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THE EMERGENCE of local networks and the ongoingvolution of the public telecommunications network todigital service have led to an increased interest in digitalsignaling techniques. Figure 1 illustrates what this termmeans. Digital data is generated by a source, which might beEDP equipment or a voice digitizer. In either case, the dataare typically represented as discrete voltage pulses, usingone voltage level for binary Oand another for binary 1. Thisis a format that we shall refer to as NRZ-L.Typically, these digital data pulses are passed through a

modem to be transmitted as analog signals. There are anumber of cases where this is not done. For example: Baseband local networks, using either twisted-pair orcoaxial cable. Digital PBX connections for terminals, hosts, and digitalphone.

Digital access to the public telecommunications net-work, over a "digital local loop." .

In all of these cases, it is expected that the digital data willbe transmitted as digital signals. It is possible, of course, todirectly transmit the digital data pulses emitted by thesource. It is often desirable, however, to encode the data insuch a way as to improve performance. The purpose of thispaper is to examine and compare various encodingtechniques.First, we will define some terms. A digital signalis a

sequence of discrete, discontinuous voltage pulses. Eachpulse is a signal element. Binary data is transmitted byencoding each data bit into signal elements. If the signalelements all have the same algebraic sign, that is, allpositive or negative, then the signal is unipolar. In polarsignaling, one logic state is represented by a positive voltagelevel, and the other by a negative voltage level. The datasignaling rate (or data rate) of a signal is the rate, in bits persecond (bIs), at which that data is transmitted. The durationor length of a bit is the amount of time it takes for thetransmitter to emit the bit; for a data rate R, the bit durationis II R. The modulation rate, in contrast, is the rate at whichsignal level is changed. This depends on the nature of thedigital encoding, as explaineq beloy. The modulation rate is.' .expressed in bauds, which' is signal elemnts per second.Finally, the terms mark and space, for historical reasons,refer to the binary digits 1 and O, respectively ...

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Evaluation CriteraThere are two important tasks involved in interpreting

digital signals at the receiver. First, the receiver must knowthe timing of each bit. That is, the receiver must know withsome accuracy when a bit begins and ends. Second, thereceiver must determine whether the signallevel for each bitposition is high (1) or low (O).A number of factors determine how successful the receiverwill be in interpreting the incoming signal: the signal-to-noiseratio (SIN), the data rate, and the bandwidth of the signal.With other factors held constant, the following statementsare true [1]: An increase in data rate increases bit error rate (theprobability that a bit is received in error). An increase in SIN decreases bit error rateo

Increased bandwidth allows increased data rateoDecember 1984-Vol. 22, :0-;0.12IEEE Communications :\Iagazine

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Nonreturn lo Zero [NRZIThe NRZ codes share the property that the voltage level is

constant during a bit interval; there is no transition (noreturn to a zero voltage level). These are the simplest codesto implement, and the simplest of these is NRZ-L. NRZ-L isgenerally the code used to generate or interpret digital databy data processing terminals and other devices. If a differentcode is to be used for transmission, it is typically generatedfrom an NRZ-L signal by the transmission system.There are also mark (NRZ-M) and space (NRZ-S) y.ersions

of the NRZ signal. These have the transmission advantagethat they are differential codeso In differential encoding, thesignal is decoded by comparing the polarity of adjacentsignal elements, rather than the absolute value of a signal

There is another factor that can be used to improveperformance-the encoding scheme-which is simply themapping from data bits to signal elements. A variety ofapproaches have been tried. In what follows, we describesome of the more common ones; they are defined in Table Iand depicted in Fig. 2. They can be categorized as follows: 'Nonreturn to zero (NRZ) Return to zero (RZ) Bi p hase Delay modulation Multilevel binaryBefore describing these techniques, let us consider the

ways of evaluating or comparing the various techniques. Anumber of evaluation factors have been proposed [2-4]: Signal spectru!f1-Several aspects of the spectrum areimportant. A rack of high-frequency components meansthat less bandwidth is required for transmission. On theother hand, lack of a direct-current (dc) component isal so desirable. With adc component to the signal, theremust be direct physical attachment of transmissioncomponents; with no dc component, alternating-current(ac) coupling via transformer is possible. This providesexcellent electrical isolation, reducing interference.

Signal synchronization capability-We mentioned the needto determine the beginning and end of each bit position.This is no easy task and may require a separate clocklead to synchronize the transmitter and receiver. Somecoding schemes avoid this problem.

Signal interference and noise immunily-Certain codesexhibit superior performance in the presence of noiseThis is usually expressed by the bit error rateo

Error,detection capability-Many signaling schemes ha vean inherent error-detection capability.

Cost and complexity-Although digital logic continues todrop in price, this factor should not be ignored.

We now turn to a discussion of the various techniques.

F/g.1. D/g/tal slgnallng of digital data.

DIGIT ALDATASDURCE

Digital DataPulses (NRZ-Lj

ENCDDERDigital Signal

element. One benefit of this scheme is that it may be morereliable to detect a transition in the presence of noise than tocompare a value to a threshold. Another benefit is that, witha complex transmission system, it is easy to lose the senseof the polarity of the signal. For' example, on a multidroptwisted-pair line, if the leads from an attached device to thetwisted pair are accidentally inverted, all1's and O's for NRZL will be inverted. This cannot happen with differentialencoding.The NRZ codes are easiest to engineer and, in addition,

make efficient use of bandwidth. This latter property isillustrated in Fig. 3 (based on [3 and 5]), which compares thepower spectral density of various encoding schemes. In thefigure, frequency is normalized to the data rateo As can beseen, most of the energy in NRZ signals is between dc andhalf the bit rateo For example, if an NRZ code is used togenerate a signal with a data rate of 9600 bIs, most of theenergy in the signal is concentrated between dc and 4800 Hz.The main limitations of NRZ signals are the presence of the

dc component and the lack of synchronization capability. Topicture the problem with the latter, consider that with a longstring of 1's for NRZ-L or NRZ-S the output is a constantvoltage. Any drift between transmitter and receiver cannotbe corrected based on the signal alone.

TABLE IOEFINITlDN DF OIGITAL SIGNAL ENCDDING FDRMATS

Nonreturn to Zero-Level (NRZ-L)1=High levelO=Low level

Nonreturn to Zero-Mark (NRZ-M)1=Transition at beginning 01 intervalO=No transition

Nonreturn to Zero-Space (NRZ-S)1=No transition0=Transit ion at beginning 01 interval

Return to Zero (RZ)1=Pulse in lirst hall 01 bit intervalO=No pulse

Biphase-Level (Manchester)1=Transition Irom high to low in middle 01 intervalO=Transition Irom low to high in middle 01 interval

Biphase-MarkAlways a transition at beginning 01 interval1=Transit ion in middle 01 intervalO=No transition in middle 01 interval

Biphase-SpaceAlways a transition at beginning 01 interval1=No transition in middle 01 interval0=Transit ion in middle 01 interval

Differential ManchesterAlways a transition in middle 01 interval1=No transit ion at beginning 01 interval0=Transit ion at beginning 01 interval

Delay Modulation.(Miller)1=Transit ion in middle 01 intervalO=No transition il lollowed by 1

Transltion at end 01 interval il lollowed by OBipolar1=Pulse in lirst hall 01 bit interval, alternating polarity Ira m pulseto pulse

O=No pulse

December 1984-Vol. 22, No. 12IEEE Communications Magazine 22

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o o o o oNRZ - L

NRZ - MNRZ-S

RZBiphase - L(Manchester)

Bphase - M

Bphase - S

DfferentalManchester

Delay Modulaton(fiiLLe r)Bipolar

Fig. 2. Digita/ signa/-encoding formats.

ecause of their simplicity and relatively low frequencyponse characteristics, NRZ codes are commonly used fortal magnetic recording [6]. However, their limitationske these codes unattractive for signal transmission

Relurn to Zero [RZ]he return to zero (RZ) convention provides no improvent over NRZ techniques. With this code, we first see theinction between data rate and modulation rateo The bit, as with all of these codes, is 1/ta, where ta =.bitation. However, the minimum-size signal element is thee for a binary 1, which is one-half the length of the bitrval. Hence, the maximum modulation rate (achieved bytring of 1's) for RZ signals is 2/ta.nother way of obtaining the modulation rate is toermine the average number of transitions that occur pertime. For some codes, this will depend on the exactuence of bits being transmitted. Table 1I shows the signalsition rate in the case of a data stream of alternating 1'sO's, and for the data stream that produces the minimummaximum value for each codeo Note that in the case of a

g of 1's, RZ has a maximum rate of twice the bit rateo

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'Because the modulation rate is higher than with NRZ, thebandwidth of the signal is greater. The same problems of thedc component and no synchronization (with a string of O's)remain. Because of its simplicity, RZ is used in someelementary transmitting and recording equipment, but is notthe technique of choice for most applications.

BiphaseThe term biphase designates biphase-L (Manchester),

biphase-M, biphase-S, and differential Manchester. Biphaseschemes are intended to overcome the disadvantages of NRZand RZ signal-encoding techniques. AII biphase schemesrequire at least one transition per bit time (Table 11),and mayhave as many as two transitions. Thus, the maximummodulation rate is twice that for NRZ and the bandwidth iscorrespondingly greater. To compensate for this, the biphaseschemes have several advantages [7]: Synchronization-Because there is a predictable transition during each bit time, the receiver can synchronizeon that transition. For Manchester and differential Manchester, there is always a transition in the middle of the

"bit interval. For biphase-M and biphase-S, there isDecember 1984-Vol. 22, No. 12IEEE Communications Magazine

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T ABLE 11SIGNAL TRANSITION RATE

NRZLNRZMNRZSRZMancheslerBiphase-MBiphaseSOifferenlialMancheslerDelay ModulalionBipolar

Minimumo (all 0'5 or 1'5)O (all 0'5)O (al l 1'5)O (all 0'5)lO (1010 ... )1.0 (all 0'5)1.0 (al! 1'5)1.0 (all 1'5)0.5 (1010 ... )O (all 0'5)

101010 ...1.00.50.51.01.01.51.51.50.51.0

MaximumlO (1010... )1.0 (all 1'5)1.0 (all 0'5)2.0 (all 1'5)2.0 (all 0'5 or 1'5)2.0 (all 1'5)2.0 (all 0'5)2.0 (all 0'5)1.0 (all 0'5 or 1'5) .2.0 (all 1's)

UlAI~

always a transition at the beginning of a bit time. Forthis reason, the biphase codes are. known as selfclocking codesoNo de eomponenl-Biphase codes have no dc component,yielding the benefits described earlier.Error deleetion- The absence of an expecled transitioncan be used to delect errors. Noise on the line wouldhave to inverl Ihe signal both before and after theexpected transition to cause an undetected error.s can be seen from Fig. 3, the bulk of the energy inase codes is between one-half and one ti mes the bit rateos, the bandwidth is reasonably narrow and contains noomponent. Also note that ail but the Manchester code areiphase codes are popular techniques for data transmis. Although NRZ is still most widely used in datamunications systems, the Manchester code is gainingnd rapidly [8]. It is already common in magnetic taperding and as an input signal for fiber-optic modulationems. Both Manchester and differential Manchesterear in local network standards. The more commonchester code has been specified for the IEEE 802.3dard for baseband coaxial cable using CSMA/CD accessIt has also been used in MIL-STD-1553B, which is alded twisted-pair bus system designed for high-noise

0.2 004 0.6 0.8 1.0 1.2 lA 1.6 1.8 2.0Nonnal iz ed f requency f JR

Fig. 3. Spectral denslty of varlous dlgltalslgnal encodlng schemes.

ember 1984-Vol. 22, No. 12E Communications Magazine 24

environments [10]. Differential Manchester has been specified for the IEEE 802.5 standard for token ring, using eitherbaseband coaxial cable or twisted-pair. Because it usesdifferential encoding, differential Manchester is preferred fora twisted-pair implementation.

Delay ModulationAn interesting alternative to the biphase technique is delaymodulation, also known as Miiler coding. With Miiler, thereis at least one transition per two bit times, and there is nevermore than one transition per bit. Thus, Miiler coding hassome synchronization capability, but requires a lowermodulation rate and less bandwidth than biphase.Figure 3 shows that the bandwidth for Miiler is signifi

cantly less than either biphase or NRZ. This figure issomewhat misleading however. For worst-case bit patterns,Miller can have a significant dc comppnent and greaterbandwidth than NRZ [6].Next, consider Fig. 4, which compares the theoretical biterror rate as a function of S/N for Miller, Manchester, NRZ,

10

10-10-20-310..Bit error rate

10'5,NRZ,Manches!er10'6

10-7O-B0-9O

26O'2146S/N in dB

Fig. 4. Theoretlcal bit error rate for various dlgitalencodlng schemes.

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nd RZ. As can be seen, Manchester and NRZ have identicalerformance, which is about 3 dB better than Miller. Putnother way, at a S/N of 12dB, the bit error rate for NRZ andanchester is two orders of magnitude better than Miller.This difference in performance falls out of the derivation ofe probability of error in white noise. (This can be found in3], but we can give an intuitive explanation here.) At theceiver, a decision must be made whether a received bitterval contains 1 or O. In the case of NRZ and Manchester,ere are only two elementary pulse waveforms to chooseom. This is also true of RZ, but RZ makes inefficient use ofignal time: either a half-pulse or no pulse. The Miller codeses four elementary pulse waveforms, making the decisionen more difficult.

Multilevel BinaryA somewhat different category of encoding is multilevelinary, which uses more than two signal levels. A popularxample of this is bipolar, which is widely used by AT&T forPCM carriers.As can be seen from Fig. 3, bipolar has a bandwidthentered on one-half the bit rateo There is no dc component:us, on the T1 carrier, repeaters can be transformeroupled. On the other hand, there is no synchronizationapability. Bipolar provides some error-detection capability,ince successive 1's must have opposite signs.

SurnrnaryThe increased availability and use of digital basebandansmission facilities have led to a renewed interest inigital signaling techniques. The NRZ codes commonly usedEOP equipment and for magnetic recording are not

fficient for transmission. Alternative techniques. particurly Manchester and differential Manchester, provideuperior performance and are coming into widespread use.

References[1] W. Stal lings. Data and Computer Communications. New York:Maemillan. 1984.[2] H. L.Deffebaek and W. O. Frost, "A Survey 01 Digital Baseband

Signalling Teehniques," NASA Teehnieal Memorandum TMX-64615. June 3D, 1971.[3] W. C. Lindsay and M. K. Simon, Telecommunication Systems

Engineering, Englewood Clif fs. NJ: Prentiee-Hall, 1973.[4] B. P. Lathi, Modern Digital and Analog Communication Systems.

New York: Holl. Rinehart, and Winston. 1983.[5] Bell Telephone Laboratories, "Transmission Systems lor Communieations," 1982.[6] R. H. Severt, "Encoding schemes support high-density digital

data reeording." Computer Design. May 1980.[7] L. Sanders. "When to preler Manchester coding." Electronics.

July 28, 1982.[8] L. Sanders, "Manchester coding gaining on NRZ." Electronic

Design, August 5, 1982.

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[9] W. Stallings, Local Networks: An Introduction, New York,Macmillan, 1984.

[10] D. Mandelkern, "Rugged local network follows military aircraftstandard," E1ectronics, April 7, 1982.

Dr. Will iam Stal lings is a Irequent lecturer on data communicationstopies. He is the author 01 Data and Computer Communications(Macmillan. 1984), Local Networks: An Introduction (Macmillan,1984). Local Network Technology (IEEE Computer Society Press,1983), and A Manager's Guide to Local Networks (Prentiee-Hall,1983).Dr. Stallings received a Ph.D. from M.I.T. in Computer Science in1971. Currently, he is a senior eommunications consultant withHoneywell Inlormation Systems, Ine. He is involved in the planningand design 01 communications network products and in theevaluation 01 communications requirements lor Honeywell customers. Previously, Dr. Stal lings was vice-president 01 CSM Corp., afirm specializing in data processing and data-communieations

technology for the health-care industry. He has also been director 01Systems Analysis and Design lor CTEC, Inc., a lirm speeializing incommand, eontrol, and communieations systems .

COMPUTER CONTROL ENGINEERSSETPONT, INC. is an acknowledged leaderin implementing advanced control and optimization in the process industries. Our continued growth has created opportunity tor atew outstanding engineers,Desired qualifications inelude: An advanced Degree in Engineering. Familiarity with modern control theory andtechniques-particularly real time identificationand estimation procedures.

An interest in applying modern controlapproaches to real industrial problems.

U.S. citizenship or permanent residency VISA.Your work will involve developing and implementinginnovative approaches to industrial control problems.You will work with a variety of companies in a variety ofindustries, Some travel, both domestic and international, is required.For turther consideration,send your resume to:

SETPf4)lNT Ine.PERSONNEL DIRECTOR950 THREADNEEDLEHOUSTON, TEXAS 77079

Principal s onlyAn Equal Opportunity Employer

December I984-Vol. 22. No. 12IEEE COInmunications Magazine