DIGITAL TRANSMISSION
Digital/Analog data conversion to Digital Signal
• Digitaltodigitalconversion – Line coding
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– Block Coding
– Scrambling
• AnalogtoDigitalconversion
– Pulse Amplitude Coding – Delta Modulation
Digital data to Digital Signal Conversion
• Howwecanrepresentdigitaldatabyusingdigital signals?
• Theconversioninvolvesthreetechniques: – line coding,
– block coding, and
– scrambling.
• Linecodingisalwaysneeded;blockcodingand scrambling may or may not be needed.
Line coding and decoding
• Theconversioninvolvescodinganddecodingatthe edges (Tx,Rx) of the line
Signal Element vs. Data Element
• Indatacommunications,ourgoalistosenddata elements
• dataelementisthesmallestentitythatcanrepresenta piece of information: this is the bit
• Indatacommunicationsasignalelementcarriesdata elements
• Signalelementistheshortestunit(timewise)ofa digital signal
• Dataelementsarewhatweneedtosend;Signal elements are what we can send
Signal Element vs. Data Element
r is the ratio between the number of data elements carried by each signal element
Data Rate vs. Signal Rate
• Thedataratedefinesthenumberofdataelements (bits) sent in 1second
– The unit is bits per second (bps).
– The data rate is sometimes called the bit rate
• Thesignalrateisthenumberofsignalelementssentin
– The unit is the baud.
– the signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate
Data Rate vs. Signal Rate
• Onegoalindatacommunicationsistoincreasethedata rate while decreasing the signal rate
• TherelationshipbetweendatarateNandsignalrateS: – S=N/r
• Thebaudrate,notthebitrate,determinestherequired bandwidth for a digital signal
• Althoughtheactualbandwidthofadigitalsignalis infinite, the effective bandwidth is finite
Aspects of Line Coding • Baseline Wandering
– In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline
– Alongstringof0sor1scancauseadriftinthe baseline (baseline wandering) and make it difficult for the receiver to decode correctly.
– A good line coding scheme needs to prevent baseline wandering
Aspects of Line Coding • DC Components
– When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies (results of Fourier analysis).
– These frequencies around zero are called DC (direct- current) components,
– DC components present problems for a system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer).
Aspects of Line Coding • 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
– A 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
Effect of Lack of Synchronization
Line Coding Schemes
Unipolar NRZ scheme
• Inaunipolarscheme,allthesignallevelsareonone side of the time axis, either above or below
• In non-return-to-zero (NRZ) scheme the positive voltage defines bit 1 and the zero voltage defines bit 0.
Polar NRZ-L and NRZ-I schemes
• Inpolarschemes,thevoltagesareonbothsidesofthe time axis
• InpolarNRZencoding,weusetwolevelsofvoltage amplitude
• NRZ-L (NRZ-Level), the level of the voltage determines the value of the bit
• NRZ-I (NRZ-Invert), the change or lack of change in the level of the voltage determines the value of the bit
Polar NRZ-L and NRZ-I schemes
Both suffer from baseline wandering , it is twice as severe in NRZ-L
Both suffer from synchronization issues, when long burst of 0 or 1 is transmitted
Both have an average signal rate of N/2 Bd, N is the data rate
Min bandwidth = 0.5N
• AsystemisusingNRZ-Itotransfer10-Mbpsdata.What are the average signal rate and minimum bandwidth?
• Solution:
– The average signal rate is S = N/2 = 5 Mbaud.
– The minimum bandwidth for this average baud rate isBmin =S=5MHz.
Return-to-Zero (RZ)
• ThemainproblemwithNRZencodingoccurswhenthe sender and receiver clocks are not synchronized
• Onesolutionisthereturn-to-zero(RZ)scheme,which uses three values: positive, negative, and zero
RZ uses three levels of voltage, which is more complex to create and discern
Min bandwidth = N
Manchester Scheme
• TheideaofRZ(transitionatthemiddleofthebit)and the idea of NRZ-L are combined into the Manchester scheme.
• InManchesterencoding,thedurationofthebitis divided into two halves. The voltage remains at one level during the first half and moves to the other level in the second half
• Thetransitionatthemiddleofthebitprovides synchronization
• DifferentialManchester,ontheotherhand,combines the ideas of RZ and NRZ-I
Polar Biphase Schemes
In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization.
Min bandwidth = N
Bipolar Schemes
• Inbipolarencoding(sometimescalledmultilevel binary), there are three voltage levels: positive, negative, and zero
• Thevoltagelevelforonedataelementisatzero,while the voltage level for the other element alternates between positive and negative
Alternate Mark Inversion (AMI).
• Inthetermalternatemarkinversion,thewordmark comes from telegraphy and means 1.
• SoAMImeansalternate1inversion.
• Aneutralzerovoltagerepresentsbinary0.
• AvariationofAMIencodingiscalledpseudoternaryin which the 1 bit is encoded as a zero voltage and the 0 bit is encoded as alternating positive and negative voltages
AMI and Pseudoternary
Min bandwidth = 0.5N
Multilevel Schemes
• Thedesiretoincreasethedatarateordecreasethe required bandwidth has resulted in the creation of many schemes.
• Thegoalistoincreasethenumberofbitsperbaudby encoding a pattern of m data elements into a pattern of n signal elements
– mBnL: m bit, n levels – 2m < Ln
Multilevel: 2B1Q scheme
2B1Q: 2 bits, 1 quad, i.e. 4 levels
Min bandwidth = 0.25N
Summary of Line Coding schemes
Block Coding – mB/nB coding
• Replaceblocksofmbitsbynbits,m