binary digits Differential PSK – Phase shifted relative to previous transmission rather than some reference signal 42 PSK 43 Differential PSK 44 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 45 to ASK and FSK Summary Various encoding schemes Some used in LANs Others more suitable in WAN with scrambling Read Stallings Section 5.1 Next: Data link layer functions. 46

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posted: | 11/24/2011 |

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