 So, welcome to the lecture series on distal switching systems, this is the first course so I will be introducing what we are going to cover in the whole course and then how I will be actually proceeding. So, basically the idea is what we understand from distal switching is, it has to do something with the switching, switching basically is an action where either there is going to be a path that is I think the most basic way of understanding most of the people understand only this way that usually they will use a switch to for example, switch on their lamp and they will usually will be a power source and they will switch on or off. So, this is what basically is the switch, but we need to talk about distal switch and basically switching functionality is what is more important how it is implemented is a different issue. So, what we are talking about is not switching in the sense of electricity, but switching in sense sense of communication. So, usually what will happen is how y y we required actually a switching let us come to that. So, essentially it is a communication requirement. So, usually what will people will do for communication there will be a transmitter. So, I am putting up a telephone here and telephone on the other end and user 1 wants to talk to user 2 they can actually lay a line and they can start working on this. So, this is a point to point communication link which gets created between them and they will communicate. So, they can just lift the handset they need to have some power source or to actually energize both the phones and that is all the communication happens whenever they want, but there are only 2 users, but this kind of communication system will actually not work. So, usually what we desire is we are going to have instead of this we would like to communicate not with 1 percent, but everybody whom I know. So, if I actually know a lot of people. So, in general they can be 1 2 3 4 N N. So, if I still want to create same kind of communication mechanism is a basically long distance telecommunication thing then what I need I need to lay my wires all the way to every user which I know, but this is true only for 1 same is going to be true for node 2 same is going to be true for node 3 and same is going to be true for node 4 and so on. So, usually what you will require is N into N minus 1 this is which is by 2 which is N C 2 bi directional links bi directional actually means where the information or voice can flow in both the directions. Now, when such a scenario actually happens the number of wires which has which are required you can actually observe is going to have a complexity of O N square. Now, this is not acceptable because as my number of users in the communication networks are increasing number of links which are required are going to be 2 I and now what will be the utility of basically of each link if we take a case where by only a user talks to only one another user it is a 1 to 1 communication in general total N by 2 communications actually can happen at any point of time. So, what you will figure out is that there only N square communications happening and I have invested money on N into N minus 1 by 2 links. So, N by 2 basically is the number of communication. So, utilization is basically number of links which are utilized divide by total number of links. So, I will be using N by 2 links and I have total number of links which is these many. So, this will be the maximum utilization factor which you can actually have in such a scenario and once you solve this will turn out to be 1 in divide by N minus 1. So, this is the utility when everybody every user is talking all the time it is that kind of situation. Now, when this happens what you will find that as my N grows in limit when N goes to infinity when number of users become very large my utilization will go to 0. Now, this is not acceptable because each cable which you are going to lay will cost you something and its utilization is going to be low. So, the cost per unit of call is going to be very high, but if you carefully observe the cost is because you are laying so many cables and a user using only one of them at any point of time. Now, this is a full mesh network actually which I have created a switching is happening here it is done by users. So, if as a user I would like to talk to say 1 wants to talk to 2 he will be now switching off basically there will be some person sitting in here which will be having a handset and a microphone they have to be connected to on to this particular line. So, other lines will not be used and tomorrow after some time for example, if he decides that I am not going to talk to node 2, but I want to talk to 3 he will stop he will take out this handset and microphone from line 1 and then will connect to line 2. So, he is doing switching in that sense actually. So, this all switching functionalities are happening at the user end not in the network network is purely passive end sense of switching actually. Now, question is my utilization also becomes almost 0 as n grows very large. So, my cost becomes very probability actually. So, this kind of communication systems cannot be built. So, only kings or top bureaucrats can actually afford a hotline running all the time dedicatedly for them. They are nothing, but dedicated hot lines being created between all possible pairs of users in the whole community. So, that is what actually is happening. So, there will be a better situation I can create I probably can ask what I can do is a better option is let me create a central switch. Now, there where the actual switch functionality will come and let every user gets connected to this. So, I am connecting all users to this thing. Now, how many links I need to actually lay for n users in this case? It is not going to be n c 2 what I require is only n links. Every problem is I need a mechanism whereby if one wants to talk to two he should be able to tell this box and it should be able to create a path between one and two. Next time for example, if this guy one decides to talk to somebody else say three. So, a path can be created in this fashion. So, I am now creating switching functionality is being implemented by this box and that is what we mean by a switch. And when we represent of my voice signals or data whatever it is being transported between users in digital formats I will call it a digital switching system. Now, in general in common sense term this is also known as exchange telephone exchange is what the word which lot of people are aware of. So, commonly it is not called as a switch is a technical term, but telephone exchange is what people commonly know of this particular system. So, this basically exchange is nothing but a building whereby you put all these kind of switches there with all power supply everything all users get connected. Now, if all users here will talk all the time for all 24 hours then what will happen? What you will find is that you will have for n by 2 possible communications n links will be consumed and there are only n links. So, utilization will be n by n which will be going to 1 in limit when n goes to infinity for that matter for whatever be the value of n it will always be going to be 1 inverse case scenario. Utilization is not 0 it is independent of n now that is a very good thing once it is independent of n. So, my cost is going to be very very low of course you can see I am not using a number of links is not n into n minus 1 which is o n square, but it is o n now. So, this system will work wonderfully well we have to still figure out how this switch will get implemented because now important thing is that the node has to talk to this switch this switch has to understand the logic or whatever link or whatever destination which one would like to connect to. In earlier case one simply decides that I am going to connect to 2 or I am going to connect to 1 and as per that it makes a decision. So, there is some intelligence which need to set in this box. Now there is another problem actually we have been assuming that all links are being used 24 hours almost all the time somebody is busy on the equipment he is talking. So, I am taking example of voice communication, but this is true even for data. Now if you are now going to work talk only for say half an hour a day. So, remaining 23 and half hours the line is empty and especially if for example line is very long say for example I am in in the capital city of Delhi we have a central exchange or central switch and all users all across India are connected to that place and I have got dedicated wires. So, think of number of wires which you have to lay is going to be huge and the cost on each wire is going to be also quite large because is about 450 kilometers from here from Kanpur if I have to lay from Kanpur 450 kilometers from Bombay 30 and so on. So, this is going to be a costly affair and you are going to use only for 1 by 24, 1 by 48 hours for my 48 of the day only that much fraction of the day you will be using it. So, can I actually take care of even this. So, idea came what we can do is we can create a smaller switch within a city and let all users gets connected to this and when they want communicate I have to ensure that this length is as small as possible. So, that my cost of this digging because the utilization of these lines are very small they are connected directly to the users. So, their cost has to be as low as possible. So, length has to be kept a small now I can create in another city another exchange of similar kind and I can connect the users there. Now, I can connect few lines in between these two cities. So, what will happen is these are known as trunk lines and I keep them in few numbers not a very large number. So, I did not actually lay a very thick cable and what will happen is whenever call this even if this guys are talking only for half an hour a day I can keep these lines busy for because everybody is not talking all the time. So, people will talk at different times of the day. So, I can probably make the utilization of these lines to be very very high I can make them to be say busy for say 14 hours in a day or 16 hours in a day kind of thing. So, the per unit call the cost is going to be lower for a trunk line in this case. So, now the thumb rule in any network design which you can actually perceive from here is that in any network whichever is a costly link your utilization factor has to be high as high as possible. So, usually we keep them about 0.8 to 0.9 roughly in between that range because this number of calls which are being made simultaneously will they will they keep on fluctuating. Sometimes there will be too many people calling sometimes less, but the average you would like to keep around about 0.85 or 0.9 roughly in between that and remaining is for taking care of if suddenly the large burst of calls will come in for that period, but that actually reduces the cost. So, wherever the cost is high reduce the length to reduce the cost. So, here utilization is low cost is high and henceforth we keep the length less. Now, this actually creates kind of a network in general the most of the telecommunication networks or data networks are created in such fashion and these kind of things where users are connected this particular portion is known as access network actually. This is access network and of course then you will have this is also going to be access network the part which is there in between this is known as trunk network trunk or core that is a word which is been used depending on whether it is a voice or whether it is a data. I will actually then again we will basically merge everything into data network ultimately. Now, in between it is also possible that I might connect to another exchange something like this. So, there are no users connected to this one, but it is permitting the calls to go through it is using like an interconnect we call it a transit exchange actually. So, now we at least understand that what is a switch functionality? Switch functionality is in this box and we need to understand how this gets designed actually how we can implement this. So, in earlier days how we used to do it. So, in earlier days this we used to have something called manual telephony I think this is not being used currently not being practiced anymore most of them are now automatic fully automatic exchanges. In fact now we are moving over to voice over IP based systems instead of manual exchanges. So, manual exchanges were typically was very simple these are all analog there is no distal thing here a simple voice is being just captured and is being transmitted. So, actually in this case a person. So, I have given these telephone instruments at the various houses they are connected to an exchange subscriber exchange we call it or a access network. Then there is a backbone or trunk there can be other exchanges now each exchange there will be a person sitting there a human being. So, earlier days this is what how the telephony actually started automatic exchange came much later actually. So, a simple thing is the protocol which can be followed here is you lift up your handset and that handset once you lift there will be a board here sitting and somebody will be sitting there are pins or jacks which are connecting to your wires for all users and for making a connection you have to create a jumper wire or you have to put in take a wire and put your pins into these slots and which will make the connection. So, that is how it can be done and usually there will be display here depending on whether you have lifted the handset or not and there will be users there will be an operator which will be operating this particular thing. So, whenever you will lift the handset the circuit will get completed and a light will glow here. So, once the light glows operator will see the light and of course, if operator is not there nobody is going to put through your call. So, once he sees he or she actually sees the light he has to put in an earphone which is maintained by this person along with a microphone. So, this guy will then actually put his jumper into your port. So, that now he or she can listen to you and you can and she can also talk to you. Now, once she greets you this light is being glowing and she knows that you have she will talk to you and she will ask you after greeting that which particular number you would like to get connected to and you will tell the number. Now, remember whatever steps I am telling are exactly followed even in current day telephony. So, this basically if gives a framework of how the protocol is going to be built and once you tell the number this lady will probably will have a music generator or some something. So, that music generator will be connected to you and this particular thing will be taken off and she will note down the number on which you would like to make the call and she will search the directory that on which particular port that number is connected. If that number is not in the same exchange it is in the next exchange she has to then talk to the person who is there in the next exchange same protocol and then she will tell that this is the number which her subscriber need to be connected to. So, in this case I am actually assuming that the other subscriber is in the same exchange. So, she will pick up the other person she will find out there is somebody there is already there on that exchange. So, there will be another box. So, meanwhile remember the music is being played back to the this guy. So, this is like search tone which we actually see. So, this music being played to this guy for the next person once you know that this is the person whom the call has to be who is that the other person whom the call has to be made. She will use a another what we call ringing generator. So, this ringing current has to be applied from here on to that particular jacket this is the restriction ringing current will be applied here which in turn cause the phone here to ring. Ringing current usually is a not in the audio band this is actually the current power which is being modulated which will cause the ringing of the phone. So, phone will start ringing at this end and once the user comes here and he will says there is a current he will lift up the handset the moment he lift up the handsets the light will glow here corresponding to that person and once the light glows the operator has to now take this particular ringing current out. So, that now there is no more ringing here and then she will put her own jacket in this case. So, this particular thing will be taken off. So, her own jacket will be put now she can talk to the destination she will tell that this is the guy who want to make a call to you would you like to take up the call. And once this person agrees that he would like to take up the call she will actually have a record register we call it a call detail record nowadays CDR and the CDR she will make an entry that who was the person who made the who initiated the call who is the person to whom the call has to be made at what time the call was made and then there is a call closing time which she will actually enter at the end of the call. And what she will do is one of the both people agree she will just put a jumper wire she will take out her own jumper and then she will connect a jumper wire between the two people and once the call is through this light will remain both glowed up because their handsets are lifted because this is being driven by the DC current circuit which is gets completed when you lift the handset and they will talk they will keep on talking and once they are talk is over any one of them can push back the handset once that handset is put back this lady will actually observe the light is off and that point of time she will note down the time when the call was off call was actually completed and once the one of the light is off she will take out all the jumpers and these guy will simply put their handsets back on the cradle and the call will get completed now this is exactly still nowadays the phone call does get completed in the same fashion except now complication will come if there is going to be another operator sitting here another console board which is sitting so if the call has to be through a trunk route so it usually takes more time because the user will tell the operator operator will take the next operator and this operator will also keep this operator on hold and then and so on and each operator will keep their own CDRs at their end and as per that they will start doing mutual charging from each other so based on this essentially then the bill has to be generated at the end of it now this is a very cumbersome process but this how the telephone actually started it became popular but remember the human cost involved is pretty high and that chances of mistake there is a chances of making a mistake in call recording and then as for generating the bills now this was the most elementary switch which we can talk about this one is manual switch okay or manual exchanges in a still defense I think some places if everything else fails this will always work kind of thing so this is still used as a backup especially in defense organizations but I think nowadays most of them have moved on to packet switch based systems now another important thing which I would like to tell here is that what I am talking about is a circuit switch okay so this circuit switch technically means I am creating a path from source to destination as it is intact throughout and once a path has been created the information or voice just simply flows okay so currently as of now it is circuit switching I will tell at some point of time where we will move over to packet switching later on because once we complete all the theory of even the circuit switching then it will become much more clear I still distillation has not started in this case it is still analog now this manual telephony is a problem because operator can always listen to what you are talking to other person so you have to either talk in code words if you are talking to that person if you want to maintain secrecy otherwise in form can leak so there was a b Stouger actually I think was the person who realized this and he started working on automatic exchanges he said I think this manual operators need to be removed and we should have fully automated system and he learned it of with something called Stouger exchanges and he it was purely an electromechanical system and it was built with very very simple devices so one of the elementary thing for example it simply uses a ratchet wheel so a B Stouger actually built up an automatic exchange which was pure electromechanical thing and basically it uses electromagnets for creating mechanical motion and thus creating contacts between electrical contacts between the input and IO ports input and output ports and thus creating switch basically voice paths so it uses basically two kind of elements one is uniselector and in fact there is a whole lot of logic which got embedded using relays actually for implementing this kind of system so or flip-flops which are actually used but the basic switching elements was only uniselector and two motion selector so I am just showing that how it would have been looked like so unimotion selector usually will consist of a hinge which will be like this and this is the hinge this can rotate around this part so there is a spring connected to this and there is a hook which is there this is again connected to the spring and there is a hard surface so it is always trying to pull this thing upward and this is being tried to pull towards this side ok and then there is a electromagnet which is here and whenever you actually supply current to this so this will actually come down and then it will go up once the pulse is over so with every pulse there is a motion of this particular bar coming down and going back up and as a result and there is a ratchet wheel which is being used on this side so every time this motion this pulse happens this wheel actually moves by one one teeth in this particular direction because when it comes down it will engage with the next teeth because of these two springs and it goes back in the return this wheel is going to rotate and this can be used to create now the central there is a current which is coming and this is a wiper electro this is going to make electrical contact and I can actually have on this circle many points where every time it rotates it keeps on moving here so I need to have mechanism to take it back to the 0 position so usually it will count from 1 2 3 n till 0 and then it will come back to and home situation when there is no connection so these are like outgoing ports so usually it will be like one input it is a switch of this kind now it will have 1 2 0 the total 10 outgoing ports and this can rotate and connect to any one of them so this was the unit selector which was actually used and this wheel actually should not go back when it is this electromagnetic is working there is a always a stint which is put so this is a very simple drawing and this what was the unit selector which was invented by him and he built up what is known as trouser exchanges and this is a reverse drive type of ratchet wheel we call it reverse drive because when the pulse is switched off that time there is a reverse thing when it is going back at that time the ratchet wheel will move reverse drive type ratchet wheel is mechanism there are other mechanisms which also people have built and this is what is known as a ratchet wheel actually what you are seeing here so this was the one element which was built and secondly what he built was a unit selector that is what he designed so the other one was actually two motion selector it required two electromagnets to actually work so one was a straight bar which is going to have gears on this so as this bar moves and then of course this is also a gear drive so this can move upward and downward this vertical motion is happens because of another electromagnetic depending on the pulse and this motion happens because of this bar so it will turn into rotation of this thing and I can then connect a wiper and these are the connection points in one plane similarly in another plane and so on so you can actually usually have this actually means this will move into these are like ten points being there in the one plane and there will be ten such planes so total 1900 actually outgoing points and one incoming point which will be there this was used to work in two dimensions using two electromagnets basically you require pulses for driving this kind of thing so if this will work from 00 technically yeah I call it 11200 that is a way because one is always what is start 0 actually means 10 pulses and of course there are many control circuits which are built using relays and these are integrated with these kind of things okay he also now started something what we call a concept of common control switches with this in this case the control circuitry usually is going to be common which is going to be then used for different unit selector and two motion selectors at different point of time depending on requirement it's not all the time so common control switches are were actually slightly complex but the amount of hardware required is going to be small now remember all these are mechanical components their failure is going to be pretty high and as a result their mentors cost is also very high so using less amount of hardware always make sense also direct control switches are also built where the control circuitry was directly integrated part of the unit selector or two motion selector but they were actually having a problem because of the lot of failures which were happening in the exchange and of course here also for the first time now the different kind of circuits came up now which we can I can just actually mention what were the circuits which were built of course with voice over IP coming in most of this will not be required so various circuits which were built were like this so one was a guarding circuit so in this case what happens that whenever you are actually trying to set up a call you actually use a one particular unit selector and call is still not through but you have made the connection so at that point of time you have to make you have to actually set the status of that unit selector to be busy so that nobody no other call should actually try to again try to capture it and use it for some other call so only when it is freed up then only it should be used so it's basically maintaining the status of each unit selector into motion selector so it was basically purpose was for that purpose second was the impulsing circuit so idea here is again remember the electromagnets are being used to drive both the systems so whenever you have to now move this particular arm on to any one of the connection points you need to give certain amount of power so when you are giving a signal the power is going to be small it's only signaling pulses and you have to now convert it to a drive current and we call it a impulsing circuit which will generate this much of power or drive current so that these electromagnets can move these arms to appropriate point so basically it's like a power amplifier then similarly we have a homing circuit which was built which is required so once you have made a call and call is over this actually should come back to its home position all either unit selector or a two motion selector both have to come to their one one position or there is a home position before one one where it they should come and then again you can give the pulses depending on which number you have to connect you want to connect the incoming port to which particular output port you want to connect to basically based on that you will give the number of pulses so that for putting all these things back to home position after a call is over we require a homing circuit. Now this is another innovation in the manual telephony the lady was actually writing on a register the source destination the call start time and call end time so that the bills can be generated here the bills were generated using a metering system so usually the things which were the way it was used is we were having kind of a pulsing system so may be say every six second there is going to be one clock pulse which will be there or every second there is a clock pulse and whenever the call is through at that point of time this timing the clock thing is also fed to a counter and there is one counter for every user actually sitting in the exchange so this counters are nothing but kind of dials so like meters so whenever call is through this meter will start counting and based on total count in the whole month the bill actually can be generated and if there is a long distance call you might probably will connect a faster clock to the meter if the short call short distance call you will be connecting a slower clock to the meter actually so that was the metering circuit then there was a ring trip these were conventionally used now so ring trip circuit is whenever you are actually applying a ringing current the bar is ringing of the at the for the destination user and it is a powerful current which can actually transfer power to the bell so that the sound can be very large and people can listen that there is a call coming in the moment he lifts the handset this information will come back in form of a electric circuit getting completed back to the exchange and that time this ring trip circuit will actually get activated and the ring generator ring tone generator will be disconnected from the subscriber earlier it was the operator which was doing it now it has to be done through this particular circuit and then of course alarm circuits were built this was for administration purposes so for certain kind of faults the alarm will start beeping and then at least the maintenance guys will figure out that there is a flaw in the system they can come and rectify it so this actually improves the maintenance this kind of system were not very very they actually have been in use for a long time but there was lot of this mechanical problem that oxidation of the contacts and then of course is a mechanical thing maintenance is going to be pretty tricky people have to be well trained for doing the maintenance thing then this need to be improved so ultimately again the invention being made to take care of this particular thing and people came up with an idea of something in the capsule and saying totally doing away with electro mechanical thing so there is no motion there is no pulse there are no gears so they will not gears will not get damaged over time and you did not replace them you did not do this all kind of machine maintenance will not be required so ultimately idea came they let there be a glass bulb which is sealed and I can put an inert gas inside so there is no oxidation problem also so connection quality will also be good and I can now actually have two simple wires and whenever they actually just snap they will connect to each other the connection is on otherwise it is off but how to control it so then you can put the electromagnets similarly you can put an electromagnet here in fact I have done it incorrectly I will just rectify the figure so this particular part will be in the glass this particular thing so this outer electromagnet will be outside the actually glass bulb so whenever you will actually put energize this electromagnet as well as this electromagnet so this will be pulled downward this will be pulled upward and there will be going to be a connection which will be snapped in and the moment any one of the electromagnet is switched off the connection will be broken and voice communication cannot take place and we call this thing as a cross point this is a cross point this is a electromagnetic thing still and this cross point now can be used to build up the basically the switching module and this particular switching module which was built is known as cross bar so how you can actually make the connections with this that is a question so very simple thing is for example there are four users which are connecting in a four users which are connected so technically I have to I am now splitting these are not bidirectional lines these are unidirectional lines so signal comes in signal goes out both are for user one so I will connect them together and I will use something called a deplexer so it converts bidirectional into unit to unidirectional things so one is where the signal enters one where signal goes out so this will be I will doing for all other users so I can call them one prime two prime three prime four prime and one two three four and now I can connect them and that is why we call it a cross bar because there are many bars they have been crossed with each other and at this point between these two elements I am going to put this cross point which I have talked about this is a cross point so n by n switch so n users connected to n users I require n square cross points now that is the only problem here this can be reduced by using only upper triangle or lower triangle because I can connect one to one through this this of course will never happen but one to two connection can be made through this cross point as well as from this cross point and if it is a unidirectional I require both the cross points to be operated and if it is bidirectional for example this link can be bidirectional because this is nothing but mechanically I am connecting the wires signal triven in both ways so I only need to operate only one of these two so it all depends on this so most of the time these electromagnetic systems are going to be electromagnetic systems are going to be nothing but bidirectional elements input IO ports so I need not use this lower half I need to only use so one to one connection I will never make because that is not required because the same user so I will be only using these elements first building up the complete switch this is known as upper triangle actually so this actually means n into n minus 1 by 2 cross points will be required in this case n is 4 so 4 into 3 divide by 2 which is 6 and you can see I require 6 cross points here to build up the cross bar if all ports are bidirectional ports now question is how to operate it how to set up the paths that is an issue so I can stop now and then probably we can discuss how the algorithm is going to be worked out for operation of this particular switch so that we will see in the next