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Module 1: Physical Layer

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Encoding and Decoding Techniques

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Encoding and Decoding Techniques
Hello, so we will be discussing our this layer on Physical Layer considerations. As wehave done for last couple of lectures; that different aspects of physical layers, we will bealso we will be seeing today also some other few more important aspects of physicallayer consideration. As I mentioned in earlier lectures also, we this is our bottom mostlayer, one of the important layer for communication where the communication aspectscome into play and so, and heavily dependent on the media considerations right.So, how you communicate whether it is through wireless, wired, what sort ofcommunication media, it plays a important role that how the data will be pushed into thethings.Finally, this layer is also at end of the thing that this layer is also in responsible that howmuch bit rate will be able to achieve right. Whatever we may be doing different sort ofencoding compression at the upper end, but nevertheless finally, pumping the trafficthrough this should be on the physical layer right.So, there are different physical layer consideration and most of the cases we will see thisphysical layer is more dictated by the communication paradigm. It is a major aspects ofcommunication paradigm and lot of things which are happening which are thereprevalent theories and practices which are prevalent in the communication systems, weare inheriting there or we are discussing therefore, that matter to for this physical layerconsideration. So, today also we will discuss about couple of things like one is theencoding techniques other is the your multiplexing techniques, encoding, modulation,multiplexing techniques right.So, those type of techniques which are there in the which are needed at the physicallayer. Again let me emphasize that this are a vast communication topics right. So, we willbasically try to see the overview upon the consideration what is required for our networkpoint of view what we have seen earlier.
So, we will just to see a overview of the different type of techniques which are prevalentinto the things right. So, if you look at the encoding decoding techniques, so what isencoding? It is a process of converting a data of or a given sequence of characterssymbols, alphabets etcetera into specified format for efficient transmission of data, right.(Refer Slide Time: 02:59)
So, that is flatly like that decoding is a reverse of encoding. So, it is a process ofconverting a data which is generated by the system into given sequence of somecharacter say symbols alphabets etcetera, in a particular format to a efficient transmissionof data.
(Refer Slide Time: 03:22)
So, what we are doing giving a digital or analog signal analog data, we want to encode iteither through a encoder, and who generate a intermediate signal which is transmittedthrough the media. And put it to the decoder which generate the signal back or generatethe data value. Now these there can be a analog data with digital signal, digital data,analog signal, analog data, analog signal, digital data. So, they are different variation ofthe things.Now, there is another technique called modulation; which is basically with that it ismodulated with respect to a carrier frequency right? Why is this is done we will come tothat right, see while I encode; while I just encode into analog signal. Now that as wehave seen earlier that the signal has some frequency phase and amplitude or a compositesignal has different frequencies a set of frequencies. Now as this frequency range shouldbe within the within the range of the frequencies which that channel can carry right.So; that means, if it is not in those range I need to modulate this frequency through athrough a carrier. So, that it fits into this channel carrying capacity right. So, I require acarrier frequency by which I can put this range into the particular things, like say thisthere this one; it could have been in some other range here or some other range, but if myparticular media can carry within this range then I put a carrier frequency f c andmodulate it such that it is within that particular range. So, we will see those
considerations. So, these are again as I mentioned (Refer Time: 05:24) these are corecommunication phenomenon.(Refer Slide Time: 05:28)
So, other thing is modulation is a process of encoding source data, encoding we haveseen encoding source data into a carrier signal with a carrier frequency of frequency f c.Frequency of a carrier signal is chosen to compatible with the transmission media usethat too at the (Refer Time: 05:46) then it should be linear thing. So, the modulationtechniques involves can be 3 different things; it can be amplitude modulated, frequencymodulated, phase modulated.The carrier frequency carrier signal is modulated that way, according to the input sourcesignal m t either analog or digital which is called the baseband signal or modulatingsignal. The carrier signal f c will be the modulated into modulated signal s t right. So,that that we have seen so we have a source signal m t carrier signal f c and modulated toa some signal s t, which is carried through this channel in a faithful manner; that means,with less with minimal distortion or minimal loss type or loss there or I can get the signalwith maximum S N R.
(Refer Slide Time: 06:30)
Now, as you are telling that encoding techniques or encoding or modulation techniquesthere are there can be 4 option, one is digital data digital signal, analog data analogsignal, digital data analog signal and analog data analog signal right. So, this can be the 4option so we can have this sort of 4 variant. So, all our this techniques what are there areif we can cater (Refer Time: 07:00) to this 4 things are absolutely fine right.So, digital data digital signal the equipment for encoding data into digital is less complexbecause it is on some levels we will see that. Whereas analog digital signal conversion ofanalog data like voice, video into digital form for communication or different type of useof modem digital transmission, and switching through using the same data network totransmit the data and sort of things digital data analog signal. There is optical systemguided media wireless that propagate analog signals where digital data needs to beconverted to analog signal.And then we have analog data analog signal that is baseband easy and cheap that is voicegrade telephone lines, what we do was signaling modulation permits frequency divisionmultiplexing or F D M like for FM radio and am FM radio.
(Refer Slide Time: 07:57)
So, digital data digital signal digital sequence of discrete continuous voltage pulses right.So, each pulse is a signal element, binary data transmitted encoding bit stream to signalthe simplest is the one is represent by some voltage say by a lower voltage. And 0 isrepresented by a higher voltage 2 voltage level that is the simplest thing, but I can havemore I mean more complex scenario with multi-level set sort of things.(Refer Slide Time: 08:26)
Such few terminologies there is a thing there is a concept called unipolar. If all signalshave the same algebraic sign positive or negative, polar one logic state represent by
positive voltage other as by negative voltage. Data rate - the rate of data transmission bitsper second, duration or length of bit already we are seen time taken for transmit or emitthe bit. Modulation rate, the rate at which the signal level changes measured in baudsignal elements per seconds. And there is a mark and space that is mark is binary onespace 0 and type of thing. So, these are some of the terminologies which are used in thistype or in this paradigm of this encoding decoding multiple signal so on.(Refer Slide Time: 09:12)
How to interpret the signal; thus receiver needs to know the timing of each singleelement right.Otherwise it will not able to synchronize when the signal element begins and ends. So,and if there are signal levels accept only positive and negative if there multiple signallevels. There are some of the factors which are important that is signal to noise ratio, wehave discussed data rate band width these are the affecting successful interpreting ofsignal if the huge degradation it may not be able to faithfully regenerate.Some principle and increase data rate, increases bit rate error right. If you increase thedata rate there can be bit rate error, an increase in SNR decreases B E R an increase inbandwidth allows an increase in the data rate. So, we are not going to the detail of thethings, but these are which can be proved and observed.
So, we request some encoding scheme right with the vanilla (Refer Time: 10:10) or theplain signal, it may not be possible to maintain all those parameters. So, some ofencoding scheme which may improve performance, so the mapping the data bits intosome signal elements. So, we have the encoding scheme. So, we have data bits data atone end and we need to map to the some signal element which need to be which can betransmitted.(Refer Slide Time: 10:34)
So, there are some of the popular encoding schemes we will just quickly look at thosenot may not be the all. So, non-return to 0 or NRZ or at NRZ L NRZ inverted, then inbipolar AMI, Pseudoternary AMI Pseudoternary things which are multilevel binary biphase Manchester encoding differential Manchester and scrambling techniques and typeof things. So, there are different type of encoding techniques right, like to give a gist ofthe things.
(Refer Slide Time: 11:08)
So, this is the different ways like non return to 0 level; is 0 is high and 0 is representedby high level. And one is represented by low level similarly if I am consider as an one ofthe popular thing is the Manchester transition from high to low level is a middle of theinterval transition with a low level to high level is the middle of the interval differentialManchester is also popular.And these are the things which are and several other things we are not going to details onthe things. The important that while encoding we need to we at what we are trying to doas we have seen that we want to ensure better bit rate, better signal to noise ratio and soand so forth.Along with that we need to have minimum error or in other sense we need to what youare looking for that it is easily identifiable right. That which with what sort of data isthere it is easily identifiable. So, that once the error is less it leads to less regenerationtransmission and saving in bandwidth.
(Refer Slide Time: 12:17)
So, this is the NRZ as you can see that it is 0 is high, 1 is low. Here also 0 is high 1 islow and this way representation is there right. So, there is a plain thing NRZ, I is avariant of the things, if you look at the Manchester encoding on the other hand likeManchester what you say that 0 transmission from high to low at the middle of theinterval, in case of 1 transmission from low to high at the middle of the integral.(Refer Slide Time: 12:49)
So, you see that at in case of 0 in the middle of the interval it is high to low and in caseof 1 again it is low to high and it goes on like this right.
So, what we see in the middle of the interval is not at the fringe where the data may besomething degraded it the, so that we can have this encoding scheme where the at themiddle of the interval.(Refer Slide Time: 13:18)
So, variant of differential Manchester, it always there is a transition in the middle of theinterval along with there is some consideration. Like always a transition in the middle ofthe interval 0 transition from at the beginning of the interval and 1 no transmission at thebeginning of the interval.So, there is always transition at the middle and 0 and at the middle at the beginning andfor one there is no transmission. See by this what we are trying to do we have a betterrepresentation of these, signal which is less prone to error less prone to degradation andtypo and faithfully construction of the things easily identified by the receiver so and soforth. So, there are few more scrambling techniques etcetera said which are there in thisparticular encoding techniques.
(Refer Slide Time: 14:15)
So, differential Manchester encoding, and as we have seen that modulation rate one bitone signal type of things. So, we can have different models and rate for different type ofencoding things right. So, we with more complex encoding we have a better modulationrate, but what is more difficult in this type of cases. Now here we the your circuitry willbe complex right.So, if you use only 0 and 1 that is one thing, but if you want to detect the transition notonly that you need to transit at the middle of the things or at the beginning of the thingsyour circuit becomes more complex. So, that your electronics part becomes morecomplex never the let us you get more advantage out of it.
(Refer Slide Time: 15:07)
Digital data analog signal so modulation involves operation of 3 characteristic of signalinto one or more like what we say; there is amplitude shift key frequency shift key. So,there are 2 variant the most that popular one is that; binary FSK. So and other one ismultiple F S K and then we have the phase shift keying that is on the phase.So, one is on the based on the amplitude another on the frequency another on the phaseright. And there is a another thing or QAM which is combination of ASK and FSK, likewhat we see that public telephonic system design to transmit a data from 300 hertz to3400 hertz right.So, use modem digital modulator or this to put the signal on a higher thing because thosewill be highly degraded right 300 hertz to 3400 hertz. This type or this frequencybandwidth low frequency band would be high deteriorated or the there will be fall in S NR. So, we can modulate the data into higher things.
Encoding and Decoding Techniques- Part 2
So, if we look at if we this is my data that 0 0 1 1 0 1 0 0; so it is 0 is represented by highand 1 is represented by 0 voltage. And if it is ASK so, what we say it is a 0 frequency issorry the amplitude is 0 for 0 and amplitude is 1 a particular value for when the data is 1.So, we see that it is like this ASK is transmitted like this right. This is easy to implement,but on the other sense other end. So, so long you have this type of flat signals or what wesay DC signal. It is difficult to maintain those things right difficult to maintain thosethings over a media or difficult to identify, where things are there where how many 0sand 1s type of things are there.Variant of or the other part is when we do a frequency division multiplication key orfrequency shift key or to be more specific binary shift key; that means, we are using 2frequency, 1 for 0 and 1 like if you see here is 1 for 0 a particular frequency f 1 anotherfrequency f 2 for 1 and go so and so forth right. So, this is based on the frequency shiftkey or FSK right.So, first one is the ASK where the amplitude of the things are considered here thefrequency and the finally, we have a PSK or phase shift key more importantly what wesay; here B P S K means there are 2 phases are used, like if you in this case one phase forthe 0 next phase for the 1 right and again for the 0 and 1 and so and so forth. This isbased on the phase of the thing the phase of the signal is considered or 2 phases are
considered for doing that. So, which are 180 degree apart so the sorry, 90 degree apart sothat you have 2 distinct things so there is there.(Refer Slide Time: 18:33)
So, what we see that amplitude shift keying values are represented by different amplitudeof the carrier frequency usually 1 amplitude is 0 in the presence and absence of thethings inefficient, up to 1200 bps on voice grade signal is possible. And ASK is used totransmit digital data over optical fiber.(Refer Slide Time: 18:58)
Binary frequency shift keying the most common form of FSK is a binary FSK or BFSK.2 binary value represented by 2 different frequency, near the carrier frequencies right. So,that instead of one you have we have 2 things.Now, carrier frequency is important because the carrier frequency is decided based on thefrequency range of the channel which can which it can faithfully transmit right. So, 2frequencies are selected, near the carrier frequency what we expect that the variations areable to these 2 are also able to faithfully transmitted along then region.So, BFSK is less susceptible to error than ASK and up to 1200 bps voice grade line alsouse for high frequency to 3 to 3 megahertz radio things right. So, it is less susceptible toerror than ASK that amplitude shift keying.(Refer Slide Time: 19:53)
And phase shift keying the phase of the carrier signal is shifted to represent the dataalright. So, binary PSK that is binary phase shifting keying 2 phase represent 2 binarydigits, right, differential PSK. So, phase shifted relative to the previous transmissionrather than some constant reference signal right.So, it is based on the previous transmission the phase is shifted rather than following aconstant reference signal to do them. So, these are the 3 predominant techniques.
(Refer Slide Time: 20:36)
There is a another technique which is called QAM, that is quadrature amplitudemodulation right or QAM which is a mix of ASK and PSK. And this QAM is used forasymmetric digital subscriber ADSL or some incase of some wireless standards right.So, combination of ASK and PSK, logical extension of QPSK, that quadrature PSK. So,this amplitude mixed with the quadrature PSK, send 2 different signals simultaneouslyon the same career right use 2 copies of the carrier once shifted by 90 degree. Twoindependent each carrier is ASK modulated alright, two independent signal over thesame medium demodulation and combination of the binary things.So, if I have 2 binary things then I can have totally 180 degree apart as I was mentioningthat 90 degree. So, it should be out of phase that is one 80 degree apart, where as in thiscase we have a quadrature like 90 degree apart right. So, quadrature QPSK, so it iscarrier modulated by thing. So, I can have independent of the amplitude I can have either2 or quadrature 4 in the PSK type of things like, 4 type of 4 phases out of this within thatsignal length. So, I can have multiple level of thing so to say right.So, 2 independent single can be same medium modulated and combination if the binaryorigin and binary output.
(Refer Slide Time: 22:29)
So, these are these are the some of the techniques where we have those these sets ofthings, where we have this digital data analog signal conversion; with the things andprimarily looking at different these are the prominent ASK, BPFSK and BPSK or binaryfrequency 2 frequency 2 level of amplitude here, we have 2 signal at 180 degree apart.And then we can have a quadrature FSK along with that quadrature PSK, and along withthe ASK to have a QAM which is pretty popular and we can have multi-level oftransmission.And we have finally, not finally, that analog data to digital signal there is anotherconversion that is the analog digital signal. So, it is a process of digitization I haveanalog data I want to digitize that signal right. So, that data can be transmitted usingconversion to analog data to a digital data and data can be transmitted using NRZL, datadigital data can be transmitted using other than NRZL and so and so forth.Analog to digital conversion is using some a concept called codec coder and an decoder.So, what we say codec there are 2 principal codec one is pulse code modulation and deltamodulation again core communication techniques those who are from thecommunication background have studied this so there is 2 techniques to convert.
(Refer Slide Time: 24:02)
So, I have a analog signal digital digitizer and a digital signal.(Refer Slide Time: 24:08)
So, in the PCM it is a sampling theorem if the signal is sampled at a regularly at a ratehigher than twice the highest frequency, signal frequency then it can be faithfullyregenerated. In other sense, if I have a voice great thing of 1000 hertz or 4 kilo hertz mysample should be somewhat the rate of the 8000 samples per second; for efficientreproduction of the voice signal right.
So, samples are analog samples called PAM samples that is pulse amplitude modulatedsamples. To convert to digital data analog must be assigned a binary code right. So, allanalog signal that should have a binary code alright.
So, this is sample is quantized to some level right. So, if I have a sample like quantizedin the some level. So, I may so a digitized value then I have a quantization value theoriginal signal is now only approximated.And cannot be recovered exactly, the effect the effect is called quantization error orquantizing noise. So, what we are doing? We have a analog signal converted to a digitalsignal, but the these values is going to be quantized right to be in some levels.So, that the more finer the quantization better is the reconstruction, but there is always achallenge of that how much data load can be there. So, say for 8 bit it gives a 256 levelsfor 8000 samples per 8 bit we can gives a 6 kbps for a singing voice channel right. So,this is for faithful reconstruction of the things right.
So, PCM block continuous time data is coming, so a sampler is there discrete continuousthis a quantizer which quantized into 2 different PCM pulses a encoder digital bit streamsout to the signal. So, this is where the analog to digital conversion is there.The variant of that or a another thing is a delta modulation. So, a analog signal isapproximately by a staircase function that moves up or down based on the thing, basedon the movement of the analog signal. If the value of the sample waveform of the
staircase one is generated otherwise 0 is this 0 is generated. So, step and the samplingrate also plays a important thing again you people are picture you see in the things.So, this is where it is follow this step or staircases. And it goes on generating this 1 andseries of 0’s and 1 type of things based on the staircase phenomenon. So, one it is herethis is 0 to 1 and then it is 0 again, then it goes on up and down and type of things.So, analog data to analog signal so modulation combining an input signal carrierfrequency. To a carrier frequency they are telling that it is a modulation and what we
have. So, we have on a carrier frequency we have a carrier frequency the analog signal isthe carrier frequency is modulated based on the analog signal.So, can be amplitude modulated where the amplitude is changed, it can be frequencymodulated frequency of the carrier frequency is changed and phase modulation phase forthe carrier frequency is changed. Why carrier frequency because we want to use therange of allowed frequency on the things right.And the usually the your normal data may be very low frequency data in a low rangewhich is which may not be faithfully transmitted to the transmission medium or prone toseveral attenuation effects.So, what we see few observations both analog digital form can be encoded either analogor digital signal. The particular encoding is chosen for the specific purpose, then we haveseen the digital data digital signal we have looked into digital data analog signal, analogdata digital signal, analog data and analog signal.So, these are the different variants of these are the different possibilities. And as Imentioned these are code communication what we are looking and is more overview, ifyou those who are interested can go deep into the any standard communication book orliterature.Other part what we thought that you are mentioning is the multiplexing and thedemultiplexing. So, is a way of sending multiple signal or stream of information overcommunication link in the same time in a form of a single complex signal and Demuxingthe reverse of the things.In most of the cases what happened I need there is several signal that need to be pumpedthrough a S N R medium. So, there is a need of multiplexing the signal and sending thethings and demux at the other end right. So, provided this signal is able to carry thethings or in another sense if I say; like it for say conversing say I have different type ofdifferent carrier frequencies.And things are being say modulated at different carrier frequency and pumped to thethings and I extract every a carrier frequency and extract the data out of things. So, that ifthe channel capacity is there then I can multiplex the data into the things right this is afrequency division.I can have different type of like that multiplexing. Frequency division multiplexing isone of the popular thing. Wavelength division multiplexing is sort of a frequency thephilosophy is same, but at the wavelength level. Other than other 2 a Time divisionmultiplexing and Code division multiplexing right; TDM and FDM are widely used.WDM is a form of FDM used for optic in optical purpose where the wavelength isimportant CDM is a mathematical approach for cell thing so it is encoded.So, frequency as I mentioned there are different channel or so to say different carrierfrequency which are thing. So, these are different channels what I have separate channel.I can plus them into different carrier frequency and carry and the receiver does that.In case of WDM, it is same the conceptually same instead of different frequency we areusing different wavelength.In time division multiplexing we have different sender and the data flow time slots aregiven and the every data is this time slot 1 2 3 4 and type of things.And there is a concept of synchronous TDM, where even that every time slot for everystation like this is no data C 1 C 2 A 3 D 2 then no data no data A 3 and like that. So,every fixed time slot that is synchronous easy to detect and type of at the other end.Where are you have statistical TDM what we send where the data is there. So, it is morebetter utilization is a easy lot of blank data or underutilized data sets will be there, but inthis case we have a more compact representation. And that helps us in better utilizationof the chair length so, that is a statistical TDM time variation multiplexing.And there is a concept of code division multiplexing it is a more mathematicallyformulation; that means, a code is generated which is which we called a chip, which isencoded based on the code which is decoded at the things it is more used in our mobiletechnology.So, used in the part of the cell telephone system and some satellite communicationspecific version of CDM is called say used in CDMA in cell phone, called code divisionmultiple access or CDMA technique. CDMA technique does not rely on physicalproperties such as frequency or time CDMA relies on a mathematical idea value for theorthogonal vectors of the things; like if you see that if the each sender is assigned to aunique code or sometimes called chip is C 1.So, there is a known chip sequence for when you every sender is assigned a chip right.So, these are orthogonal vectors and; that means, that their dot product will be 0; thatmeans, that the other end as we know, the this chip sequence that can be extracted. So,this is a very interesting phenomenon that handling 3 handling the different data set withorthogonal binary code sequences right. So, that is exploited in the in case of in for codedivision multiplexing or CDMA technique.So, what we try to see today is that different type of encoding modulation andmultiplexing techniques which helps in faithful communication or of the data throughthese communication channel right. So, though again I mentioned though is acommunication related phenomenon, but it is we tried to have a overview of the what arethe different aspects which are there. So, with these let us conclude our discussion today.Thank you.