UNIT – I BASICS OF COMMUNICTAION
DCWSN - UNIT – IBASICS OF COMMUNICTAIONDepartment of Computer Engineering,Neminagar, Chandwad
Local Communication
Remote Communication
A Communication system ModelSourceDestinationTransmitterReceiverCarrier3 March 2004 Peter Komisarczuk &VUW, 2004.4
A Communication system Model
Elements of A Communication systemSource of information Transmitter Channel (transmission medium) Receiver User of information (sink) Noise source
Simplified Data Communications Model*
Two Basic Modes of Communication Broadcasting: single powerful transmitterand numerous receivers, one direction. Suchas radio, television. Point-to-point communication: a pairtransmitter and receiver, usually bi-direction,such as telephone.
Base bandThe original band of frequencies of a signal before itis modulated for transmission at a higher frequency. A type of data transmission in which digital oranalog data is sent over a single unmultiplexed channel,such as an Ethernet LAN. Baseband transmission useTDM to send simultaneous bits of data along the fullbandwidth of the transmission channel.
BroadbandThe term broadband is used to describe a type of datatransmission in which a single medium (wire) can carryseveral channels at once. Cable TV, for example, usesbroadband transmission. In contrast, basebandtransmission allows only one signal at a time. Most communications between computers, includingthe majority of local-area networks, use basebandcommunications
Data transmission modesThe transmission is characterized by:1.The direction of the exchanges.2.The transmission mode: the number ofbits sent simultaneously.3.Synchronization between the transmitterand receiver.
There are 3 transmission modescharacterised according to the directionof the exchanges:A simplex connection A half-duplex connection A full-duplex connection
Transmission ModesSimplexHalf DuplexFull Duplex3 March 2004 Peter Komisarczuk & VUW, 2004.13
A simplex connectionData flows in only one direction, from thetransmitter to the receiver.E.g. - computer to the printer or mouse to yourcomputer.half-duplex connection(alternating connection orsemi-duplex)Data flows in one direction or the other, but notboth at the same time. Each end of the connection transmits in turn. It is possible to have bidirectional communicationsusing the full capacity of the line.
A full-duplex connectionData flow in both directions simultaneously. Each line can thus transmit and receive at the sametime. Bandwidth is divided in two for each direction of datatransmission if the same transmission medium is usedfor both directions of transmission.
Synchronous and asynchronoustransmissionAn asynchronous connection in which each character is sent at irregular intervalsin time. To remedy this problem, start and stop bits are used. synchronous connection the transmitter and receiver are paced by the sameclock. transmitter and receiver are paced at the same speed
Digital TransmissionPCM DM ADM DPCM ADPCM
ANALOG-TO-DIGITAL CONVERSIONA digital signal is superior to an analog signalbecause it is more robust to noise and caneasily be recovered, corrected and amplified.For this reason, the tendency today is tochange an analog signal to digital data. In thissection we describe two techniques, pulsecode modulation and delta modulation.4.18
Analog to DigitalEncoding
What is Sampling 4.20In signal processing sampling is thereduction of a continuous signal to adiscrete signal
Aliasing 4.21aliasing refers to an effect that causesdifferent signals to becomeindistinguishable (or aliases of oneanother) when sampled.It also refers to the distortion
What is Aliasing? False frequencies createdsampling of an input signal.bypoorSampledsignalInputsignalSampling Rate
Effects of Aliasing in Graphics Jagged effect in rasterised graphics:Vector representation of a circleJagged edges due to aliasingduring the rasterisation process
Avoiding Aliasing? The aliasing problem can be avoided whensampling audio by applying Nyquist’sTheorem. Sampling should be done at TWICE therate of the maximum frequency present inthe original signal.
Avoiding Aliasing? Use of Nyquist’s theorem produces amathematically identical output signal.SampledsignalInputsignalSampling Rate (2x input frequency)
Sampling rate (Frequency) 4.26The sampling rate, or samplingfrequency () defines the number ofsamples per unit of time (usually seconds)taken from a continuous signal to make adiscrete signal
Nyquist rate & Nyquist Interval 4.27In signal processing, the Nyquist rate, istwo times the bandwidth(W).i.e. 2WNyquist Interval, is the time intervalbetween ant two adjacent samples whensampling rate is Nyquist rate 1/2W
PCM (Pulse Code Modulation) PCM consists of three steps to digitize ananalog signal:1. Sampling2. Quantization3. Binary encoding 4.28Before we sample, we have to filter thesignal to limit the maximum frequency ofthe signal as it affects the sampling rate.Filtering should ensure that we do notdistort the signal, ie remove high frequencycomponents that affect the signal shape.
Figure Components of PCM encoder4.29
Sampling Analog signal is sampled every TS secs.Ts is referred to as the sampling interval.fs 1/Ts is called the sampling rate orsampling frequency.There are 3 sampling methods: 4.30Ideal - an impulse at each sampling instantNatural - a pulse of short width with varyingamplitudeFlattop - sample and hold, like natural but withsingle amplitude valueThe process is referred to as pulse amplitudemodulation PAM and the outcome is a signalwith analog (non integer) values
Figure Three different sampling methods for PCM4.31
Quantization 4.32Sampling results in a series of pulses ofvarying amplitude values ranging betweentwo limits: a min and a max.The amplitude values are infinite between thetwo limits.We need to map the infinite amplitude valuesonto a finite set of known values.This is achieved by dividing the distancebetween min and max into L zones, each ofheight (max - min)/L
Quantization Levels 4.33The midpoint of each zone is assigned avalue from 0 to L-1 (resulting in Lvalues)Each sample falling in a zone is thenapproximated to the value of themidpoint.
Quantization Zones 4.34Assume we have a voltage signal withamplitutes Vmin -20V and Vmax 20V.We want to use L 8 quantization levels.Zone width (20 - -20)/8 5The 8 zones are: -20 to -15, -15 to -10,-10 to -5, -5 to 0, 0 to 5, 5 to 10, 10 to 15, 15 to 20The midpoints are: -17.5, -12.5, -7.5, 2.5, 2.5, 7.5, 12.5, 17.5
Assigning Codes to Zones Each zone is then assigned a binary code.The number of bits required to encode thezones, or the number of bits per sample as itis commonly referred to, is obtained asfollows:nb log2 LGiven our example, nb 3The 8 zone (or level) codes are therefore:000, 001, 010, 011, 100, 101, 110, and 111Assigning codes to zones: 4.35000 will refer to zone -20 to -15001 to zone -15 to -10, etc.
Figure Quantization and encoding of a sampled signal4.36
PCM Decoder To recover an analog signal from a digitizedsignal we follow the following steps: 4.37We use a hold circuit that holds the amplitudevalue of a pulse till the next pulse arrives.We pass this signal through a low pass filter witha cutoff frequency that is equal to the highestfrequency in the pre-sampled signal.
Figure Components of a PCM decoder4.38
(DM) Delta Modulation 4.39This scheme sends only the differencebetween pulses, if the pulse at time tn 1 ishigher in amplitude value than the pulse attime tn, then a single bit, say a “1”, is used toindicate the positive value.If the pulse is lower in value, resulting in anegative value, a “0” is used.This scheme works well for small changes insignal values between samples.If changes in amplitude are large, this willresult in large errors.
Figure The process of delta modulation4.40
Figure Delta modulation components(Transmitter )4.41
Figure Delta demodulation components(Receiver)4.42
Pros and cons of Delta Modulation(DM) Main advantage – simplicitySampled version of the message is appliedto a modulator (comparator, quantizer,accumulator)delay in accumulator is “unit delay” onesample period (z-1)
Noise in Delta Modulation Systems slope overhead distortiongranular noiseDigital Communication Systems 2012R.Sokullu44/45
Adaptive DM Adaptive delta modulation Quantization Error 1.45A better performance can be achieved if thevalue of δ is not fixed.The value of δ changes according to theamplitude of the analog signal.DM is not perfect.Quantization error is always introduced in theprocess.Much less than that for PCM.NDSL Copyright@2008
Delta PCM (DPCM) 4.46Instead of using one bit to indicate positiveand negative differences, we can use morebits - quantization of the difference.Each bit code is used to represent the valueof the difference.The more bits the more levels - the higherthe accuracy.
DPCM - How it works We know the signal up to a certain timeUse prediction to estimate future valuesSignal sampled at fs 1/Ts ; sampled sequence– {m[n]}, where samples are Ts seconds apartInput signal to the quantizer – differencebetween the unquantized input signal m(t)and its prediction:ˆ [n]e[n] m[n] mprediction of theinput sample
DPCM Predicted value – achieved by linearprediction filter whose input is thequantized version of the input samplem[n].The difference e[n] is the prediction error(what we expect and what actuallyhappens)By encoding the quantizer output weactually create a variation of PCM calleddifferential PCM (DPCM).
DPCM system. (a) Transmitter. (b) Receiver.49/45
Comparison DPCM and DM DPCM and DM DPCM includes DM as a special caseSimilarities Differences subject to slope-overhead and quantization errorDM uses a 1-bit quantizerDM uses a single delay element (zero prediction order)DPCM and PCM both DM and DPCM use feedback while PCM does notall subject to quantization errorDigital Communication Systems 2012R.Sokullu50/45
Adaptive Differential PCM (ADPCM) PCM for speech coding requires high bandwidthsome applications (secure transmission overradio channel – low capacity)requires speech coding at low bit rates butpreserving acceptable reliabilitypossible using special coders that utilizestatistical characteristics of speech signals andproperties of hearing
ADPCM Design Objectives Adaptive differential pulse-codemodulation (ADPCM) is a variant ofdifferential pulse-code modulation (DPCM) thatvaries the size of the quantization step, to allowfurther reduction of the required bandwidth for agiven signal-to-noise ratio.
ADPCM Adaptive prediction in ADPCM adaptive prediction with forward estimation(APF); uses unqunatized samples of the inputsignal to calculate prediction coefficients;disadvantages similar to AQFadaptive prediction with backward estimation(APB); uses samples of the quantizer outputand the prediction error to compute predictorcoefficients; logic for adaptive prediction –algorithm for updating predictor coefficientsDigital Communication Systems 2012R.Sokullu53/45
Digital-DigitalConversionLine CodingLine Coding SchemesBlock Coding
DIGITAL-TO-DIGITAL CONVERSIONIn this section, we see how we can represent digital databy using digital signals. The conversion involves twotechniques: line coding, block coding.Line coding is always needed;block coding may or may not be needed.Topics discussed in this section:Line CodingLine Coding SchemesBlock Coding
Signal Element versus Data Element Data element– The smallest entity that can represent a piece ofinformation: this is bit. Signal element– The shortest unit (timewise) of a digital signal. In other words– Data element are what we need to send.– Signal elements are what we can send.1.56NDSL Copyright@2008
Figure Line coding and decoding1.57NDSL Copyright@2008
Figure Signal element versus data element1.58NDSL Copyright@2008
Data Rate versus Signal Rate(Bit and Baud rate) Data rate– The number of data elements (bits) sent in 1s– The unit is bits per second (bps)– Called bit rate Signal rate– The number of signal elements sent in 1s– The unit is the baud– Signal rate is sometimes called the pulse rate, the modulation rate,or the baud rate Relationship between data rate and signal rateScN1rbaud S: number of signal elements, c: the case factor, N: datarate (bps), r: data elements per signal elements
Figure Line coding schemes1.60NDSL Copyright@2008
Figure Unipolar NRZ schemeNon-Return-to-Zero (NRZ)It is called NRZ because the signal does not return to zero at the middle of the bit.1.61NDSL Copyright@2008
Figure Polar NRZ-L and NRZ-I schemesNRZ-L (NRZ-Level), NRZ-I (NRZ-Invert)1.62NDSL Copyright@2008
NoteIn NRZ-L the level of the voltage determines thevalue of the bit.In NRZ-I the inversionor the lack of inversiondetermines the value of the bit.1.63NDSL Copyright@2008
Return to Zero (RZ) :The main problem with NRZ encoding occurs when the sender and receiver clocks arenot synchronized. The receiver does not know when one bit has ended and the next bitis starting. One solution is the return-to-zero (RZ) scheme, which uses three values:positive, negative, and zero. In RZ, the signal changes not between bits but during thebit. In Figure 4.7 we see that the signal goes to 0 in the middle of each bit. It remainsthere until the beginning of the next bit. The main disadvantage of RZ encoding is that itrequires two signal changes to encode a bit and therefore occupies greater bandwidth.The same problem we mentioned, a sudden change of polarity resulting in all Osinterpreted as is and all ls interpreted as Os, still exist here, but there is no DCcomponent problem. Another problem is the complexity: RZ uses three levels of voltage,which is more complex to create and discern. As a result of all these deficiencies, thescheme is not used today. Instead, it has been replaced by the better-performingManchester and differential Manchester schemes (discussed next).Figure 4.7 Polar RZ scheme4.64
Figure Polar RZ schemeRZ: Return-to-Zero1.65NDSL Copyright@2008
Polar biphase: Manchester and differential Manchester schemes In Manchester and differential Manchester encoding, the transition at the middle ofthebit is used for synchronization. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ.The Manchester scheme overcomes several problems associated withNRZ-L, and differential Manchester overcomes several problemsassociated with NRZ-I. First, there is no baseline wandering. There isno DC component because each bit has a positive and negativevoltage contribution. The only drawback is the signal rate. The signalrate for Manchester and differential Manchester is double that for NRZ.The reason is that there is always one transition at the middle of the bitand maybe one transition at the end of each bit.4.66
Figure Polar biphase: Manchester and differential Manchester schem1.67NDSL Copyright@2008
NoteIn Manchester and differential Manchesterencoding, the transitionat the middle of the bit is used for synchronization.1.68NDSL Copyright@2008
Bipolar Schemes Bipolar encoding (sometimes called multilevelbinary)– Three voltage levels: positive, negative, and zero variations of bipolar encoding– AMI (alternate mark inversion) 0: neutral zero voltage 1: alternating positive and negative voltages1.69NDSL Copyright@2008
NoteIn bipolar encoding, we use three levels: positive,zero, and negative.1.70NDSL Copyright@2008
AMI AMI (alternate mark inversion)––––1.71The work mark comes from telegraphy and means 1.AMI means alternate 1 inversionThe neutral zero voltage represents binary 0.Binary 1s are represented by alternating positive andnegative voltages.NDSL Copyright@2008
Figure 4.9 Bipolar schemes: AMI and pseudoternary1.72NDSL Copyright@2008
B8ZS Bipolar with 8-zero substitution (B8ZS)– Commonly used in North America– Eight consecutive zero-level voltages are replaced bythe sequence 000VB0VB.– The V in the sequence denotes violation; that is anonzero voltage that breaks an AMI rule of encoding(opposite polarity from the previous).– The B in the sequence denotes bipolar, which means anonzero level voltage in accordance with the AMI rule.1.73NDSL Copyright@2008
Figure Two cases of B8ZS scrambling technique1.74NDSL Copyright@2008
Block Coding Use redundancy to ensure synchronization and toprovide some kind of inherent error detecting. In general, block coding changes a block of mbits into a block of n bits, where n is larger than m. Block coding is referred to as an mB/nB encodingtechnique. For example:– 4B/5B encoding means a 4-bit code for a 5-bit group.1.75NDSL Copyright@2008
Block CodingWe need redundancy to ensure synchronization and to provide some kind of inherenterror detecting. Block coding can give us this redundancy and improve the performance ofline coding. In general, block coding changes a block of m bits into a block of n bits, wheren is larger than m. Block coding is referred to as an mB/nB encoding technique.Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an nbit group.Figure Block coding concept4.76
Block CodingBlock coding normally involves three steps: division, substitution, andcombination. In the division step, a sequence of bits is divided into groups of mbits. For example, in 4B/5B encoding, the original bit sequence is divided into 4bit groups. The heart of block coding is the substitution step. In this step, wesubstitute an m-bit group for an n-bit group. For example, in 4B/5B encoding wesubstitute a 4-bit code for a 5-bit group. Finally, the n-bit groups are combinedtogether to form a stream. The new stream has more bits than the original bits.Figure Using block coding 4B/5B with NRZ-I line coding scheme4.77
4B/5B Encoding 5-bit output that replaces the 4-bit input No more than one leading zero (left bit) and nomore than two trailing zeros (right bits). There are never more than three consecutive 0s. If a 5-bit group arrives that belongs to the unusedportion of the table, the receiver knows that there isan error in the transmission.1.78NDSL Copyright@2008
Table 4.2 4B/5B mapping codes1.79NDSL Copyright@2008
Figure 4.16 Substitution in 4B/5B block coding1.80NDSL Copyright@2008
Figure 8B/10B block encoding1.81NDSL Copyright@2008
Multiplexing
Multiplexing: Sharing Medium A link is divided into channelsn signals(input)MUXMultiplexer831 link, n channelsDEMUXDemultiplexern signals(output)
Frequency Division Multiplexing(FDM)An analog multiplexing technique to combine signals Medium BW Channel BW Each signal is modulated to a different carrierfrequency E.g., broadcast radio Channel allocated even if no data84
Conceptual View of FDMChannel 385Channel 2Channel 1f1f2f3
FDM: Multiplexing Process86
FDM: Demultiplexing Process87
Wavelength Division Multiplexing(WDM)An analog multiplexing technique to combine optical signals WDM is a special case of FDM11WDM21 2 3WDM233111 2 3Fiber-optic cable2388Multiplexer2Demultiplexer3
Time Division Multiplexing(TDM)A digital multiplexing technique to combine data Medium Data Rate Signal Data Rate Multiple digital signals interleaved in timeone unitABFrameTDMC B A C B A C B ACTime slot Time slots are preassigned to sources and fixed are allocated even if no data89 do not have to be evenly distributed among sources
CDMA (Code Division Multiple Access) One channel carries all transmissions at thesame time Each channel is separated by code90
CDMA: Chip Sequences Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors Inner product of any pair must be zero With N stations, sequences must hav
4.30 Sampling Analog signal is sampled every T S secs. T s is referred to as the sampling interval. f s 1/T s is called the sampling rate or sampling frequency. There are 3 sampling methods: Ideal - an impulse at each sampling instant Natural - a pulse of short width with varying ampli
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