describe type of 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.