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Circuit Switching and Packet Switching
Hello. So, we will be discussing on the course on Computer Networks and InternetProtocol. So, today we’ll be discussing on the topic of Circuit Switching and PacketSwitching, right. So, what we see when we look at a any network or any communicationnetwork it is a switch network. That means, if I have two nodes connected directly or twostation, if we consider the station, that source station and destination directly connectedthen they can communicate to each other directly. But however, if they are in the someother network then what I require? I need require to switch between different devicesright, network-enabled devices.It is true for our telecom connection also, when you are calling somebody it is someother exchange, then there should be some sort of a switching between these exchange.So, it will be first the first the phone my phone is directly connected to my homeexchange, then there can be other trunk connection and go and so on so forth, it go onswitching to the so long to the end, right. Similarly, for data network also, when we lookat when two station, source and destinations are apart then there they are equal numbermultiple hop which are being switched, right.So, what we try to look at is the preliminary of what are the switching technique,irrespective of what sort of network is there. It can be added a network, it can be atelecom network and type of things. So, what sort of switching techniques are there? So,that is our goal and will try to look more deep into the thing, when we actually see whenwe discuss about the protocols, right.(Refer Slide Time: 02:05)So, what we are discussing about is a switch network. So, communication betweendistance stations or end devices is typically done over network of switching nodes. So,the so, intermediate nodes are switching nodes which allows me to switch to the thingsor in other words what we say, we find a path between the source and destination, right.So, from the source to destination we get a path and that is that is that path need to beestablished or and the your packets or the data or need to be information need to betransmitted from the source station to the destination through this switch network.So, switching nodes do not concerned with the content of the data. The switching nodestypically not concerned about the data. The purpose is to provide a switching facility or apath between the source and the destination. So, they are not what data is being carried,right. Later on we will see that the different data has different type of necessities, right.So, a collection of node and connections forms a communication network. So, thiscollection of nodes and intermediate connections forms network. So, the nodes and theerrors or the connection between the nodes from the communication network that wealready we known.So, in a switch communication network the data entering the network from the sourcestation are routed to the destination via being switched from one node to another, right.So, that is when we natural that if I have two nodes far apart and intermediate set ofconnection and nodes, so the data need to be switched or hopped between one node toanother. So, couple of things one is that there can be multiple paths, right; there can beno path, right. I cannot able to switch or there can be a path at times while thecommunication is on, there can be one of the node fails or edge fails and there can becommunication problem right. So, what there can be the several issue; so to for this typeof switch network.Nevertheless, if we keep those challenges and issues little bit apart like will look at thosething that how still reliability whether it can be attained, if at all what are themechanisms, but what we try to see is that how these information or data from the sourcestation or destination can go from one hop to another.(Refer Slide Time: 04:45)Now, typically this is a example of scenario from the switch network I am - you can referthis data communication by Williams which I am also referring in this case. So, this is aswitch network if you say, right. So, there are there are end systems right end systems,there are there can be mainframe, server, personal computer and variety of things, therecan be mobile devices and so on so forth. So, if so for them this is a switching cloudright, or so to say, that some way if I am connected, I will be able to communicate withthe things, right. So, I am not bothered about this or a user is not bothered about thisswitching things, but if you look try to look a little deep into things, so there are differentcategory of switching nodes. So, if a packets come, if I want to communicate from B toE, so, B is this is the host or the connected switch or the connected node in case oftelephone we connect to our nearest telephone exchange and then it decides in order togo to D which path it should follow, right. It can follow this path, this, this, this, this itcan follow this path, sorry. Or there can be other paths, right this, this, here, here, hereand D right, B to D. So, there can be different path. So, this intermediate switch need todecide that which path there should be followed or there can be a predefined path. If Iwant to go from B to D I need to follow 1 4 5 3 D somebody defined it, right,nevertheless this route or path need to be decided or need to be established by this switchnetwork. So, this is the switching network typical switching network thing and there arethe external things which are going.(Refer Slide Time: 06:51)Now, there are what are the there are different switching technologies? So, switchingnodes may connect to other nodes or to some station. So, in this case what we mean tosay there are two category of nodes what we say these are switching nodes and these aresome other nodes right, these are stations. So, this is a terminology which are beingfollowed here, but you in some places you will find that though all are considered assource node, destination node, it doesn’t matter.A network is usually partially connected, like it is not that all are all routes areestablished. So, that it is the routes will be established on demand and type of things andhowever, there can be some redundant connections are desirable for reliability also, right.So, I can connect that network, like I can have only these path, only this path could Ihave been there for B to C, but there can be there can be redundant paths so that thereliability if 5 goes down in this case I could have gone through 1 2 3 D, if 6 4 goesdown also this path can be possible, right. So, there can be reliable path there can bemultiple paths, right.So, two predominant switching technologies are there: circuit switching, one is packetswitching, right. At as the name suggest or roughly suggest, circuit switching isestablishing a circuit or establishing a connectivity between source and destination this isthe predominant thing; whereas the packets switching says means that individual in thefrom the source station data can be divided into different packets which are beingtransmitted through this switch network to the destinations. We will try to see that boththe things quickly.(Refer Slide Time: 08:533)Now, circuit switching it says that there is a dedicated communication path between twostation. Usually there is a three phase things. So, for circuit switching I should have adedicated path from the source station to the destination right. So, how I how a dedicatedpath can be established? So, there are how this from communication can be goes on?There are three phases; first one is a path establishment phase, then next is the transfer ofdata or information form the source station to destination and the third one is thedisconnection phase.Or, in other words if you try to see this establishment phase is basically acquire theresources right, suppose I want to talk to somebody so, I basically some other part of theworld, so, I what I am trying to do is allocating the resources from my starting from mytelephone connection to my nearest exchange and there are different other intermediateswitching nodes so, I am allocating the resources. And interesting and then I can go oncommunication it can be bi-direct communication or full-duplex communication andonce the communication is over, then I teardown the connection or disconnect the thingright, in other sense I release the resources, right.So, if there are there are possibility of ten connections can be established, if when I amone is already in the communication path. So, one is blocked one is already occupiedother nine are available. So, there from there are another type of things come up that isblocking architecture and non-blocking architecture. So, if the connections if the all tenconnections are blocked then we then they are when may not be the eleventh connectionwill get a that “no connection available at this point of time” type of things; that means,it is a blocking things. Unblocking or nonblocking means I have enough resources tohandle, that we’ll look at this.So, circuit switching must have switching capacity and channel capacity to establishconnection, right. So, it should both have switching capability; that means, capacity; thatmeans, that number of paths are free and not only that, the data which is being carried bythe channel that should have also the capacity to carry the information or the data. So, itshould have both channel capacity and switching capability to establish connection; musthave enough intelligence to work out routing. So, in some of the cases or many of thecases it should have a typical intelligence to work out the routing. So, if a B wants cometo connect to D what should be the path intermediate through the switching node, thatneed to be find out. So, it can be, there can be some algorithm, routing algorithm, someway of intelligence which work out the routing, in some cases there can be dedicatedpath if B to D this is the path which is there, right. So, there can be dedicated path oralgorithm to look at it.So, these packets are pumped into the network and these go on the network andthey are delivered at the destination.So, there can there are also can two approaches; one is what we say datagram approachwhere packets moves on independent things, one packet may follow say route 1, onepacket may follow the route 2 and so on so forth. Nevertheless, there is the destinationand not only that, once this type of situation come you cannot guarantee that they willmay reach sequentially, because the one say path one or the route one may be morecongested then the path 2 so, the delay will be varying and so on so forth. Whereas thereis another approach called virtual circuit approach, where a virtual circuit is establishedbetween the source and destination and the packets moves in that particular path.So, these are typically in case of a packet switching network.So, like a typical circuit switching network is like these that from the like, which is this isas we know that circuit switching predominantly for that voice type of connectivity. So,the connectivity is like a telephone. If you consider a telephone then it is this first officeor this nearest exchange then a switch to that, if there is a call from this A to destinationB so, it goes on different switching circuit, circuit so like things are established. So, apath is established from source to destination. Now, this how this connectivity’s will beestablished by at the intermediate things is decided by some protocols or somealgorithms or some way intelligent techniques that it how things will be established.Once established the this is dedicated for this things. So, that nobody can basicallyintrude into this path in other sense this path is reserved or the resources are reserved atdifferent exchanges, at different with different type of what we say resource allocationsschemes, right.Now, if you just look at the space division multiplexing. So, or in the other sense there’swhat we say this sort of cross connect or cross points. So, 1 connects to 4 or 2 wants toconnects to 4 then I switch on this type of things. So, there is a controller or controlstation which basically tries to see that that which is connected. So we have this sort of amatrix type of things so anybody can connect to the any other resources.(Refer Slide Time: 16:38)So, it is a that was a single stage, it can be a multi stage thing where you have this sort ofswitches in different multi stage and in doing so, there can be optimization of this type ofspace division switching, otherwise if it is a whole matrix everybody is connected toeverybody by these things like here if you see there are 10 to 10 this side 10 nodes thisside 10 nodes. So, in ideally we could have a needed a 10 cross 10, or if this sort ofsituation was there. So, around means 10 cross 10 or 100 nodes right like 3 cross 4 herewe are having 12 nodes; whereas, here the number of points have much less. So that wecan have multi stage switching and then I can have a, we can established here establishedconnectivity into the things, but this still the then we have to looked at to whether it is abecoming a blocking architecture or nonblocking architecture, how many cansimultaneously talk each other and type of things. So those consideration comes intoplay. So there is a need of overall planning when we go for this type of switchingstrategies.(Refer Slide Time: 17:50)So in case of time division sorry there was a type of in case of time division multiplexingso what we have? There is so, one is that no switching. So, I send it BCDA and it isreceived by BCDA, right or there can be some sort of a switching, right. I want to sendfrom D the connectivity is that this has to go for to 4 has to communicating with 2 then Ican basically here change this packets sequence. So that there is a switching. So, BCDADCBA now become BADC based on that where the packets should go, right.So, these sort of things are possible there this is a time division multiplexing and insteadof these we can have TDM bus or a mix of the different things. Nevertheless for all thesepurposes what we have there is a resources location, a connection establishment isrequired, then the communication and then a connection teardown process required. So,these are these things are required when we look go for a this sort of circuit switching.The circuit switching on the other hand is extremely useful or extensively used for ourstandard telephone connections, right. All our exchanges etcetera are circuit primarilycircuit switched network means this our traditional telephone connectivities.
Circuit Switching and Packet Switching- Part 2
Like we are talking about time slot interchange, so that I can have a particular switchingthings which basically with a control unit based on the things we can interchange orselect, so this is this can we select sequentially control and this can be a selectivelycontrol that which data is coming in the thing. So, that is 1 2 3 4 whereas, I canselectively control from where the date has to come out. So, in other sense I have Isimulated switching techniques where things will be there.(Refer Slide Time: 20:06)So, if you look at the switching circuit switching properties and issues, first of all onceconnected, it is some sort of a transparent, right. So, once connected it is thecommunication goes on in a transparent mode, like what we do in our telephoneconnection once it is connected, I am talking about our traditional telephone connections.So, once it is connected over this exchange and trunks and type of things then it istransparent it is as dedicated line between the source and destination with appropriateresources, so that you can go on communicating with the things, right. So, this is adedicated connection.Secondly, what we have primarily developed for voice traffic. So, this was primarilydeveloped for voice traffic or our traditional phone connections, right. In some scenariosor in several scenarios it considered to be not efficient, right. Channel capacity dedicatedfor a duration of the connection, even a whole channel capacity which could have beenproperly multiplexed right, I could have done a frequency division multiplexing etceteraor other type of multiplex it is fully dedicated for the things right, or if even no data isthere, the channel is wasted, right. Even if you are not communicating holding the phoneor even not no data communication is not there, the channel capacity is wasted - no otherparty can use the things. So, it is a dedicated resource type of things.So other things other issues are there at times there are set up or connection takes time.So establishment phase takes time to have connectivity, because unless the connection isestablish you cannot start communication. The basic assumption or basic condition isthat there should be connection between the source and destination, a full connectionshould be established between the source and destination, unless it is established thenyou cannot do.Another things is the data rate is fixed. Both the ends must operate at the same data rate.As the as it is dedicated path and flowing the data flow is in a particular data rate, so,your source and destination should that it should be fixed and must operate at the samedate rate otherwise the sum there will be accumulation of the data all over flow of datarate is come will come to play which will adversely affect the connection. So, the datarate need to be fixed. So long as the voice things are concerned with fine right becausefirst of all voice as this own means, that communication has its own restriction with thedevices at the end, secondly, the human interaction can handled this data rate and type ofthings, right if there is at all need is there.(Refer Slide Time: 22:50)On the other hand, which is our primary interest for our data network is the packetswitching. So, basically what it tells it is typically small packets the data from the stationto station it typically source to destination typically broken down into small packettypically it is 1000 octets or one 1000 8 bits - 8 bit packet or type of packet. The longermessage split into series of packets. Each packet contains a portion of the data plus somecontrol information. Now here what is happening as we understand that it is as it is apacket and packets are moving independently, so the packet contains the data and somecontrol information which helps it in get routed to the destination, right. So, there is oneinformation.So, what is the control information primarily for? It’s basically for the routing oraddressing informations, where I am going type of things, or where to where I am it ismoving and there are other informations which are there in the things when weparticularly discuss about this packets then we’ll be discussing those things right like.So, packets are received stored briefly or what we say buffered and passed on to the nextnode. So, once say intermediate node receives a packet, it once a packet reaches aintermediate switching node, it receives it, store it and forward it. So, typically it is thatstore and forward mechanism receive it, store it and forward the packet to the nextdestination.So, there are advantage, first of all line efficiency - single node single node to node linkcan be shared by many aspects of the many packets over the time because now it is nodedicated path, I have small packets. So, the single node to node edge can be shared bydifferent packets. So, packet queued and transmitted as fast as possible, right. So, ateverywhere what we are doing, you are receiving, storing forwarding, so, it is queuing,queued and pass to the next destination or next hop as fast as possible.There is a there is a possibility or there is a what we say chance of data rate conversionright, each station connects to the local node at it is own speed, right. So, nodes bufferthe data, if require equalize rates. So, as there is a store at forward type of mechanism thenode has a option of synchronizing the speed, right. So, different communicationbetween the different edge and different speed of different node may be different. So, ittakes and basically make a rate conversion on the things, right.So, that is possible because this these are small packets and are handled independentlyand each other. Packets are accepted only even when the network is busy even when inthe intermediate network is busy, that the packets can be accepted and delivery may beslow, right. Which may be which will be difficult in case of a circuit switch things if thenetwork is not there it is basically a dedicated circuit we cannot do anything on thethings. Now, in case of a datagram each packet is treated independently. Packets can take anypractical route. So, if there are ‘n’ number of routes or say there are 4 routes, 10 packetsthis anything can. Packets may arrive out of order, it should not arrive, it not necessarilyarrive in sequence, packet may get lost or delayed. So, once some packet may getdropped or delayed may not reach the things on time or even may not reach at all.So, there can be other mechanism to look at it, but it there this is possibilities are there.Up to the receiver to reorder packet and recover from the missing packets. So, thereceiver only sequence it, and if there is a missing packet, receiver can take appropriatemechanism, like receive can request for retransmission of the packet and so on so forth.So, other technique for packet switching is the virtual circuit. So, preplanned routeestablishment before any packet is same. So there is some sort of a route establishment isthere. Call request and call accept packet establish connection or hand shaking. So, thereare there are request and call accept packets and type of things. Each packet contains avirtual circuit identifier or VCI instead of the destination address, right. So, it is moredecided instead of destination address it goes on locally like if I want to transmit herewhen next thing I have to deliver, which in turn things will be there right, but my routingis now somewhat table based, right. I at every node, I say that if I received packet theseVCI received from port number received at port number 1 will go to that with portnumber so on so forth.So, no routing decision required for each packet once the packet is established, right. Soyou do not have to individually you don’t have to take the routing decision. So, it followsthe things. So, it can be at times more efficient. Clear request to drop circuit. So, if thereis a if the once the communication is over, circuit may have to drop. So, there can be asome sort of a circuit means tear of request and it may not be truly dedicated path. So thecircuit once they are if there is disruption or some other reason there can be anothercircuits can be established. So, it is not necessarily truly dedicated path not only that thepaths can be shared by other virtual circuit also, right.So I can have some sort of a multiplexing on the path, like here same thing what we areseeing at a thing is established and then it goes on communicating in this particularsequence right, in case of a virtual circuit.So, if you try to look at the packet switching virtual circuit by the datagram. Virtualcircuit network can provide sequencing and error control, as they are having a establishpath. Packets are forwarded more quickly, no routing decision is required forindividually at the packet level. Less reliable, loss of node loses as the all the circuits tothat particular node and because a particular node can have more than one circuit whichis established, and if that particular node is faulty or not working, then all the circuits tothe node is lost it goes for not only reliability, it also have extra cost of reestablishing thecountry circuit and transmitting the things.Whereas datagram one has these has no call setup. So, each packet moves independently.Better if the number of packets is less, if huge volume of packets are there then there arelot of calls there. So, if we try to now look at circuit switched and packet switched network or packetswitching network, right; so in case of circuit switch, bandwidth is guaranteed right. So,that you have a establish connectivity and your data flow through that so, the bandwidthis guaranteed; whereas, in case of a packet switch, bandwidth dynamically allocated asneeded as need basis right, or as and when needed type of things. So, that is that is beingallocated as there is a need, right. So, it is not guaranteed bandwidth. It may so happenthat the bandwidth availability is not there or when you need because you are notpreoccupying the resources. So, while communication you may face problem ofdropping of packets etcetera lose of packets.Circuit capacity not reduced by other network traffic. So it is not, as it is a dedicatedcircuit the circuit capacity once allocated or once established, are dedicated for thissource and destination. Whereas, may have concurrent transmission over physicalchannels, right. In case of a packet switches, may have concurrent transmission overphysical channel so one physical channel there can be number of things.Circuit costs independent of the amount of data transmitted right, resulting in wastage ofbandwidth. So, whether I if I established a connectivity whether I transmit or not it is adedicated connection. So, I pay for it, right. So, in other sense, there can be effectivelythere is a waste of bandwidth whereas, there is more effective and better performancebecause there is no such sort of a blocking the channel even if you not communicating,but they may have some delay and congestion because of its typical characteristics. Andat we see that this type of circuit switch network is more amicable for voicecommunication, right. So, it is more amicable for voice communication where as thissorry this circuit switch network and where as the packet switch network are moreamicable for data communication or how data is communicated between the things. Sofor our computing this particular course for computer network and internet protocols; wewill be primarily looking at this sort of a packet switched network, right where this nodesor the packets are independently moving from the source to destination and in number ofcases this is a unreliable, best effort service, it depends on how things are there.If you want to have a reliability on the things, you should have a different mechanism ofthe things. That we will gradually study on in this particular course when we go on moredeep into the things and specifically look at different protocols, their characteristics andtype of things. So, what the primary objective of this particular lecture is to that to brushup that already most of you may be knowing that packet switch and circuit switchednetwork. There are several other things we mentioned here. So, we have a overview ofthe things, but we primarily now look at that packet switched based things what we go onsubsequent things, right.So with this, let us stop today and we will continue our discussion in subsequent lecture.Thank you.
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