Physical Layer: Overview
Hello. So, we are discussing on Computer Network and Internet Protocols, today we willbe looking at the our level layer 1 of the OSI or in TCP/IP model that is the physicallayer. So, rather couple of lectures we will be discussing on physical layer phenomenathough in some of the literature on in several standard some of the standards theyconsidered physical and data link as a single layer like if you put the NIC card it comeswith the both things.And though again the physical layer as we will see is more related to the so, to say quoteunquote electronics and communication perspective, nevertheless we need to try we needto understand that physical layer basic philosophy, which allows me to carry the trafficfrom one network to another network and type of things.So, as we discuss several times that, any upper layer phenomena need say lower layer tocommunicate; that means, unless the lower layer is there it is not able to do it not able tocommunicate and end of the thing is the physical layer. That should be a physicalconnectivity between the source and the destination or in our cases the physicalconnectivity between two hops. So, whether it is wired, wireless it based on thetechnology, nevertheless or fibre or whatever, but nevertheless I require a physicalconnectivity.And there are several consideration like what should be the encoding scheme becausefinally, whatever we are doing with the system or at the end systems are generated digitalsignals right, but your communication path may not be suitable to carry that digitalsignals right. So, in that case you need to convert this digital to some sort of analog andthen communicate and do a other way around who will do that right. Who will providethe physical infrastructure to work on the thing work on this communicate path right. Ibuy a laptop I want to communicate with the rest of the world right through say mail,chat, browsing and type of things right. Either in a interactive mode or either I read orwrite type of things right.
Now in order to do that, I need to a connection to the overall networking cloud right oroverall this internet; how do I connect? So, one the best way at the home what we areusing is using the mobile data network. So, what you are doing? You are using themobile as a hub right or hotspot or Wi-Fi hotspots or a type of things, then you connectto the mobile your device mobile device from your laptop and that in turns use thebackend data network to connect to the internet right.So, this is something which is need to be connected or physically connected whateveryour protocol and whatever you are working at the upper layer, this thing is needed rightor you can connect your PC or laptop through a cable if you are having a connection inyour office with RJ 45 through a cable. So, there is a physical connectivity in turn thatpoint it is connected to a switch in turn to the backbone network and so and so forthright.So, what we require at our end at the device end is a network interface card; the NICtypically takes care of both your data link layer phenomena for more to say MAC layerphenomena and the physical layer standard, whether it is having a wireless connectivity,whether it is having a fibre connectivity whether it is having a coaxial connectivity or aRJ 45 Ethernet cable connectivity that is decided by your NIC card. So, NIC card hasthose interfaces right and then you can connect to the rest of the world the physicalconnectivity will be there.So, there is the physical connections what is there, lot of things goes on and its a verycomplicated not only complicated its a very complex communication procedure whichgoes on which is another purview of communication. So, it would not be possible to dealwith all communication phenomenon, but we will try to hit on those important stuff. So,today we are trying to see that what makes this overall process works together and thenslowly in next 1 or 2 lectures we will see that what are the basic features which we needto understand when you will when we are looking at a computer network or protocolstack in a following some standards. Now, the picture which is available in severalliterature also we are seen in at the beginning of this lecture series.
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So, what we see at the end we have this physical layer, which is mostly responsible fortransmission of binary data through a medium right very straight forward and simpleright. So, if we go up, you have transfer of unit frames etcetera then delivery model fornetwork layer and so and so forth right.So, physical layer is concentrated with the things medium maybe wired wireless or anymedium which allows to communicate right. So, this is the down the layer OSI again Iam repeating in case of TCP/IP, some of the cases we considered this two as a singlething in some cases the physical layer is kept apart because this is more of acommunication phenomena, nevertheless it requires for us to communicate.
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Now, two things are coming whenever we are discussing two stuff is coming one iscommunication, let us put it as a data communication because we are looking as, we aretrying to look at data where we will communicate another aspect is a networking right orother time is what we do once the communication is there. So, data communication as wesee transmission of signals in a reliable effective manner.So, want to transform one node to another node. So, that is the way what we do get thedata communication and it involves signal transmission, transmission media, signalencoding interfacing with different transmission media and interfacing with the NIC cardwith the transmission media and type of things and multiplexing and so and so forth.So, these are the different aspects which are there with the data communication whereas,the networking it mostly deals with technology, architecture, protocols, protocol ofcommunication network used to inter connect communicating device or entities or endpoints or intermediate points per say. So, its the technology, the other should be aarchitecture and the protocol or what we say standards so, that the two devices cancommunicate right. Here mostly what we have seen topic like LAN WAN networkingprotocols of different levels say routing and at the network level that a transport levelcommunication TCP or UDP there are communication at the and finally, we havenetwork applications which talk to peer to peer communication right.
So, these are mostly concentrated in the networking. What it tries to; what it assumes inwhen we will worked on the network? We assume that the underlining media is thereright when you try to browse a particular site or do some networking do some networkrelated operations with your systems PC or laptop, what we assume that the as if thephysical media is there right or there is a physical communication is there.(Refer Slide Time: 09:20)
So, these are the two aspects and again in the literature, we will find that when we see.So, we have a source where the data is generated, there should be a something which is atransmitter then a transmitter carries the thing to a transmission systems right then at theother end I have a receiver and the destination which are there right.So, this way source to destination is there, if it is a full duplex or both waycommunication and there should be a communication path when the destination sourcewhen it acts as a source and this is the distance. So, what we have this thing one is thesource like typically PC or laptop can be a source, a transmitter that some devices whichallows me to transmit to the backbone of the network.Then the transmission systems it can be internet or it can be a rather I should say that itcan be a some sort of a communication system like the most prevalent communicationsystem what we are having is the telecom network right; so, the internet with thebackbone as the telecom network and then we have other things right. There is a what
call modem come into coming into picture we will seen coming one or two slides thatwhat is there and that the other end of the computer.So, I have some way to communicate with the things right, it can be a mobile device, bythis systems connect to this say particular mobile device and then this in turns at the datanetwork at the backbone and then it the data traffic goes out. It can be like this, thissystem my system can have a RJ particular Ethernet cable, which is connected to thewall mounted socket which is in turn connected to the backbone layer two network, aback backbone layer two switch or the layer two switch off our IIT Kharagpur networkwhich in turn connect to the hierarchy of switches finally, goes to the gateway to go outof that particular network right.If it is required or if it is within that the same switch network, then it as whatever is thereto be do in a layer two switch right. So, that is the way is here. So, there is some deviceswhich allows me to push the network thing.(Refer Slide Time: 11:43)
And if you look at the communication task, there is a variety of things, some of them Ilist transmission system utilization, interfacing or rather than utilization if you saytransmission system utilization is definitely another aspect transmission systemutilization, interfacing, signal generation, synchronization of the things. Once wecommunicate there should be a way of synchronisation there is a exchange management
or there should be when we exchange the thing there should be a in management thingslike I send a data I get acknowledgement and type of thing.Error detection and correction mechanisms, if there is error how do we recover from theerror? Addressing and routing - how to the route packets? Recovery from a particularfailed situation, message formatting. Another aspects which come into play though it notdirectly may not be possible to do cover those things, but what we plan that at least one ifpossible two lectures we will talk on network security aspect right.Become this is becoming extremely important so, what are the different aspects ofnetwork security that we will take which is so, that is also a thing, and then we have anetwork management overall network management process to handle the things. So,these are the different aspects of communicate task or so to say computer datacommunication components right. So it is a when you do looking at the datacommunication aspects of it.(Refer Slide Time: 13:17)
So, we have two type of things one is a Local Area Network or LANs right. So, there areseveral protocols which are predominantly for the LANs, we will see that there is a basicconsideration for LAN another category is a WAN Wide Area Network which is connectto the internet. Typically what we say this LANs are pretty high speed thing, like you geta backbone of a connectivity of 1 Gbps connection etcetera through your system
provided your NIC card supports that right 100 Mbps is very common is the defectothings or communicating in the things.Whereas, if you look at your other side of the router, getting 8 Gbps links is pretty costlyright so, even means to that tune of order of 100 Mbps as also costly to maintain. So; thatmeans, in other sense the LAN connectivity is much higher, much much higher than thebackbone or the internet router level connectivity. So, and if you look at the source andtransmitter at together we put in the sources like your NIC card is sitting on your systemitself. So, the both the transmitting and the typically the source of the data are together.So, out of what we typically see you have the source which is having source data sourceand the transmitter, you have the destination which has the receiver and the destinationfor the data right an intermediate thing is that transmitted media and we will we will tryto see that what are the different consideration for this transmission media for in one ortwo lectures.(Refer Slide Time: 15:14)
So, what is the predominant things what you are having is the telephone network right.So, that is the something which is omnipresent when the communication network cominginto play.
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So, this is very much there this telephone network and what we see rather it is what is seebasic telephone network. What rather we see there are not only there, there can bedifferent parties which are there right, there are different telecom companies or telcoswhich are in your serving in your area or serving in your region, state, country and typeof things right.So, these telecom operators are primarily initially started or primarily started for voicecommunication, but now they graduated to a scenario of to data communication ratherstreaming data type of thing. So, voice data multimedia along with mobility right. So,this sort of communication are happening into the things right definitely we have deviceswhich can talk to each other using these things.There are one thing come into play immediately that this different telcos should be ableto communicate with each other; that means, there should be interoperability betweendifferent telecom company right or if you look at our things that mobile service providersor there should be some way, how they communicate with each other. So, that is oneimportant aspect, now if I can put somewhere other my data network or my datacommunication paradine into this enter this telecom cloud, then at least the backbonecommunication I can ensure with their data communication right. And if you practicallysee its a huge infrastructural investment when you go for this sort of a thing; its a hugeinfrastructure investment when to when things are there right if you look at.
So, we can utilize the things; what we require? We require some connectivity with thisthing with your system or your network or your organisation network to the nearesttelecom centre or what we say nearest exchange right, if you are a organisation right.When we use as individual then my system should be able to communicate to mynetwork suppose I have a BSNL or Airtel or Vodafone or something Reliance andetcetera etcetera connections what I require is the how I communicate to the system tothe they are backbone telecom network. Once it is there then the connectivity everybodyis connected things are there right.
Physical Layer: Overview- Part 2
So, another thing as we are discussing so there is a the usually our communication pathare analog right wired or wireless audio tones info conveyed through a signal amplitudefrequency phase or analog signal right where as the working of our system etcetera areprimarily digital or 1’s and 0’s. So, what is the problem happening? You are basicallygenerating a series of 0’s and 1’s needs to communicate through a media, whichunderstand that analog signal.So, you a generating a digital signal, but need to be pumped through a media whichunderstands a analog signal right. Consider a wire that previously how used tocommunicate, there is a many of you might have not seen that. So, there was landlinestill it is there through the landline phone cables, there is a device used to connect and
that device connects to this system or the computer, when I used to we used tocommunicate we communicate this through this.So, this device is typically a modem right or what we say a modulator and demodulator.So, the modem at the source end or one end is does that modulation signal getting abinary set, it modulate and pumped into the analog by this analog backbone whichunderstand the analog, that is carried to the other end of the signal and then it is againdemodulated and push it to the end system. So, the two system talks each other at thephysical layer itself they communicate that one is modulation another is demodulation atthe other end right so that two terms we is called it is as a modem.(Refer Slide Time: 20:14)
So, it is like this I have a analog network, it’s can be a phone or type of the cloud itself.So, the digital signal through the modem goes to this analog and goes out right. So, thisanalog to the another end while at the in system it is the other way around right. So, thatis a very effective device and we could able to easily connect it is not there are lot ofother hindrance as like your call drops like it goes can disconnected say the neverthelessthe phenomena is straight forward. You need to have a modem at your disposal, whichconnects to the rest of the network where at the other end also somebody who iscommunicating is having another modem so and so forth.
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So, what we do effectively? So, we have a system through a modem, it communicatesthrough this basic network and communicates here again a modem and a basic systemright and a computing system. So, I can use these backbone network or the backbonetelcom network to communicate with the two systems right so using modulator anddemodulator or modem.(Refer Slide Time: 21:36)
Now if you look at the modem, modem translate the digital computer signal to analogsignal which are which is the telecom network or the telephone backbone understands
and carry it to the destination and vice versa right. So, that is the things are there andthere is a like what we have that plain old telephone systems or the pots. So, whattelephone service there is a pot, and then what this modem allow is to communicate withthis things right. So, that is the way we communicate.(Refer Slide Time: 22:14)
So, what we see if there are multiple systems need to communicate with the othersystem; like there is a bunch of system out here, which wants to communicate with thissay this is the server at the other end right. So, there can be database server, applicationserver different type of different type of clients which wants to communicate with thisserver; and for that what we require? We require individual lines to communicate withthe things right. So, to say if I am communicating distance where modem 1, modem 2,modem 3, modem 4 and communicating with the things or even it is communicating witha wired mesh to connect if means a wired connectivity from the source and destination.
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Now this becomes a cumbersome process if multiple things are there. So, we have athere is a concept in the communication called multiplexer or multiplexing where aallows multiple signals to be carried across the single physical dimension so, that is themultiplexing. And if you look at these are there can be different type of things we willdiscuss in our subsequent lectures, that is one is time division multiplexing or TDMfrequency division multiplexing and so and so forth. So, these are through popular thingsand we will see that different variants of the things.Now so I have different signals which are generated by this different systems and thenwe have a multiplexer which multiplex this things and usually this when the signal thingsare communicated these are through baseband communication whereas, these arethrough broadband communication. So, apply able to carry single multiple signalssimultaneously, in a baseband it carries only one signal right. So, this is the way it isthere. So, I have broadband because there are multiple signals. So, there are multiple. So,those are multiplex using some technology and that has be communicated to a broadbandsignal.
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So, just to quickly do the letters are pretty little small in size, but nevertheless we justhave a quick look at the baseband and broadband. What the baseband said? Basebandtransmissions typically use digital signalling over a single wire right so; that means, thetransmission themselves take the form of either electrical pulse or light the thing. So,whether it is a what sort of media is there, that digital signal used in basebandtransmission occupies the entire bandwidth of the network. Band to the network mediaand transmit a as a single data signal right. So, typically the baseband multiplexing is theTDM.Whereas in case of a broadband uses the analog signal in the form of a optical andelectromagnetic wave over multiple transmission frequency. So, I have multipletransmission frequencies or carriers by which this signals are carried. For signal to beboth send and received the transmission media must be split into two channels. If I havea send receive type of path then we have a things that is true for they are also that if youhave to both send and receive data using single cable, then that type some sort of amechanism that with is there (Refer Time: 25:54). However, the sending and receive canoccur on the same whereas, the same time. So, either I to have a separate two separatecables can receives or some arrangement of doing some sort of a time divisionmultiplexing, where multiple things can be shared.
So, in case of a broadband also, multiple channels are created by the broadband systemfor multiplexing technique, known as using some a approach for frequency divisionmultiplexing, we will just see those things for in subsequent lecture.(Refer Slide Time: 20:34)
So, early communication so there is a pots and it able to communicate. So, there is abranch office or head quarter or a location 1 and location 2. So, it has some multiplexer,then if I have a multiplex a modem which communicate to the plain old (Refer Time:26:59) telephone service, which in turns communicate to other modem to or this is formodulation one end, this is demodulation and then if there is a multiplexing. So, there isa demarks and the signals are formed into the at the server end or the other end.
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Now, with if the multiple locations are there, then we have multiple modem connectionswhere say and so and so forth. So, with these things also its works fine do the achievablespeed is not that high secondly, the management etcetera also challenging right. So, thereare different architecture, different protocols of managing them and type of things rightthough there are lot of standardizations will occur. So, these are three locations wherethese are communicating with the modem and then it is communicate into the plain oldtelephone services or pots framework or pots backbone.(Refer Slide Time: 28:02)
So, what we are looking and that if we look at the physical layer consideration one isyour NIC or Network Interface Card as we discussed. So, NIC provides both this MACand the physical layer as we are discussing it gives a connect a way to connect through aRJ 45 cable or through a wireless media or even through your say coax cable if it is likethere coaxial cable is there is prevent. So, two systems, we to connect two computerswhat I need to do? Suppose I have two computers how do I connect? Install NIC at eachright and then connect the thing right? I can have a single cable to connect to the twosystems because I if my networking is based on these two system, I do not requiresomething to do that.But it is how, there is a small catch that we need to do using a crossover cable; thatmeans, crossover means trans of this source is receive of other things and trans of the bin the receive cable of the thing. So, it is cross over to the thing. So, that is a crossovercable or popularly what we say cross cable right we require a cross cable means there is.So, two system can communicate so it is consider at the thing. So, this physicalconnectivity is that through a say RJ 45 cable.(Refer Slide Time: 29:45)
So, typically this cable can be stretched up to 100 metres or 300 feet that for say fullyconstruction of things, but in reality we may not able to go that path it is little less thanthe 100 metres considered as a, otherwise signal degradation will be much higher tohandle that.
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If it is more then we require something call every repeater right. So, a repeater whichregenerates the signal right so that is a repeater can be used to increase the distance. So,if I have repeater, again the distance is increase there right. So, repeater regenerates thesignal.(Refer Slide Time: 30:31)
And one we look at the concept of hub, if there a number of points needs to be connectedthen we have a hub or multi port or also a sort of a multi port repeater right. So, these arethe devices which are at the manifestation at the network here the layer 2 or the physical
layer. So, any other layer thinks like instead of the hub if I have a layer two switch, thatalso works as a signal regenerator and have this physical layer connectivity. So, thatinstead of hub, I could have a have a layer two switch or I even can have a layer 3 switchor where we have the routing for the network layer level active activation right.So, any upper layer has all the lower layer phenomena. So, that is absolutely fine itworks with right. So, another things what we previously also discussed we just tomention again that, see at the layer 1 I am in the same broadcast level or at the same as athing the same collision domain also. So, as we are in the same collision domain. So, themore is the collision or more simultaneously the stations want to exchange information,there will be collision and retransmission of the signal right.Suppose considered this a four port switch, a four port hub of say 100 Mbps for our somearguments thing right. Now with the four systems are connected. So, and they all arecommunicating at the simultaneously. So, there will be huge amount of collision orretransmission of the signal or effective bandwidth will form right. So, even if it is abackbone of 100 Mbps. So, effectively if or in a worst case situation we will get a around25 Mbps right. So, divided by across the thing. So, the hub the overall bandwidth isdivided by the number of ports of the end type of things. So, that is a as in the samecollision domain all right.But the other side is that, more you go on the higher devices more cost increases right thehub is the switch is much costlier than hub, layer 2 switch a layer 3 router is costlier thanthe layer 2 switch and so and so forth as you go on the things right. So, hubs are good forsome very small network or for extending a network and type of things, but may not bethat efficient to do that. But our basic consideration is not that immediately finding theoverall performance efficiency, but to look at that how to connect systems across right atthe physical level right. So, this is the thing.So, what we what we have seen today is more of looking at the different aspects or ofthat of at the physical layer of the layer 1 and in our subsequent lecture or what we willlook it to some more phenomena and go little deep into the some more phenomena ofthis physical layer like that including multiplexing and so and so and forth, how theywork and type of things right. So, that in subsequent lectures in this. So, with this let usconclude today and for and we will continue in the in our next class. Thank you.
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