Line coding
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The document discusses line coding, which is the process of converting digital data to digital signals for transmission over a communication channel. There are several types of line coding discussed, including unipolar (e.g. NRZ), polar, bipolar, and Manchester coding. The goals of line coding are to increase data rates while decreasing signal rates and bandwidth requirements, provide error detection capabilities, and allow for self-synchronization between the transmitter and receiver clocks. Characteristics of line coding include distinguishing between data elements (bits) and signal elements, and how data and signal rates differ.
Line coding
- 1. Data communication Digital Transmission Line coding Line coding: Line code is a code chosenfor use withina communicationssystem fortransmittingadigital signal downa line. It isthe processof convertingdigital datatodigital signals. Line codingconvertsdigital datatoa digital signal for the purpose of transmissionoverthe channel. PropertiesofLineCoding: As the codingisdone to make more bitstransmiton a single signal,the bandwidthusedismuch reduced. For a givenbandwidth,the powerisefficientlyused. The probabilityof errorismuch reduced. Error detectionisdone andthe bipolartoohasa correctioncapability. Powerdensityismuchfavorable. The timingcontentisadequate. Long stringsof 1s and 0s is avoidedtomaintaintransparency. Characteristics of Line coding: 1. Signal Element versus Data Element: A data elementisthe smallestentitythatcanrepresentapiece of information.Thisisthe bit.In digital datacommunications,asignal elementcarriesdataelements.A signal elementisthe shortestunit(time wise) of adigital signal.Inotherwords,dataelementsare whatwe needto send;signal elementsare whatwe cansend.Data elementsare beingcarried, signal elements are the carriers.
- 2. 2. Data Rate versus Signal Rate: The data rate definesthe numberof data elements(bits) sentin1s.The unitis bitspersecond (bps).The signal rate isthe numberof signal elementssentin1s.The unitisthe baud.The data rate issometimescalledthe bitrate;the signal rate issometimescalledthe pulse rate,the modulationrate,orthe baud rate. One goal in data communicationsistoincrease the datarate while decreasingthe signal rate. Increasingthe datarate increasesthe speedof transmission;decreasingthe signal rate decreasesthe bandwidthrequirement. 3. Self-synchronization: To correctlyinterpretthe signalsreceivedfromthe sender,the receiver'sbitintervalsmust correspondexactlytothe sender'sbitintervals.If the receiverclockisfasteror slower,the bit intervalsare notmatchedand the receivermightmisinterpretthe signals.The followingfigure representsthe synchronizationproblem.
- 3. Types of Line Coding: Unipolar: Unipolarsignalingisalsocalledas On-OffKeyingorsimplyOOK.The presenceof pulse representsa 1and the absence of pulse represents a 0. There are one variations in Unipolar signaling Non-ReturntoZero(NRZ) Unipolar Non-Return to Zero (NRZ): In thisscheme the positivevoltage definesbit1 andthe zerovoltage definesbitO.Itiscalled NRZ because the signal doesnotreturntozero at the middle of the bit
- 4. Advantages: Simplicityinimplementation. Doesn’trequire alotof bandwidthfortransmission. Disadvantages: Presence of DClevel (indicatedbyspectral line at0 Hz). Containslowfrequencycomponents.CausesSignalDroop. Doesnot have anyerror correctioncapability. polar: polar Non-Return to Zero (NRZ): In polar(NRZ) voltageare onbothside.InNRZ positive voltage means0andnegative voltage means1. polar (NRZ) have two versions NRZ-LandNRZ-I. NRZ-L: In NRZ-Lthe polarityof the signal changesonlywhenthe incomingsignal changesfromaone to a zero or froma zeroto a one, positive voltage means0andnegative voltage means1. NRZ-I: In NRZ-La '0' isencodedasno change inthe level.'1'isencodeddependingonthe currentstate of the line.
- 5. Advantages: It issimple. No low-frequencycomponentsare present. Disadvantages: No error correction. No clock is present. The signal droop is caused at the places where the signal is non-zero at 0 Hz. Polar RZ: In RZ the signal change foreach bitand signal backto level 0.Positive levelmeans1and negative means0.example: Advantages It issimple. No low-frequencycomponentsare present. Disadvantages No error correction. No clock is present. Occupies twice the bandwidth of Polar NRZ. The signal droop is caused at places where the signal is non-zero at 0 Hz.
- 6. Polar (Bi-phase): Bi-phase encoding is a variation on polar encoding and is an effective answer to synhronisation problems.Bi-phase encodingworksbychangingthe signalinthe middleof the bitinterval,however, the signal doesnot thenreturn to zeroit continuestothe opposite pole.Thismid-interval changeis perfect for synchronization purposes. Bi-phase (Manchester Coding): Manchester encoding uses inversion at the middle of each bit interval for both synchronization and bit representation. Negative to positive transition represents binary 1 whereas a positive to negative transition represents binary 0. Bi-phase (Differential Manchester Coding): Differential Manchester encoding uses inversion at the middle of each bit interval for synchronization, but the presence or absence of an additional transition at the beginning of the interval isusedtoidentifythe bit.Transitionrepresentbinary0and no transitionrepresent binary1.
- 7. Bipolar: Thisisanencodingtechniquewhichhasthreevoltage levelsnamelypositive,negative and0level. Such a signal is called as duo-binary signal. Three types of bipolar encoding are use data communications: AMI, B8ZS, and HDB3. Bipolar(AMI): AMIisa bipolarencodingsystemwhere0voltage representsbinary 0andalternatingpositiveand negative voltages represents binary 1.