Encoding Techniques - Lecture 19

Encoding TechniquesLecture 19OverviewData RepresentationSignalingEncoding TechniquesDigital Data, Digital Signals (Line Coding)Advantages of Digital TransmissionDisadvantages of Digital TransmissionSignal Elements Vs. Data ElementsData Rate Vs. Signal Rate (Bit Rate Vs. Baud Rate/Pulse Rate/Modulation Rate)Unipolar Encoding2Polar Encoding and its TypesNRZ-L NRZ-IRZBiPhase EncodingManchester and Diff ManchesterBipolar EncodingAlternate Mark InversionMultirate 2B1QMultirate 8B6T4D-PAM5Multitransition MLT-3Block Coding4B/5BIntroductionThe user data can be in one of two formats:Analog: Human voice as converted by typical home telephones Digital: Computer filesThe transmitted signals, representing the data, can also be in one of two formats: Analog or DigitalThe conversion of the user data into a transmission signal is called Encoding.3Encoding TechniquesIn data communications, the user data must be put in a format (signal) suitable for the transmission media (nature, quality, length, etc.)4Encoding: Data-Signal ConversionThere are four possible cases:Digital data, digital signals: We use Line Coding. Less complex and less expensive.Analog data, digital signals: We use A/D conversion for voice and video.Digital data, analog signals: We use Digital Modulation for optical fiber and unguided media.Analog data, analog signals:We use Analog Modulation to transmit base-band signal easily and cheaply.5Digital Data, Digital Signaldigital signaldiscrete, discontinuous voltage pulseseach pulse is a signal elementbinary data encoded into signal elements6Advantages of Digital TransmissionDigital technologyLow cost LSI/VLSI technologyData integrityLonger distances over lower quality linesCapacity utilizationHigh bandwidth links economicalHigh degree of multiplexing easier with digital techniquesSecurity & PrivacyEncryptionIntegrationCan treat analog and digital data similarly7Disadvantages of Digital Signals8greater attenuationdigital now preferred choiceDIGITAL-TO-DIGITAL CONVERSIONhow we can represent digital data by using digital signals. The conversion involves three techniques: Line coding Block coding Scrambling. Line coding is always needed; block coding and scrambling may or may not be needed.91010 Line coding and decodingLine coding is the process of converting digital data to digital signals. We assume that data, in the form of text, numbers, graphical images, audio, or video, are stored in computer memory as sequences of bits.Signal Element Versus Data Element In data communications, our goal is to send data elements. A data element is the smallest entity that can represent a piece of information: this is the bit. In digital data communications, a signal element carries data elements. A signal element is the shortest unit (timewise) of a digital signal. In other words, data elements are what we need to send; signal elements are what we can send. Data elements are being carried; signal elements are the carriers. 11Signal element versus data elementWe define a ratio r which is the number of data elements carried by each signal element. Figure 4.2 shows several situations with different values of r. Data Rate Versus Signal Rate The data rate defines the number of data elements (bits) sent in is. The unit is bits per second (bps). The signal rate is the number of signal elements sent in Is. The unit is the baud. The data rate is sometimes called the bit rate; the signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate. 12A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?SolutionWe assume that the average value of c is 1/2 . The baud rate is thenExampleWe now need to consider the relationship between data rate and signal rate (bit rate and baud rate). This relationship, of course, depends on the value of r. It also depends on the data pattern C. If we have a data pattern of all 1 s or all Os, the signal rate may be different from a data pattern of alternating Os and 1 s. where N is data rate c is the case factor (worst, best & avg.)r is the ratio between data element & signal element13Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.we can say that the bandwidth (range of frequencies) is proportional to the signal rate (baud rate). The minimum bandwidth can be given as We can solve for the maximum data rate if the bandwidth of the channel is given. 14Baseline Wandering In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline. The incoming signal power is evaluated against this baseline to determine the value of the data element. A long string of Os or 1 s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly. A good line coding scheme needs to prevent baseline wandering. DC Components When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies . These frequencies around zero, called DC (direct-current) components, present problems for a system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer). For example, a telephone line cannot pass frequencies below 200 Hz. Also a long-distance link may use one or more transformers to isolate different parts of the line electrically. For these systems, we need a scheme with no DC component. Self-synchronization To correctly interpret the signals received from the sender, the receiver's bit intervals must correspond exactly to the sender's bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals. 15 Effect of lack of synchronizationA self-synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. If the receiver' s clock is out of synchronization, these points can reset the clock. 16In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps?SolutionAt 1 kbps, the receiver receives 1001 bps instead of 1000 bps.At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.ExampleTerminologyunipolar – all signal elements have the same signpolar – one logic state represented by positive voltage and the other by negative voltagedata rate – rate of data ( R ) transmission in bits per secondduration or length of a bit – time taken for transmitter to emit the bit (1/R)modulation rate – rate at which the signal level changes, measured in baud = signal elements per second. mark and space – binary 1 and binary 01718Unipolar uses only one signal level (one polarity)High voltage is binary “1”No voltage is binary “0”Unipolar encoding is easy to implement. However:Not self-synchronizedHas a DC componentCompared with its polar counterpart, this scheme is very costly. The normalized power (power needed to send 1 bit per unit line resistance) is double that for polar NRZ. For this reason, this scheme is normally not used in data communications today.Unipolar Encoding19Unipolar uses only one signal level (one polarity)High voltage is binary “1”No voltage is binary “0”Unipolar encoding is easy to implement. However:Not self-synchronizedHas a DC componentCompared with its polar counterpart, this scheme is very costly. The normalized power (power needed to send 1 bit per unit line resistance) is double that for polar NRZ. For this reason, this scheme is normally not used in data communications today.Unipolar EncodingTypes of Polar Encoding20Polar encoding uses two signal levelsPositive & Negative Polarities21Non-Return to Zero (NRZ) EncodingNRZ encoding can be of two types:NRZ-Level (NRZ-L)“0” is encoded with one polarity, say “+5V”“1” is encoded with another polarity, say “-5V”NRZ-Invert (NRZ-I)“0” is encoded with no change in polarity from previous bit“1” is encoded with a change in polarity from previous bitNRZ-I provides better synchronization than NRZL if “1” bits exist in data streamA stream of many “0” can still cause synch. problems22NRZ-L and NRZ-I EncodingNRS Pros and ConsProsEasy to engineerMake good use of bandwidthConsdc componentLack of synchronization capabilityUsed for magnetic recordingNot often used for signal transmission23Return to Zero (RZ) EncodingWe have seen that:NRZ-L has poor synch. PerformanceNRZ-I has better synch. for streams of “1” but faces the same problem for streams of “0”RZ encoding overcomes this synch. issue by using three voltage levels: Positive, Negative and Zero“1” is encoded as: (“+V”, Transition “+V ↓ 0V”)“0” is encoded as: (“ −V”, Transition “−V ↑ 0V”)RZ is less spectrally efficient than NRZ because it has more transitions i.e. higher freq. components.24RZ Encoding25Types of Polar Encoding26Polar encoding uses two signal levelsPositive & Negative PolaritiesManchester EncodingManchester uses a polarity inversion in themiddle of each bit period Low to high represents oneHigh to low represents zeroThis transition is used for bit representation as well as synch. purposes.Manchester achieves the same level of synch. asRZ but with two voltage levels only27Diff. Manchester EncodingPolarity inversion in the middle of each bit period (Tb) is used for synch. onlyTransition at start of a bit period represents zeroNo transition at start of a bit period represents oneDiff. Manchester requires two signal changes to represent “0” and one signal change to represent “1”28Manchester and Diff.Manchester Encoding29Manchester vs. Diff. ManchesterBoth Manchester and Diff. Manchester encoding rely on signal transition to encode dataBoth have better performance in the presence of noise than any encoding scheme that relies on the absolute voltage level to encode dataHowever, it is easy to lose sense of the polarity of a signal in a complex transmission layout30Biphase Pros and ConsConAt least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandwidthProsSynchronization on mid bit transition (self clocking)No dc componentError detection31Bipolar EncodingBipolar encoding uses three voltage levels: Positive, Negative and Zero“0” is encoded as: (“0V”)“1” is encoded by alternating between (“+V”) and (“−V”)If the first “1” is encoded as (“+V”) then the next “1” is encoded as (“−V”), and so on.This alternation occurs in the case whether these “1”s are consecutive or notTypes of Bipolar EncodingAlternate Mark Inversion (AMI)Bipolar n-Zero Substitution (BnZS)High Density Bipolar 3-Zero (HDB3)32Alternate Mark Inversion (AMI)“Mark” means “1” in telegraphyAMI means Alternate “1” InversionAMI alternates the voltage polarity for successive “1” bits“0” bits will be represented by “0V”AMI lacks self-synchronization for long streams of “0”AMI encoding has no DC component33Bipolar AMI Encoding Example3435Bipolar schemes: AMI and pseudoternaryIn bipolar encoding (sometimes called multilevel binary), we use three levels: positive, zero, and negative.The bipolar scheme was developed as an alternative to NRZ. The bipolar scheme has the same signal rate as NRZ, but there is no DC component. The NRZ scheme has most of its energy concentrated near zero frequency, which makes it unsuitable for transmission over channels with poor performance around this frequency. The concentration of the energy in bipolar encoding is around frequency N/2. 36Multilevel: 2B1Q scheme (two binary, one quaternary). uses data patterns of size 2 and encodes the 2-bit patterns as one signal element belonging to a four-level signal. Using 2B1Q, we can send data 2 times faster than by using NRZ-L. However, 2B 1Q uses four different signal levels, which means the receiver has to discern four different thresholds. 37 Multilevel: 8B6T scheme eight binary, six ternary The idea is to encode a pattern of 8 bits as a pattern of 6 signal elements, where the signal has three levels (ternary). In this type of scheme, we can have 28 = 256 different data patterns and 36 = 478 different signal patterns. The mapping table is shown in Appendix D. There are 478 - 256 = 222 redundant signal elements that provide synchronization and error detection. Part of the redundancy is also used to provide DC balance. Each signal pattern has a weight of 0 or +1 DC values. This means that there is no pattern with the weight -1. To make the whole stream DC-balanced, the sender keeps track of the weight. The minimum bandwidth is very close to 6N/8. 4D-PAM5 4D-PAM5 is called four dimensional five-level pulse amplitude modulation. The 4D means that data is sent over four wires at the same time. It uses five voltage levels, such as -2, -1, 0, 1, and 2.However, one level, level 0, is used only for forward error detection . The technique is designed to send data over four channels (four wires). 3839 Multitransition: MLT-3 schemeThe signal rate is the same as that for NRZ-I, but with greater complexity (three levels and complex transition rules). It turns out that the shape of the signal in this scheme helps to reduce the required bandwidth. 1. If the next bit is 0, there is no transition. 2. If the next bit is 1 and the current level is not 0, the next level is 0. 3. If the next bit is 1 and the current level is 0, the next level is the opposite of the last nonzero level. Summary of line coding schemes40DIGITAL-TO-DIGITAL CONVERSIONIn this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: Line coding Block coding Scrambling. Line coding is always needed; block coding and scrambling may or may not be needed.41Block Coding 42Block coding can ensure synchronization, provide error detection and improve the performance of line coding. In general, block coding changes a block of m bits into a block of n bits, where n is larger than m. Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group.Block CodingBlock coding normally involves three steps: Division : In the division step, a sequence of bits is divided into groups of m bits. For example, in 4B/5B encoding, the original bit sequence is divided into 4-bit groups. Substitution: In substitution step, we substitute an m-bit group for an n-bit group. For example, in 4B/5B encoding we substitute a 4-bit code for a 5-bit group. Combination: The n-bit groups are combined together to form a stream. The new stream has more bits than the original bits. 434B/5B mapping codes44Substitution in 4B/5B block coding45AMI used with scramblingBipolar AMI encoding, has a narrow bandwidth and does not create a DC component. However, a long sequence of Os upsets the synchronization. If we can find a way to avoid a long sequence of Os in the original stream, we can use bipolar AMI for long distances. To provide synchronization but not increasing the number of bits, a solution is to substitutes long zero-level pulses with a combination of other levels to provide synchronization is called scrambling. Note that scrambling, as opposed to block coding, is done at the same time as encoding. The system needs to insert the required pulses based on the defined scrambling rules. Two common scrambling techniques are B8ZS and HDB3. 46ExampleWe need to send data at a 1-Mbps rate. What is the minimum required bandwidth, using a combination of 4B/5B and NRZ-I or Manchester coding?Sol:First 4B/5B block coding increases the bit rate to 1.25 Mbps. The minimum bandwidth using NRZ-I is N/2 or 625 kHz. The Manchester scheme needs a minimum bandwidth of 1.25 MHz. The first choice needs a lower bandwidth, but has a DC component problem; the second choice needs a higher bandwidth, but does not have a DC component problem.478B/10B block encoding48More bits - better error detectionThe 8B10B block code adds more redundant bits and can thereby choose code words that would prevent a long run of a voltage level that would cause DC components.49ScramblingThe best code is one that does not increase the bandwidth for synchronization and has no DC components.Scrambling is a technique used to create a sequence of bits that has the required c/c’s for transmission - self clocking, no low frequencies, no wide bandwidth.It is implemented at the same time as encoding, the bit stream is created on the fly.It replaces ‘unfriendly’ runs of bits with a violation code that is easy to recognize and removes the unfriendly c/c.50Two cases of B8ZS scrambling technique51B8ZS substitutes eight consecutive zeros with 000VB0VB.HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.B8ZS52B8ZS substitutes eight consecutive zeros with 000VB0VB.Different situations in HDB3 scrambling technique53HDB354HDB3 substitutes four consecutive zeros with 000V or B00V dependingon the number of nonzero pulses after the last substitution.55Summary56Data RepresentationSignalingEncoding TechniquesDigital Data Digital Signals (Line Coding)Advantages of Digital TransmissionDisadvantages of Digital TransmissionSignal Elements Vs. Data ElementsData Rate Vs. Signal Rate (Bit Rate Vs. Baud Rate/Pulse Rate/Modulation Rate)Unipolar EncodingPolar Encoding and its TypesNRZ-L NRZ-IRZBiPhase EncodingManchester and Diff ManchesterBipolar EncodingAlternate Mark InversionMultirate 2B1QMultirate 8B6T4D-PAM5Multitransition MLT-3Block Coding4B/5B5758Key Data Transmission Terms59Interpreting Signalsneed to know:timing of bits - when they start and endsignal levelsfactors affecting signal interpretation:signal to noise ratiodata ratebandwidthencoding scheme60Digital Signal Encoding Formats61Encoding Schemes62Manchester Encodingtransition in middle of each bit periodmidbit transition serves as clock and datalow to high transition represents a 1high to low transition represents a 0used by IEEE 802.363Differential Manchester Encodingmidbit transition is only used for clockingtransition at start of bit period representing 0no transition at start of bit period representing 1this is a differential encoding schemeused by IEEE 802.5 64SummarySignal encoding techniquesdigital data, digital signalNRZ, multilevel binary, biphase, modulation rateanalog data, digital signalPCM, DMdigital data, analog signalASK, FSK, BFSK, PSKanalog data, analog signalAM, FM, PM65