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1 Data Communications Prof. A. Pal Department of Computer Science & Engineering Indian Institute of Technology, Kharagpur Lecture-11B Multiplexing (Contd.) Hello and welcome to today s lecture on multiplexing techniques. You will find that these multiplexing techniques will have widespread application in data communication. Here is the outline of today s lecture. (Refer Slide Time: 2:23) First we shall discuss why we really need multiplexing then we shall be introduced to the basic concepts of multiplexing and as we shall see there are two basic approaches of multiplexing; first one is known as Frequency Division Multiplexing and second one is known as Time Division Multiplexing. One variation of this Frequency Division Multiplexing is Wavelength Division Multiplexing which is used in the context of optical communication. Whenever we send optical signal through optical fiber light signal through optical fiber then we call it Wavelength Division Multiplexing although it is basically same as Frequency Division Multiplexing. Then the Time Division Multiplexing has again two different types; the synchronous and asynchronous. We shall discuss about them in details. There is another technique which is known as inverse TDM which is also used in data communication today. So we shall conclude our lecture by discussing about inverse TDM.

2 On completion the students will be able to explain the need for multiplexing as why multiplexing is needed, they will be able to distinguish between multiplexing techniques, what are the different multiplexing techniques, they will be able to explain key features of Frequency Division Multiplexing and also the key features of Time Division Multiplexing. They will be able to distinguish between the two types of Time Division Multiplexing, one is known as synchronous and another is known as asynchronous. And finally they will be able to explain the concept of inverse Time Division Multiplexing. (Refer Slide Time: 3:10) To discuss about why multiplexing let us consider the observations that we have in our day to day data communication. First one is most of the data communication devices typically require modest data rate. As we shall see data an individual user requires very small bandwidth. For example whenever we send wires we require a bandwidth of may be up to 4 KHz or sometimes 3 KHz. Similarly when we send data that time also you may not require high bandwidth. On the other hand the communication media which are used nowadays have much higher bandwidth. For example, if we use coaxial cable or optical fiber or if we use microwave technique then the bandwidth of the medium is quite high. So whenever two users are communicating through a link usually the full capacity of the link is not used, it is not utilized. So how do you make full utilization of the link capacity? In fact as we shall see higher the data rate the most cost effective is the transmission facility. That means to make the transmission facility cost effective we want the medium which has high data rate and high transmission capacity on the other hand individual users have smaller capacity. The best one is observation where the multiplexing techniques have been developed. Basically it can be used when the bandwidth of the medium is greater than individual signals to be transmitted through the channel. That means since the bandwidth of the individual signals is small then a medium can be shared

3 this is the key idea sharing by more than one channel of signals by using multiplexing. That means what we are trying to do is we are trying to do share the bandwidth of a channel by a number of users. It is just like in a city the water comes through a bigger pipe and that water gets distributed through narrower pipe to individual residences so it is somewhat like that. And for efficiency the channel capacity can be shared by a number of communicating stations. That means this also increases the efficiency of communication of the transmission media. And since we are concerned about the cost we will find that most common use of this multiplexing will be in long haul communication using coaxial cable using microwave and optical fiber. So these are the three transmission media which have quite high bandwidth and which are used for long distance communication. So this is where we can use multiplexing because there we should make use of the bandwidth in a very efficient manner. (Refer Slide Time: 6:14) Let us take up a very simple example even when the bandwidth is not really high say telephone line.

4 (Refer Slide Time: 8:01) In a telephone line as you know the analog telephone line has very small bandwidth of which about 2400 Hz can be used for data. Now even this small bandwidth can be shared for data communication in two directions as it is shown in this diagram. The first part 600 to may be 1800 means up to 1200, so 600 to 1800 then 1800 to 3000 so 600 to 1800 is the part used for data communication in one direction may be from user A to B and the other one can be used from user B to user A. So you see that even the small bandwidth can be shared by making use of this multiplexing and here the multiplexing can be made possible by encoding the data by using say PSK that is PS shift keying. So PSK increases the data rate with a smaller baud rate so even with 2400 Hz bandwidth the data rate can be quite high. This is a simple example. This can be used for bidirectional communication between two users using the same telephone line. Now let us look at the basic concept of multiplexing.

5 (Refer Slide Time: 8:40) Here we use a device known as multiplexer. This multiplexer is combining the signals coming from n channels. As you can see this is channel 1, channel 2 and this is channel n. so from n channel signals are coming and it is combined with the help of a multiplexer. The multiplexer is sending the combined or composite signal through a single medium. So you have got only one medium and the signals of n channels are being sent through the medium.and at the other end we have an opposite device known as demultiplexer. This demultiplexer separates out the signals of different channels by using filtering and here we get back the signal corresponding to channel 1, signal corresponding to channel 2 and so on up to channel n. So, this is by making use of two devices; one is known as multiplexer which combines signals of n channels into a single composite signal which can be sent through a single medium and at the other end a demultiplexer separates out these signals of different channels that can be sent to different stations. This is how the multiplexing and demultiplexing can be done. Now as I mentioned there are two basic approaches. First one is known as Frequency Division Multiplexing. And as we shall see Frequency Division Multiplexing is possible because of the analog transmission that is possible by modulation. As we have seen the modulation performs narrow banding of this signal. As a consequence it is possible to send n different signals each of them having a small fraction of bandwidth through the transmission media. So here what you are doing is the n channels are multiplexed and you are creating a Frequency Division Multiplexing and here we are sending all the n different signals simultaneously. Obviously they should be at different bands so that one does not mix with the other and the other end by again filtering you can separate out the different channels like channel 1, channel 2 and channel n. So this is Frequency Division Multiplexing (Refer Slide Time: 11:20) so here everything is sent in parallel.

6 (Refer Slide Time: 11:33) The second one is Time Division Multiplexing. Here the approach is little different. In the previous case all the signals were sent simultaneously but here it is not so. as you see the signals are coming from different channels and what is being done is the time is divided into slots and in slot one signal one signal from channel 1 is sent, in slot two signal is sent from channel 2 is sent, in channel three signal from channel three is sent and in slot n signal from channel n is sent. so here all are not sent in parallel but as you can see signals from different channels are sent one after the other in a sequence and each is sent in a particular slot of time and at the other end they can be again separated by reading data from this slot to channel 1, the data from slot two to channel 2 and so on so it is some what like switch. This switch first selects this one and transmits then it selects this one and transmits, then it selects this one and transmits (Refer Slide Time: 12:40) and so on. Similarly at the other end it performs the opposite operation, first this signal is sent to channel 1, then to channel 2 and then to channel n so in this way it goes on so this is your Time Division Multiplexing. Now as we shall see the multiplexing will require two different signaling.

7 (Refer Slide Time: 13:10) As I have mentioned analog signals are used for Frequency Division Multiplexing and also for Wavelength Division Multiplexing. Although basically they are the same thing but this is used in the context of transmitting through optical fiber. On the other hand digital signaling is used in Time Division Multiplexing which has got two versions; one is Time Division Multiplexing this is essentially synchronous and the other one is Asynchronous Time Division Multiplexing in which we use digital signals. So this approach the Frequency Division Multiplexing makes use of analog modulation techniques and this Time Division Multiplexing makes use of the encoding techniques. We have discussed this already. Let s see how the Frequency Division Multiplexing is implemented. What is being done is the available bandwidth of a single physical medium is divided into a number of smaller independent frequency channels. So the available bandwidth is divided into a number of smaller independent frequency channels. Look at this figure smaller and independent frequency channels so this will not overlap with each other.

8 (Refer Slide Time: 15:19) Then using modulation independent message signals are translated into different frequency bands. As we have seen by using the modulation technique this can be done because modulation allows narrow bending. And by narrow bending they can be translated to different frequency bands. All the modulated signals are combined into a linear by using a linear summing circuit to form a composite signal for transmission through a media. Obviously you will require a number of carriers to modulate the individual message signals which are known as sub-carriers. We shall illustrate with the help of example and we shall make use of different modulation techniques. (Refer Slide Time: 15:36)

9 Sometimes we shall make use amplitude modulation, sometimes we shall make use angle modulation which has got two types namely frequency modulation and phase modulation. Let s see how it is being done. Similarly when the signal is digital then we have to make use of Amplitude Shift Keying or Frequency Shift Keying or Phase Shift Keying. As we know the Amplitude Shift Keying and Phase Shift Keying can be combined to form the Quadrature Amplitude Modulation QAM. So these are used whenever the data is digital and we would like to convert it into analog signal which has to be done whenever we want to do Frequency Division Multiplexing. (Refer Slide Time: 17:42) Here the signals are coming from different sources; source one source two and source n and here you have got the modulator and as you can see different sub-carriers f 1 f 2 and fn. So these are the sub-carriers used to modulate different signals and if this is the bandwidth of the modulating signal, the bandwidth of the modulated signal is shown here it is around that carrier on both sides. if the bandwidth here is B the analog signal then bandwidth of the modulated signal as we know is 2B and after these combined together as you can see the total bandwidth is summation of the individual bandwidths so f 1 plus 2B plus 2B in this way so obviously there should be some separation between f 1 f 2 and fn so that there is no overlap. This is the transmitted signal bandwidth. As you can see (Refer Slide Time: 17:44) the transmission bandwidth is sum total of the individual bandwidths of different signals. At the other end that composite signal is received and then demodulated. Demodulation is nothing but some kind of filtering and here individual filters essentially band pass filters are having center frequencies f 1 for this case, f 2 in this case and f n so on. So you can see these are the filters. After doing this demodulation and filtering the signals can be sent then of course that signal has to be converted back into..(refer Slide Time: 18:27) actually this filter should be here and demodulator should be here. After demodulation we get the original signal which has to

10 be sent to the destination. So this will be here (Refer Slide Time: 18:46) this will be here so this is how the Frequency Division Multiplexing is done. And as I mentioned there should be some separation between different frequency bands. So this is corresponding to the channel 1 (Refer Slide Time: 19:04) with sub-carrier f C1 and this is corresponding to the bandwidth of the channel 2 f C2 and so on. In this way you have got channel 1 if its sub-carrier frequency is f cn. Now as you can see between each band there is a small gap and this is known as guard band. This guard band is necessary so that the channels are separated by strips of unused bandwidth that is your guard band to prevent inter channel cross talk. (Refer Slide Time: 19:40) So if there is no separation there is a possibility of cross talk. If you place them side by side without any separation there will be some overlap which will lead to cross talk. Hence to avoid cross talk these guard bands are used and obviously it is an extra overhead. So, apart from the sum total of the bandwidths some additional bandwidth is wasted for these guard bands this is an extra overhead that is used in Frequency Division Multiplexing.

11 (Refer Slide Time: 20:17) And this Frequency Division Multiplexing has many uses as we know. For example, we have the transmission of AM and FM radio signals. Everyday we are listening to AM Amplitude Modulated radio stations and FM radio stations. FM has become very very popular because of the quality of the signal nowadays and both are based on Frequency Division Multiplexing FDM. And our TV broadcasting is also based on Frequency Division Multiplexing because you have different TV stations and they use different bands for transmission of their signals. In the TV receiver we can select different channels with the help of filtering. And also we are familiar with cable television where the signal is distributed with the help of coaxial cable. There also we use Frequency Division Multiplexing. Later on we shall discuss about it in more details. So these are the three important areas where Frequency Division Multiplexing is used. Nowadays cable television is used not only for signaling video but the cable modem can be used for the transmission of data.

12 (Refer Slide Time: 21:36) As I mentioned there is one special type of Frequency Division Multiplexing called Wavelength Division Multiplexing that is whenever we are sending light signals through optical fiber. Why we call it Wavelength Division Multiplexing? The reason is the frequency is very high so wavelength is small. So instead of stating in terms of frequencies we state in terms of wavelengths. And particularly this Wavelength Division Multiplexing is becoming very very popular because of the enormous bandwidth provided by optical fiber media. To make use of the enormous bandwidth Wavelength Division Multiplexing is the most viable technology that overcomes the huge opto-electronic bandwidth mismatch. As I have told the optical fiber can send very high bandwidth. On the other hand individual users who are sending either audio or video their bandwidth requirement is smaller. And only by using Wavelength Division Multiplexing we can share optical fiber and we can make use of the huge bandwidth mismatch or make use of the enormous bandwidth and it also overcomes the huge opto-electronic bandwidth mismatch. Wavelength Division Multiplexing optical fiber network comprises optical wavelength switches or routers interconnected by point-to-point fiber links. The end users may communicate with each other through either all-optical multiplexing channels which are known as lightpaths which may span over more than one fiber link. That means over a very long distance the signal can be sent in the form of light and at the other end with the help of suitable transducer. We can use the pin diodes for conversion from light signal to electrical signal and then get back the original data. So the basic approach as you can see is same. Here you have got a number of n sources coming in as 1, 2 and n (Refer Slide Time: 24:04) and here is the bandwidth of the optical signal and these are multiplexed and this is the bandwidth represented in terms of

13 wavelengths. this is the bandwidth of the first signal lambda 1 to lambda 2 then lambda 3 to lambda 4 coming from source two in this way 2n minus 1 to lambda 2 that is going from source n. so these are expressed in terms of wavelengths essentially these are small frequency ranges and these light signals are transmitted through optical fiber. (Refer Slide Time: 24:34) So here you can see the bandwidth is much more which can be easily sent through optical fiber. And the optical fiber with the help of demultiplexer we can separate out different optical signals having different frequencies and then they can be sent to destinations. (Refer Slide Time: 24:55)

14 You may be wondering how really it can be done this multiplexing and demultiplexing. This can be explained very easily with the help of this simple diagram where we have used two prisms. As you know the light signal has two properties reflection and refraction so you can make use of the refraction property to combine light signals coming from three different sources then as you can see a single composite signal is here (Refer Slide Time: 25:37) which can be sent through optical fiber through this region and at the other end with the help of another prism they can be separated out and we can get back all the three different signals. This is how the Wavelength Division Multiplexing can be done. Now let us focus our attention to Time Division Multiplexing. (Refer Slide Time: 26:00) As I mentioned Time Division Multiplexing is used when we are using digital signals. Digital signals as you know are generated by different encoding techniques. We have already discussed about them in detail. This Time Division Multiplexing is possible when the bandwidth of the medium exceeds the data rate of the digital signals to be transmitted. That means here there is a possibility of sharing.

15 (Refer Slide Time: 26:33) The multiple digital signals can be carried on a single transmission path by interleaving portions of each signal in time. What we are doing is essentially we are interleaving signals first we are sending signal from channel 1 then signal from channel 2 then signal of channel three in this way we are interleaving then we are sending it through the medium having higher bandwidth. This interleaving can be done at the bit level or in the blocks of bytes. That means we can take one bit from channel 1 then one bit from another channel then one bit from another channel so you can do it this way this is called bit level interleaving or we can take one byte from channel 1 then second byte from channel 2 then third byte from channel three and so on. So this way we can do interleaving in terms of bits or in terms of blocks of bytes. Obviously in this case as I told we shall be using digital signal and digital signals are generated by using suitable encoding technique.

16 (Refer Slide Time: 27:45) If the data was digital then the digital signals are generated by using three different types of coding such as unipolar, polar or bipolar. Similarly if the original data was in analog form then digital signal can be generated by pulse code modulation or delta modulation. In either case ultimately we have got digital signals. These digital signals are expressed in terms of bits per second or Kbps that is the data rate.and these digital signals can be sent through a medium by using Time Division Multiplexing. However, this will require some kind of buffers. (Refer Slide Time: 28:29)

17 The incoming data from each source are briefly buffered and each buffer is typically one bit or one character in length depending on the interleaving level. The buffers are scanned sequentially to form a composite data stream. So, by sequentially scanning the different bits a composite data stream is created and the scan operation is sufficiently rapid so that each buffer is emptied before more data can arrive. Let us see how this is being done. (Refer Slide Time: 30:14) Here data is coming from source one source two and source n so there is some kind of switch you are taking from this source then from this source so by this way we are taking and we are creating a frame. So as you can see here in this frame first we have got the data from source S1 then from source S2 then source S3 and so on up to S minus n. After taking data from n sources again it is started from source S1. as you can see here (Refer Slide Time: 29:41) second frame is started from source S2 then to S3 and so on then this is sent in terms of time. So this frame is sent then this frame is sent and so on. So as you can see here there are n slots in each frame. This is slot 1 this is slot 2 so in this way you have got n slots and each slot corresponds to a particular source. That means slot 1 corresponds to data from this source slot 2 corresponds to data from this source and slot n corresponds to this data from this source. However, whenever we are doing the framing some additional bits are necessary for synchronization. Usually per frame one bit is used for the purpose of synchronization. So a special bit or a bit pattern is added in a control channel so this is used for synchronization. For example if this is the frame (Refer Slide Time: 30:48) at the beginning of each frame 1 is added then if it is the next frame then at the beginning of the next frame 0 is added so in this way alternately ones and zeroes are added for the purpose of synchronization. These bits are used for the purpose of synchronizing the frame.

18 (Refer Slide Time: 31:01) Essentially these synchronization bits state that this is the beginning of the frame and data from different slots are coming from different sources and they are demultiplexed at the receiving end. However, sometimes the data rate from a particular source does not match or is not multiplied at the rate at which the scanning is done while doing multiplexing. In such a case some additional bits are added which is known as pulse stuffing or bit padding both the terminologies are used to facilitate synchronization of different data rates. For example, you are sampling at the rate of say 8 Kbps so here the data is coming at the rate of 8 Kbps but here it is coming at the rate of 7.2 kbps then obviously these two cannot be synchronized. So what is done is additional bits are stuffed into this so that the data rate for this is 8 Kbps and then at the receiving end those dummy bits are taken out or separated out because it is known that the data from this source is coming at the rate of 7.2 Kbps so those extra dummy bits can be taken out. This is known as pulse stuffing or bit padding where signals from different data sources having different data rates can be combined by using Time Division Multiplexing.

19 (Refer Slide Time: 33:12) Here is an example of Synchronous Time Division Multiplexing. So here for example and here the transmitter data is coming from four sources say this is the source one (Refer Slide Time: 33:27), this is source two, this is source three and this is source four. And as you can see here source A has got four characters four characters, source B has got three characters, source C has two characters and source four has only one character to be sent. Therefore as you do the framing the first frame has got four characters coming from four different sources. So here we have put it as coming from source A, then you have put B in slot two corresponding to second frame then here you have put C coming from third source and D coming from fourth source. So this is how the first frame is created. Now as you go to the second frame the second A is put here in the first slot then the second B is placed in the second slot then second C is placed in the third slot and so on. Here as you can see there is no data so this slot goes empty (Refer Slide Time: 34:38). And when we go to the third frame the first two slots are filled up by the third characters from source one and source two respectively. So here in slot one from this third character we have A, and in the second slot the third character B but these two slots remain empty because there is no data. And at the receiving end however it can be received and they can be sent to different sources. As you can see this will go here, this B will go here, this C will go here this D will go here so there is no problem in multiplexing and demultiplexing. The problem is somewhere else. What we are observing in this particular case is that the data that is generated by framing has got some redundancy. That means if a particular source has no data to be sent that particular slot goes empty because each slot is dedicated for a particular source.

20 In this case as you can see in frame 1 one slot goes empty (Refer Slide Time: 36:00), in frame 2 two slots goes empty and in frame 3 three slots goes empty so this is essentially wastage of bandwidth because here as you can see the bandwidth of this medium is higher, this composite signal is sent through a medium of higher bandwidth so the data rate here is much higher than this data rate. so the transmission medium having higher bandwidth is used for sending multiplexed signal. However, in Synchronous Time Division Multiplexing as we find the bandwidth is not fully utilized and there is some wastage of bandwidth. Therefore we have to overcome the problem of the wastage of bandwidth. (Refer Slide Time: 37:15) This is the limitation of Synchronous Time Division Multiplexing. As I have seen many of the time slots in a frame may be wasted. This problem is overcome by using a new technique which is known as Statistical or Asynchronous or Intelligent Time Division Multiplexing. Actually there are three different names to refer to the same thing. So it is referring to Asynchronous Time Division Multiplexing but sometimes it is called Statistical Time Division Multiplexing or Intelligent Time Division Multiplexing. In this Asynchronous Time Division Multiplexing slots are allocated dynamically on demand. In the previous case as you have seen the slots are pre assigned dedicated to each channel but here it is not so. Depending on whether a particular channel has some data or not a slot is allocated dynamically. so no slot is assigned to a particular source. Any slot can be used by any source and it takes advantage of the fact that not all the attached devices may be transmitting all the time. For example, whenever we talk over telephone line all the time we are not speaking, sometimes we are listening and sometimes we are thinking so that silence period is wasted whenever we talk through telephone line. Hence that wastage can be overcome by using Statistical Time Division Multiplexing.

21 Let s see how it can be done. This is illustrated with the help of this simple example. (Refer Slide Time: 40:02) Here data is coming from four different sources A B C and D and here you have got the high speed multiplexer. Here obviously the data rate is four times that of the data rate of these inputs. Here there are different time slots so this is going to source A, this data is going to source B, (Refer Slide Time: 39:25) this is going to source C and this is going to source or channel D. Now as you can see during this time slot t 0 to t 1 only channel C and channel D has data and similarly during slot two channel B channel C and channel D has data, during slot three channel B and channel D has data, during slot four channel A and channel C has data so if we use Synchronous Time Division Multiplexing the framing will be done in this way. In frame 1 we shall have this data (Refer Slide Time: 40:16) this is A1 and C1, in frame 2 we shall have B1, C1 and D1, in frame 3 we shall have B2 and D2 as you can see and in frame four we shall have A2 and C3. And we can see these are the time slots which are wasted. Although we have large bandwidth from here to here the bandwidth is quite high we are not making use of it. Let s see what we can do in Synchronous Time Division Multiplexing. In Synchronous Time Division Multiplexing we reduce the bandwidth. So instead of four slots we have only two slots coming out from this multiplexer. Therefore in a frame we have got only two data. So in the first frame we are sending A1 and C1, in the second frame we are sending B1 and C2, in the third frame we are sending D1 and D2, in the fourth frame we are sending D2 and A2 and then in the fifth frame C3. So we see that wastage is much less and the remaining slots here these frames and slots can be used for sending data coming from the other time slots. Thus we are making much better use of the available bandwidth.

22 (Refer Slide Time: 42:02) So in this Asynchronous Time Division Multiplexing since the data arrives from different sources and are distributed to IO lines unpredictably address information is required. There is a problem in this. Although we are able to make use of the bandwidth in a more efficient manner or we can say in a different way with a transmission medium of lesser bandwidth we can send signals coming from different sources provided they don t generate data continuously. However, there is a problem. At the receiving end it is necessary to identify which data is coming from where. For example, at the receiving end this slot is not meant for data only from source A or source B so in this slot data can be sent by any one of the channels or data can be taken from any one of the channels. But at the receiving end how the receiver will know that data of a particular slot belongs to a particular channel? For that purpose you have to incorporate address information. So, for proper delivery at the receiving end it is necessary to have address information embedded as part of the data so there is an overhead. So apart from the data we are adding this information here and since it is an overhead we want to minimize it. If we are sending say one source per frame then we can do the framing in this manner like data and address which we can send in a particular frame. And whenever there is a frame coming from multiple sources we shall also require address information and length of data if we use variable length data coming from different sources for each of the sources. So for each of the sources we require address, length and data, that was not so in Synchronous Time Division Multiplexing. There the number of bits to be taken was fixed and the slot allocation was fixed but here it is not so. Hence this additional information that is needed to be sent through the transmission medium has to be minimized and to do

23 that sometimes we make use of relative addressing so that the number of bits required to specify the address is smaller wherever we use relative addressing. Sometimes we can use fixed length or length can also be specified in some special way so that the length field is smaller so that the efficiency of Asynchronous Time Division Multiplexing is more. (Refer Slide Time: 45:13) In Asynchronous Time Division Multiplexing the data rate at the output is less than the data rate at the inputs. We have seen that the data rate at the inputs is higher than the data rates at the output because the inputs are not always sending data. However, in peak periods the inputs may exceed capacity. Because the output bandwidth is smaller then the sum total of the input bandwidth so in peak periods there will be some kind of overflow and it will exceed the capacity. Therefore how can we overcome this? To overcome this we can use buffers of suitable size to store the data then they can be selected at later time slots. So some experimentation has been done by which one can decide what should be the buffer size for achieving efficiency. Let s assume n is the number of inputs, r is the data rate of each of the source, M is the effective capacity of the output and alpha is the mean fraction of time each input is transmitting. That means the sources are not transmitting all the time that is the important property that we are exploiting and obviously the alpha is less than 1 so it lies between 0 and 1. Then a measure of the compression, that means the bandwidth of the transmission media compared to the maximum bandwidth that is required that is the value of C and M is the effective capacity of the output and n into r is the maximum bandwidth because you have got n sources and r is the data rate of each source. Hence this factor is again less than 1 and it lies in the range alpha to 1 so C lies alpha to 1. So depending on the statistical behavior of the inputs the value of alpha and

24 value of C will depend that is the reason why it is called Statistical Time Division Multiplexing. We have discussed the Frequency Division Multiplexing and we have discussed Time Division Multiplexing. (Refer Slide Time: 47:57) Here we have another very important technique which is known as inverse multiplexing. This is opposite of the multiplexing technique. In the previous case what was done was the individual inputs of lesser bandwidth then we are combining to form a composite signal of higher bandwidth. Here it is opposite. Here we are receiving signal of higher bandwidth then it is divided into a number of channels of smaller bandwidth and at the other end opposite operation is done. in what situation it can be used. Let us see an application. Suppose you have to send voice which will typically require 64 Kbps then let us assume we have to send data which will require say 128 Kbps and video which will require say Mbps. Now the user can higher medium of transmission capacity mega bits per second then whenever it wants to send the video it will make fully make full use of the transmission bandwidth. However, whenever it is sending data that bandwidth is not utilized or whenever it is sending voice the transmission bandwidth is also not utilized. Let us consider the other alternative. The other alternative is that here you have got a number of channels of smaller bandwidth and these bandwidths are only 164 kilo bits and you have got a large number of 64 Kbps bandwidth available on demand so here we are making use of the property bandwidth on demand.

25 (Refer Slide Time: 50:31) So whenever we are sending voice only 164 Kbps bandwidth is demanded and one channel is assigned. Whenever we are sending data two channels are made available to send 2 Kbps and obviously we are doing some kind of demultiplexing where we are dividing this data into two separate channels and sending them and again at the other end we are combining them. Whenever we have to send video we will require a number of channels which are demanded and the video data is divided and sent through a number of channels and at the other end they are combined to get back the data. So here there is a more cost effective use of the bandwidth and the facilitator is providing you bandwidth on demand and as a result as and when the bandwidth is required that is being utilized that s why this technique is known as inverse multiplexing. Nowadays these kinds of facilities are available. We have discussed various multiplexing techniques. Now it is time to give you the review questions.

26 (Refer Slide Time: 52:38) 1) In what situation multiplexing is used? 2) Distinguish between the two basic multiplexing techniques? 3) Why guard bands are used in Frequency Division Multiplexing? 4) Why synchronization pulse is required in Time Division Multiplexing? 5) What limitation of Time Division Multiplexing is overcome in Asynchronous Time Division Multiplexing and how? 6) Design a Time Division Multiplexing system having output bandwidth of 128 Kbps to send data from four analog sources of 2 KHz bandwidth and 8 digital signals of 7200 bandwidth. Here we have to do pulse bit stuffing so that synchronization is possible. The answers for these questions will be given in the next lecture. Here are the answers to the questions of lecture-10.

27 (Refer Slide Time: 53:44) 1) Which modulation technique is used in optical communication? As we know On/Off Keying is used in optical communication. It is some kind of Amplitude Modulation technique. It is a special case of amplitude modulation or Amplitude Shift Keying ASK technique which is known as On/Off Keying which is used in optical communication. 2) What are the three modulation techniques possible in modems? We shall discuss about modems in detail later on. The three modulation techniques are Amplitude Shift Keying, Frequency Shift Keying and Phase Shift Keying. We have discussed it in detail in lecture minus 10.

28 (Refer Slide Time: 54:25) 3) Why Phase Shift Keying is preferred as the modulation technique in modems? In PSK scheme it is possible to send signal having more than one digital value and this approach is known as Quadrature PSK. That means here the baud rate is less than the data rate. As a consequence we can send more data through a transmission medium of smaller bandwidth that s why PSK is preferred. (Refer Slide Time: 54:31)

29 4) Out of the three digital to analog modulation techniques which one provides higher data rate? For a given transmission bandwidth higher data rate can be achieved in case of PSK. In other words in PSK higher channel capacity is achieved although the signaling rate is lower. That s the end of all the questions. We have discussed various multiplexing techniques. As I told the multiplexing techniques have wide spread applications in different areas. In the next two lectures we shall discuss about applications of multiplexing like telephone system and so on. Thank you.