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Introduction to Computer NetworksHello. So, welcome to this course on Computer Networks and Internet Protocols. So, thiscourse we’ll be taking, I will be taking jointly with Dr. Sandip Chakraborty of ComputerScience and Engineering of, of IIT Kharagpur. So, as the name suggest, we will beprimarily looking around all sorts of aspects encompassing Computer Network andInternet Protocols, right.So, though all of us are somewhat means rather, everybody is accustomed with usingcomputer network in some form or other and it has become a part and parcel of our life,right, anywhere any disruption in the computer network is as a disruption of power orwater supply like that, right everything moves around the networks. And with the moreE-enabled services like starting from banking to E-marketing to any aspects we talkabout is so, these networks become all-pervasive and we need to look into this, meansworking principles of the network.So, this course primarily aims at looking into that backbone or background or the, whatthe, what activities go out the back of the network which helps us in having a, this wholeinter-networks, right.(Refer Slide Time: 01:50)So, if we try to look at that objective of this course, so, we try to understand how twocomputers in a network talks to each other, right like, if I say if I type “www” or if youtype “www iitkgp ac dot in”, so, what are the activities goes on at the background thatthe page get displayed, right. Or if I do a, if I send a mail, what it goes on type of things,right, we need to understand.Go through the, we like to go through the basic functionalities of the computer networks,what are the various component.Learn about how to write my own application programs or own programs in a networkand also try to look at the other aspects, like what is, what is in future, what we arelooking for, given our different aspects of, with our needs and different proliferation ofseveral network enabled applications. So, what type of designs or what are the thingswhat we need to look at. So what sorts the, what in other sense, what is the future of thiscomputer network, we set to open up.This also may help you, in some of you who are looking for some of the researchactivities around this, may help you in finding out some research directions.So, so what we look at, we have a, in a broadly, we have a set of functionalities one sideand set of protocols, right. We will discuss about both of them in details in thesubsequent lectures. But protocols, what do you mean by protocols? That’s a, in very flatterms or layman terms, it is a set of rules which allows me to execute something, right.So, if I want to do something, so this is the protocol I want to follow. So, network alsohave a set of protocols. And using this protocol, I want to achieve some functionalities,right, like I want to transfer a file given my underlying network. I want to, this, thisparticular lecture is broad-casted or multi-casted across different sender using theseunderlying networks, right.So, there are functionalities I want to achieve from this network, right. The network assuch the architecture of the network should be supportive for the things, right. Thoughthere are general, general architecture backbone, I may look into some specialized thingwhen I do some - (Refer Time: 04:31) like multimedia transmission may have little bitdifferent requirement from say general texting machine or email services or internetservices, right. Anyway, we will, we will be looking at those different aspects when wego through the course. And in between to, what this tie up these functionalities and theseprotocols is basically the network architecture. This is a very very broad view of thethings. We want to look into deep into the aspects.(Refer Slide Time: 05:01)Now, so obviously, comes that what is this network architecture?So, a way to visualize, how two remote computers talk to each other, right. So, it is givesme a way to visualize that how that underlying things will be there. I require some sort ofa protocol stack to handle that, right. So, even if I look at our, even not in case of thenetwork, forget about the network, if I want to communicate with somebody else at theother things or some other party, I need to follow some, I need to follow some set ofprotocols. When I, when I am delivering lectures I am following some protocols and typeof things and there is a way of looking at it, right.So, these protocols or the network protocol stack and having a underlying technology ofthe things provides me a way to communicate between two any systems - whether it is inmy same network or whether it is in the different network, right. One may be here IITKharagpur; one may be somewhere else in the world.(Refer Slide Time: 06:14)So, so, our basic objective is that two systems talking to each other, right, these twosystems can be wired connections, directly connections, we will see that whether it ispossible or not and where these two systems can be somewhere at the different things.So, the basic vanilla thing is that I have two systems and somewhere I, I connect a wirebetween these two network interfaces, like there may be some, some way of connectingthem and then these two systems talks to each other, right. This can be two directsystems.(Refer Slide Time: 06:49)Or so, what we have for these, we require a need to convert this digital data to analogsignal and vice versa, right.So, when we connect by a wire, this is a physical connection, right. This is physicallyconnected between the two systems. So, what I require? I require a physical wire and asthe signal transmitted by the typical wire, say, say even if you consider our, previouslywe used to communicate through telephone lines and type of things or still we do insome cases.So, what is there, it basically converts this digital data of these generated by thesecomputer systems to analog signal which is carried out by this wire, at the other end it isreconverted to digital data and placed it to the systems. So, I have two applications attwo systems. They are generating some data. Underlying network interface convertingthat to the analog signal, being carried out things etcetera. This basic communicationpath which allows things to communicate is the physical layer.So, whatever may be the way of protocol etcetera followed at the high levels, I need tohave some physical connection. It can be wired, it can be wireless, right, it can be cable,it can be fiber, it can be normal, it can be Bluetooth, it can be Wi-Fi etcetera, but I requirea physical connection, right. There should be a connectivity which carries the signal fromone part to another, right.(Refer Slide Time: 08:27)And then, if I have more than one, like only not two party, more than more than twoparties are communicating with each other, like three systems are there, then I cannotconnect this wire like this.So, I require a somewhere, concentrator, right or somewhat we say in our terms, aconcept of hub or switch in between, where it has a multi-port things, I can connect tothe things and then I communicate with the other three systems, can communicate. Theproperty of this device is allows it to communicate to one other.So, one way of looking then, is that they when everyone talking to other, there is a thereis a collision between the things. So, what we say they are in the same collision domain,right. They can - they are in the same collision domain in the same physical network andor they one communicating others can listen, or everybody can listen. So, they are in thesame broadcast domain.So, this sort of things at a vanilla things what we say that, there is a say I can think thereis a concentrator of these wires, where we - where the wires basically terminates andallows to communicate to each other, right. So, another property usually these type ofdevices has, is that they are more, they acts as an amplifier. So, if there is a degradationof the signal, they act as a amplifier. Sometimes, we call that at the physical layer theyare hub or repeater. So, they repeats, they basically energize the signal, right.So, this physical layer, here the requirement is, ensure proper scheduling in the mediaaccess, right. Now, the underlying media say like A communicates, A communicates withB, B communicates with C, A communicates with C. And so, there will be somecollision. So, there should be some media access protocol need to be scheduled or needto be ensured that how this access will be there, otherwise what will happen there shouldbe lot of retransmission.So, if at the physical layer, if all things are connected at the first layer, then we have lotof retransmission. Whenever we retransmit, we lose bandwidth. Right, like 4 persontalking or 3 person talking to each other and everybody wants to talk, nobody is waitingfor others to the thing. So, there will be lot of collision. If there are collision, then say Iam one of the party, I need to again tell my own story, whatever I have told. So thatmeans, there is a retransmission and in doing so, what I am doing, I am losing that, I amtaking more time - that overall communication things is more time or in other sense, it islosing bandwidth of a communication it is becoming inefficient, nevertheless we cancommunicate, right.Now, the next type of things what we have a layer 2 switch, what we say where thesecommunication are at a, what we say data link layer, right, where these collision domainsare divided. So, they, though they listen to the same broadcast, but collisions are reduced.So, effectively, we increase the available bandwidth of the things. So, this layer 2 activityallows us to have a better bandwidth realization of the things. So, initially we are havinga physical layer or layer one type of devices. Now, we are having layer 2 type of devices.So, it I can that that is, that communication between A to B, B to C, C to D or D to Eetcetera are not colliding. So the retransmission is less and effectively we are gettingmuch more bandwidth, what we say in network terms, they are in different collisiondomain, but still in the same broadcast domain. Listen to, they are in the same broadcastdomain, but they are in the different collision domain.(Refer Slide Time: 12:40)Now, it may so happen that I have number of things, like consider your office oruniversity institute type of things. So there are several departments. Every departmentshas several labs, class room etcetera a different labs, etcetera which are having differentset of computers and we require, so, there are, there are huge, huge number of systemscome into play and as such there may be two groups who are talking, right. But they aretalking among themselves, they are talking among themselves, but as such, there is, theyare not, this group is not talking with each other. In other sense, their conversation beingheard by them or their conversation being heard by this group will create a collision ornetwork congestion, right and it may lead to again, bandwidth loss.So, I need to so, if you look at a data link layer, so we could have avoided that collisiondomain. But even we listen to each other, right. So, they are in the same broadcastdomain, right. So, though we could handle this collision, but they are in the samebroadcast domain, right.(Refer Slide Time: 14:10)So, in order to handle this, so we need to find out a suitable way to forward this datafrom one to another, right and they are what we require a network layer. I, let us think ofanother case, like here in a class room I am, we are discussing about the network,computer network, internet, internet protocols. In the next class room, there may besomething going on, say something on a class on economy. So, economics or a class onsay chemical bonding, right. So, had it been a one class, say two parallel system in alarge class. So, they are in the same broadcast domain. So, my broadcast is heard there,but now if it is a different physically card, so, my broadcast is not going to their otherthings, right. In order to communicate now from here to here what I require, I requireanother way to go there. I have to go out from this particular gate or room, catch hold ofsome person, if somebody is there, then tell that I need this communicated to becommunicated to this. I require a pen from the other class room right. I need tocommunicate something, right.So, I require so, this not only the collision domains are divided, there the broadcastdomains are divided. So, I in networks terms, we say these are two different networks. Inorder to connect these two networks, I require a network level device which will allowme to connect, right. So, what we say this is a layer three or the network layer type ofthings, where routing is possible. So, one network is routed to the things and ratherfinding a suitable path to forward a data from one network to another, right.So, we require a level 3 or network layer, right or mostly they are known as layer 3switches or routers. So, whenever we have different networks like in this case if you seethis is a network this forms a network, this forms a network, in order to communicate Irequire a, L3 switch or routers. So, that routers can communicate this. In this case, thenthe picture it shows that two port that can be multi-port, typically routers are 4 portrouters and type of things. So, there can be other networks connected from other portsetcetera. So, they can communicate with each other, right.So, this is this way, I have a network layer type of things. So, what I, what we try to seethat initially, from the physically we can connect, right; two things, they are in the samecollision domains etcetera in order to improve that situation, we have a data link layer orlayer 2 type of switches where, this, though they are in the same broadcast, collisions aredivided, now we divide both the things. So, they are separate networks in order to, inorder to connect them, we require a layer 3 switch or a router.
Introduction to Computer Networks
So, sorry, so if we try to see, so, there are different networks communicating to eachother and there are some inter-between links, right or what we say data links or this likethe road networks, these are high speed connected links or gateway or data pathsbetween the things. So, I have different networks across the world and they areconnected with different type of mechanisms and these routers across this world layer 3or layer 3 plus switches distributed over the network allows these things to communicatewith communicate anything to the anything.So, within the network, there are devices, right, there are several computers. So, onecomputer here wants to communicate to here, it needs to find the path; either this path orthis path or this path. There may be multiple paths like in case of our road network. So,different cities, different regions are connected by these road networks. You can havemultiple paths and you chose a optimum path, right, based on your requirement, right.Here also, if the particular path is chosen based on the, based on the overall traffic flowmanagement and other type of things, right. And if there is a interruption or, if there is asome congestion in the path, so, there some other paths can be chosen etcetera. So, thereare multiple paths and so and so forth. Rather, if you like to see that they are allsomewhat independent things, they are, there is this particular network at 4 or network at2, 1 they do not, they do not control each other, right. So, they are distributed networkautonomous systems or autonomous networks and still it can communicate because theyfollow a underlying protocol and there is a different network architecture for that. So, Ican, I can say this is the way I architect the internet, right or I architect my network.So, it can be at a small scale, right. It is in a department, how you things make thenetworking things, it can be at a institute scale, it can be as a region scale, it can becountry it can be over internet, right.So, the core things which binds them is that agreed upon protocols, right. How theycommunicate is the agreed upon protocols and there should be a network architecturewhich may vary from installation to installation, and even within a installation, we canhave different type of reverse of the things, but nevertheless they allowed you, theyfollow that standard protocols, right.We see, we have seen this in our telecoms scenarios also, then we can communicate toanything to anything because they follow from protocol. Anyway these days, segregatingdata, voice and other things are slowly becoming, they are becoming converged networksand type of things they follow is becoming more, what we say consolidated so to providea better service.(Refer Slide Time: 20:50)So, what we see that with this 3 layer, we try to have a end-to-end some networkconnectivity or some sort of a traffic control in the network, right. So, there should besome physical connection, there should be some data link for hub to hub connection, andthere should be a network layer to have a network to network connection, right. Over andabove, we require a layer called transport, right or mostly what we say it is a process toprocess connection and it has lot of property: error control, traffic management, trafficcontrol and different other properties.So, it is above the network layer, right. So, it gives a process to process communication.So, one is physical connectivity, one is hub to hub connectivity, one is network tonetwork, even to system to system connectivity, but there can be multiple process in thesystem, process to process communication is handled by this transport layer type ofthings, right.So, we see that this different type of layers gradually will be dealing all of them in thisparticular course. We will be going details into the things, but we try to have a overviewthat these are the stacking of the layers which are there in our realization of a, anycomputer network or inter-networking or what you say data communication, thewhatever way you try to put it. Whenever there is a, there is a movement of a some datafrom one node to another node from another place, I require this type of things will bethere.But you see, I may not require always all the things, right. If I am, my data is onlytransmitted in between and to the other point, like in between router, may not want toopen up to the transport layer, right. If there is a hub to hub connection, the data linklayer may not require that needs to be open up to the, this up to the data link layer so, noteven the network layer is not required.So, sorry, intermediate devices can act up to the level which connect and carry, right.(Refer Slide Time: 23:10)So, finally, what we looking for, end user is the application, right. So, end of the day I amlooking for, if I am a having a mail application, I am looking for the mail application,that in turn uses some transport layer that, in turns uses some network layer; in turn datalink and physical - all stack is there, but the at the top it is the application layer. This isthe, this is the typical protocol stack popularly referred at TCP/IP protocol stack whichare, which is the predominant protocol across this over all, over inter network, right.So, this stack is the what we will try to look into the things, right, into one by one intothe all the aspects of this particular protocol stack in this particular course.(Refer Slide Time: 24:03)So, what we try to do now, the if there is a source and the destination of somewhere inthe network, then somewhere other a path should be established between the source anddestination, and the communication should go on into the these paths, right. So, there canbe, we will see that what are the subsequently the nitty gritty of these having these pathsto the things, but nevertheless I require a path to communicate from the source todestination, right and this, overall different layer of structure. So, at the source anddestination, what the things are going on? These are basically applications what the useris using primarily, right like I say “www iit kgp ac dot in”. So, what I am doing? I amopening up a browser it can be a say, some some browser like this Firefox or Chrome orInternet explorer explorer and then that’s a client thing and at the other end somewhere,the server is there, iit kgp server is there. So, it hits through the things.So, the applications things are going on, but it is taking it is using this intermediate layerto communicate, right. That will, we will look into these aspects.(Refer Slide Time: 25:21)So, what we will see, if we look at from this picture to this picture. So, it is a this is aapplication stack where this is the whole protocol stack down there, to application istalking to the application, it is reduced to the transport, network, data link, physical, thenit go on, in the in between there is a layer 2 switch. If there is layer 2 switch, it can openthe packet up to the data link layer, right. Rest of the things is a payload for the data linklayer, then it goes this some routing device and it have a up to the network layer.So, it network layer means, it can see this by which network it transmits and find out thepath. Then it can go to another layer 2 switch and finally, hits to this server or means toother party A and B, right. And similarly, one packet moves from here to here, otherpacket moves from here to here and things goes on into in the communication thecommunication, but the intermediate devices can open the packets up to the things whichis which it can handle, right, rest is a payload for the things, right. Had it been there is ahub, so it could have opened only up to the physical layer, rest are is a payload of thethings.So, you see though these devices can be from different sources etcetera, that we can stillhandle a communication between source and destination, right.(Refer Slide Time: 26:52)So, protocols at different layers if we try to see or what we are looking at, is morepopular protocols at different layer; if we look at the application layer, the popularprotocol is HTTP for what by which we access web pages etcetera that is thepredominant protocol, there are protocol for FTP: file transfer protocol and there is apredominant protocol for SMTP: simple mail transfer protocol, right.So, there are TCP, UDP, RTP and these are the predominant protocol in the transportlayer, right. We will see that some are connection-oriented, some are connectionless,some are real time protocol and so and so forth. Similarly, at network layer we have IP,they what say we call it also IP layer different protocol like IPV4, IPV6, MPLS and soand so forth. Similarly, data link layer Ethernet, Wi-Fi, Bluetooth, UTMS, UMTS andLTE type of things different set of protocols. Major predominant protocol definitely isEthernet and Wi-Fi, but we have other protocols to communicate. At physical layer ismore of the physical connectivity is mostly the physical characteristics of the things,whether it is wired, wireless, if it is wired what set of wiring and type of things.So, there are standards for, there are well defined standard for their communications orwhat we say signal and data communications that which are used by the things, right.So, if you, there are some of the, what we say protocols or which are, which we do not,we cannot place in a particular layer rather like if you talk about DNS, it is somewherebetween application and transport; SNMP, this is between transport and network; ARP,DHCP, these are again in between. They’re some in some reference they are consideredat in say network layer ARP and so on and forth.But nevertheless, they use the cross layer phenomena. So, these are what we say, crosslayer protocols, they connect between two layers, right.(Refer Slide Time: 29:13)So, what if you want to look at computer network, there can be two way of looking at itlike if I want to study things, one is going from physical, data link, transport, applicationetcetera, other is going on the, from the top to bottom. So, one is bottom-up approach oris the top-down approach. So, both are fine, both are accepted and type of things. In thisparticular course, we will be taking this top-down approach, we will start fromapplication, transport, network and goes to the physical layer type of things. So, we willbe going in this means bottom means, top-down approach, right. From the top we willstart and then go on the down, right.(Refer Slide Time: 29:29)So, we will come to this history of networks subsequently, I will just give you some(Refer Slide Time: 30:07)references which will be good for you to refer. Like one book is by Kurose and Ross,Computer Network.(Refer Slide Time: 30:16)There are, this is primarily having the way we will follow that is top-down. There arebottom-up they primarily follow this bottom-up, but nevertheless it really hardly mattersthat which book you refer.Computer Networks, so, this is with Tanenbaum and the computer networks, there is asystem approach by Peterson.(Refer Slide Time: 30:40)And there are different sources; one good source is IBM Redbook, there is a nicereference which is on the internet. Again you can refer TCP/IP guide, this is alsoavailable online. So, this will be some of the books.(Refer Slide Time: 30:55)And there are several internet sources like one is IETF is one of the major source, thereare RFCS related to this network protocol requests for comments.So, it will be nice to look at those sites like Internet Engineering Task Force. I think it isfrom 1986 or so, it has been established and there are several resources which give yougives you a things that how this protocols are developed, what new things are coming upand so and so forth. So, with this, let us compute our this first introductory lecture. Wewill be continuing with the subsequent lectures on those on the on this internet andinternet technology.Thank you.
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