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Module 1: Data Link Layer

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Data Link Layer: Ethernet Frame

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Data Link Layer-Ethernet
Hello. So we will be continuing our discussion on Ethernet. What we are discussing inthe last lecture on Data Link Layer phenomena under our this Computer Networks andInternet Protocol course right. So, as Ethernet is one of the predominant technology inthe layer two devices and omnipresent across the network. So, we already discussedsome of the aspects we will be trying to cover some more aspects here. So, it gives as aoverall idea that how the Ethernet works right.(Refer Slide Time: 00:55)
So, just to a recap slide it was developed by Bob Metcalfe in Xerox PARC and his team(Refer Time: 01:02) in 1970. So, basic motivation from the Aloha packet-radio networkstandardized by Xerox DEC in 78. And there are few standards which became prettypopular 802.3 which use’s CSMA/CD. 802.3 u for 100 Mbps standard Ethernet and803.z 802.3 z which is for gigabit Ethernet or 1,000 Mbps or gigabit Ethernet and itemploys CSMA/CD.
(Refer Slide Time: 01:42)
So, just to give a Ethernet frame last lecture also we discussed, but there are little bitmaybe some ambiguity, but to clear it off the Ethernet frame is as far as 802.3 standardconstitutes a preamble. If you remember one some slide some of the literature means inpreamble consisting of a 7 plus 8 bit. But as per the standard preamble is a 7 byte; sorry,it by 7 byte, SDF is 1 byte, destination MAC is 6 byte, source MAC is 6 byte, type is 2bytes, payload MAC’s is 1,500 bytes, 46 to 1,500 bytes. So, if the there is no payload Iwe should have at least a padding of 46 right. So, if I say the payload varies from 0 to1,500 bytes, the varies from 0 to 46 bytes and we have a CRC check or FCS of 4 byteright. So, preambles inform the receiving system that a frame is starting and enablesynchronization right.So, that start of that it say it for the synchronisation it says the receiving systems orinform the receiving system the frame is starting. SDF or the start frame delimiter issignifies the destination MAC address field begins at the next byte. So, it says that thenext byte onwards the destination MAC address is there or that the hardware address onnetwork address, online address whatever we say.So, destination MAC address identifies the receiving system, source MAC addressidentifies in the sending system, type identifies the protocol inside the frame. That meansthe next higher level protocol. Typically for example, it maybe IPv4 and IPv6 right anddata and pad contains the payload data right. What is received from the higher level
protocol? The payload data and padding data is added to meet the minimum lengthrequired for the field. So, the minimum length required for this field is 46 byte. So, thepadding is added for that if the pad there is no payload; that means, 0 thing is iftheoretically. Then the padding of 46 will be there if the full payload 1,500 bytes arethere then I may it may not have any padding right. And the frame check sequencetypically the CRC contains a 32 bit, CRC redundancy check which allows the detectionof the corrupted; so that is the frame check sequence or FCS. So, this constitutes thetypical Ethernet frame a preamble 7 byte, SDF 1 byte, destination MAC address 6 bytes,source MAC address 6 byte, type 2 bytes, payload 46 to 1,500 bytes and payload pluspadding so it should be at least 46 bytes and FCS of this right.So, if we keep away this preamble and SDF so we required 14 bytes at the beginning and4 byte at the end. And these are primarily two more of synchronisation and say thatwhere the start of next address is there and like sort of this right.(Refer Slide Time: 05:20)
Now how do I know the MAC address; many of you might I tried. In Linux there is acommand call ipconfig if you give slash all then it keeps on MAC address of the thingright. In windows there is there is ipconfig in windows it is ifconfig let just try if it worksright. Now if you let me see whether I can expand it will let us see you can use thecommand called ifconfig. And you see here the MAC is MAC address is all if you seeipconfig minus slash all so where is the MAC here is the physical address right 64 00 6a
ic 1 is 63 right. So, this is the typical MAC of this particular systems where from wherethis we I am using the systems right. So, it is in case of this is I if this is ipconfig slash allhere also we have used this ok. It is in windows only in the in case of Linux it is ifconfigright; anyway that commands you can get easily.So, in other sense what you want to say that you can basically get in good get around thisMAC address or the physical address of the things. And we see that is the 48 bit structure18 to 6 we signifies the MAC address of the particular systems. So, when wecommunicate we require the source and the destination and as let me repeat it, but whenevery hop that MAC need to be resolved right. So, when I hop from one system or onerouter or any network device to network device every every link the hop the MAC itneed to be resolved for the destination and it is transmitted to the destination right.(Refer Slide Time: 07:43)
So, the major challenge when we have a shared media is collisions right that. So that iswhy carrier sense multiple access with collision detection scheme right when you say fora wired media. We later on will look at in sometime we will look at the other mechanismfor wireless media; where we have a instead of a collision detection a collision avoidancescheme. Because there are detecting collision in a wireless media is much difficult in thatwill come to that sometime later in this particular course. So, collisions are caused whentwo adaptors transmit the same time adaptor sense collision based on the voltagedifferences right. So, the how the collision is caused when the two adaptors without
knowing that both are thinking or both are sensing channel to be free and communicateand then it is collide.And there is a once the collision is there is a fluctuation in the voltage the adaptorunderstand the voltage like A is sending to B. So, at time 0 it starts where as more onlyset the same time or after a typical delta T time the B starts where, also it sense this sosomewhere here the it collides in between collides. And then the voltage fluctuationsdifference among the things unless A gets a sense of that voltage difference is it thing itis gets A and B both things that there is a collision. It sends a jam packet and resend theretransmit after the sometime right.(Refer Slide Time: 09:28)
So, how can A know that a collision happens? There must be a mechanism to ensuretransmission retransmission on collision right. Otherwise we do not know that where thedata it at send and then there is no data where I think so there otherwise retransmission isthere. If that is not there so it is typically some sort of a connection or best effort serviceyou transmit and then forget that whatever as gone one right. So, A is message reaches Breaches B at time T so B is message from the B to A it is 2 T. So, A must still betransmitting at 2 T; so after 2 T time it may know that there is collision or not.So, 802.3 specifies max value of 2 T to be 51.2 micro second ok. Last class also we haveseen this relates to a maximum distance of 2,500 meter between the host which at 10Mbps speed in 0.1 micro second 1 to transmit 1 bit to. So, 512 bits takes 51.2 micro
second. So, it takes point in 10 Mbps to transmit 1 bit it take 0.1 micro second for in 51.2micro second. It will be some 512 bits or 64 bytes of data to transit. So, the Ethernetframe must be 64 byte long so that you can it can successfully recognise or identify acollision. So, out of that 14 goes on the header excluding that first 8 byte of preambleand SFD and the 4 byte goes for the FCS or the CRC check. So, 46 byte of the payload isminimum required otherwise I cannot guarantee that whether I can find out the things.So, that is why what we see that the payload length has been specified, specified fromnot 0 to 1,500, 46 to 1,500 bytes right. So, or the in other sense 0 to 1,500 is the paddingor 0 to 46 is the or in a sense 46 to 0 is the hey sorry data 0 to 1,500 is the data 0 to 46 or46 to 0 is the padding of the data. So, that is the requirement of the of that particularpayload field. So, if there is a collision it sends a jam jam signal after collision to detectto ensure that all host know all host know about the collision or see the collision. So, it isa bit 48 bit signalling mechanisms.(Refer Slide Time: 12:30)
.
So, there is a same picture, there is a collision, and there is a retransmit this there is after2 T time a gets that informational collision and then it goes on retransmission.
(Refer Slide Time: 12:41)
So, if a collision is detected delay and try again; that is the philosophy it is not thatimmediately retransmit delay, sense, try again right or try again in the sense again sendsthe channel and then plumb that in. So, first time choose K how much time you will thereK from 0 one then delay for K into 51.2.So, second time choose K from 0 1 2 3; nth time K 0 to 2 to the power n minus 1. Themaximum value of K will be 1023 this we have seen also right. If the delay is notrandom there is a chance that the source would retransmit in lock step. And if it is a verysmall then it is fine for small network, but it may be problematic or large number ofnodes will result in more collisions in a (Refer Time: 13:36).
(Refer Slide Time: 12:36)
So, from the receiver side send a handles all access control right from the receiver issimply read the frames and acceptable address. So, address to host, address to broadcast,address to multicast, to which the host belongs all frames if the host is in thepromiscuous state right; so this is the receiver things right if it wants to do with thereceiver end.(Refer Slide Time: 14:10)
So, there is a another; so what we are seen sometimes will down the today’s talk itselfwill see that gradually how it evolved just to recap the things that how it evolved putting
all this together. So, with the increase of requirement of bandwidth and more bandwidthhungry applications etc that so we the overall graduate it from fast Ethernet and gigabitEthernet. So, fast Ethernet which gives a 100 Mbps has technology very similar to 10Mbps uses different physical layer encoding 4B 5B will see that when we will discussabout the physical layer sometime in this lecture series. And can be many NIC’s has both10 slash 100 compatibility.So; that means, it switches based on the things it negotiate and find out that where thingswill be there. So, other thing is the 100 Gbps or gigabit 100 Mbps of the gigabit Ethernetcompatible with lower speed uses standard framing and CSMA/CD algorithms. Distanceare severely limited typically used in the backbone and inter router connectivity. So, it isa high speed things where the backbone and inter router connectivity become becomingcost competitive that become the cost is slowly coming down and it is becoming moreviable. And that finally, it need to be seen that how much effective bandwidth is realablerealizable that is also need to be calculated on need to be evaluated type of things.
Data Link Layer: Ethernet Frame-Part 2
So, if you look at the Ethernet; Ethernet works best under light loads, utilization over 30percent is considered sometimes heavy nodes load. Network capacity is wasted bycollision what I am telling that traditional Ethernet. Most network are limited to about200 nodes to specification allows up to 102 for nodes, but mostly when we say thatwithin a particular network then we have a 200 nodes. Then we will see the concept of
bridged a land that we have a bridge to connect the other land. Most networks are muchshorter 10 to 5 to 10microsecond that RTT and round trip time. (Refer Time: 16:40)Transport level flow control helps reduce load. So, as we are talking about that there arethe other layers what they are doing it is totally not keeping independently the transportlayer flow control mechanism may help in reducing the load Ethernet is inexpensive soto say fast and easy to administer right. So, Ethernet found to be cost effective quite fastand easy to administer overall. So, it is a from the network administration station point ofview it is always a good thing to do to look at the things.(Refer Slide Time: 17:20)
There is a few challenges issues or with this Ethernet, I am that some of them askEthernet peak utilization is pretty low right. So, it is low peak throughput worst withmore hosts. So, if you connect more hosts that the throughput will fall more collisionneeds to identify by a single sender smaller packet size more frequent arbitration. Longerlinks collision take longer to observe more wasted of bandwidth if it is a link is longerthan it is a more collision takes in a large time to works we have seen that 2 into Tefficiency sorry there is a typo can be improved by avoiding this condition.
(Refer Slide Time: 18:19)
So, see these are these are may be administratively controlled and later on we will seethat there are things which we can which can be managed we can manage with thisEthernet. So, this can be improved by avoiding these conditions. So, if we come to thisoverall evaluation of the Standard Ethernet. So, we have Standard Ethernet, Fast
Ethernet, Gigabit Ethernet, Ten-Gigabit Ethernet. Now a days we are talking about Ten-Gigabit Ethernet we see that there is a issue of bundling this things to larger bandwidth
and so and so forth.Mostly use for higher things are mostly used for backbone traffic because in becauseneither the application nor the most of the devices can exploit that that very high level.Because I have a laptop with some 100Mbps capability connecting to a 1 gigabit so it iswastage of the resources right. And secondly, all are not used even your connectivity is at1 Gigabit or 100Mbps. Finally, your application may not be using more than 10Mbps or20Mbps and type of things like that so it is a again a wastage of resources. And wheneveryou go up on the scale on the speed the cost increases manageability becomeschallenging and it is a overall pressure on the backbone of the network.
(Refer Slide Time: 19:33)
So, Ethernet addressing already we have seen just to recall. So every each station on aEthernet network or every system on an Ethernet network such as PC, workstation,printer etcetera. Which on network enabled having a network interface card has it is ownnetwork interface card or NIC right. So, in these systems like this system’s out I amusing here. And it is a own NIC your network interface card has adapted to connect tothe physical media, it can be wired connection, it can be wireless connection, it can becoaxial cable, it can be twisted pair, it can be fibre optic, it can be some wireless devicesBluetooth, etcetera. NIC fits inside the station and provides the station with a 6 bytephysical address which is unique for every NIC ideally.So, typically like 17 6E 10 this is a typical thinks 6 byte address right and there is ascheme at the layer two level is a Unicast multicast and broadcast address right. So, asource address is always Unicast right so fine it is generating it is always Unicast theframe comes from only one station. So, anyway generating a Ethernet or layer two frameit comes from a one station. The destination address however, it can be Unicast multicastor broadcast if the least significant bit of the byte in the destination is 0 the address isunicast right or otherwise it is multicast right. So, say this address what is this 7; 7signifies 0 1 1 1 so; that means, the least significant bit of the; a as you have seen the thatis similarly of the first byte in the destination address is not 0 in this case 1 so thatmeans, it say multicast. Broadcast address is a special case of multicast in which all bits
are 1 what does it mean that all these are ones in other sense this is a FF:FF:FF:FF:FF:FFright: 6 FF right. So, that is the multi there is a broadcast a special case of a multicast.(Refer Slide Time: 21:56)
Like here, if we have this A as the BC so A is a Unicast because A is 0110 right. Any waylast bit is 0 for this first this one these the last bit of the first byte is 1. So, it is multicastand these are all ones so that is broadcast right. And if I have a address like this there ismaybe there is some typo because this is not representing correctly here because this 47it is not representing 47. So, just please re calculate this one it should be some a typo isthere any way. So, what to be mean to say when we took at address like this it is decipherinto the binary form right.So, again I am repeating this 40 yes please check the representation. This 47 should berepresenting here, 20 representing here, 1B representing here, 2 here at the 47 is not thisrepresentation right. Because 4 is 0 0100 0100 and 7 is 0 111 so it is should have been01000111 so there is a mistake here there please correct it.Anyway these are the bit wise representation and when it goes it goes in this direction.So, the when did you send that is going and receiver receiving it is in this direction. Nowif you look at the standard Ethernet already we have discussed. So, it is 10 base 5 busthick coaxial 10 base 2 it is say again bus structure with thin coaxial 10 base T is thetwisted pair that is which allows star connects start type of topology 10 base F is a fibrewith star topology where 10 represent the speed of the thing.
(Refer Slide Time: 24:04)
Ethernet evaluation bridge Ethernet dividing LAN’s using bridges increase bandwidthand separates collision domains correct. So, if I have a large number of systems into thethings so there is a concept of bridge Ethernet right. So, I we put a bridge in between sowhat happen that instead of this one whole thing I put we can put a bridge here. So, thesetwo LAN’s are now connected with a layer two device or bridge. What it does itbasically as we know layer two device divides a collision domain the collisions are theyare separate collision domains and in a effect it increase the overall bandwidth.So, this is the bridge Ethernet where we have better bandwidth things. So, if you look atyour own network in your office; or in your colleges, offices and so where the number ofnetworks are there. So, at least we have layer two devices which are which basically adivide the collision domain effectively increase the bandwidth of the network righteffective bandwidth of the network is increase or in other sense the wastage of thebandwidth due to collision are reduced. So, that is a very standard practice to do that sodivide LAN’s into bridges.
(Refer Slide Time: 25:34)
So, we have the next phase the bridge to switch can be thought of what we can say amultiport bridge. So, instead of one bridge dividing into two; I can have now a switch of12 ports which is dividing into 12 domains right. So, as per domain which again canconnect to the bridge so, I can have a hierarchical structure of this layer two devices rightwhich allows me to increase the effective better utilisation of the bandwidth of theavailable network right. Like here we have one set of switches which goes on thingsanother a switch which connect a set of systems say this is a router or layer three deviceright.So, it is a in this connection it is a full duplex connection it come to that so trans receivedis difference. So, there is a not a challenge of collision, but a effectively we can managethe things and as you know that these are there are manage switches and type of thing.So, we can basically understand manage the switch in effective manner that how theswitches can be managed and type of things in effective more effective manner. So, therewe have the overall health of the switch or management of the switch.Next one is a full duplex Ethernet which is popular these days which are switch aremostly used these days in most of the today’s network. So, full duplex network modeincreases the capacity of the each domain to 10Mbps to 20Mbps. So, this is full duplexas we are showing the pixel that is full duplex. So, or it increases in a double insomewhat double in other sense we have a separate transrecieve line. So, they are not
colliding each other in the transmitter and the receiver or not colliding with one another.So, that that increases effectively so something 10Mbps effectively increased to 20Mbps.So, all are transrecieve lines are different.(Refer Slide Time: 27:46)
So, in other sense what we say in a full duplex Ethernet that may not be need of anyCSMA/CD because there is in a separate transreceive. So, there is a no collision parts sayright in a full duplex switch Ethernet there is no need of CSMA/CD method in a fullduplex switched Ethernet is station is connected to the switch via to separate links right.So, we have a transline and a receive line so each station or switch can send or receiveindependently without worrying about the collision each link is a point to point dedicatedpath between the station and the switch. And the MAC layer becomes much simplier inthis case, because into do not have to take care all those things of the collision avoid acollision detection and type of things are.So, in other sense what we are saying these days whenever this system is connected to aswitch it is as a two’s trans two separate line one for trans and another for receives right.So, in other sense it has it has independently that transrecieve things goes on there is nocollision so there is no need of CSMA/CD per say. So, there is a MAC control layerstandard Ethernet was designed as a connectionless protocol for MAC sub layer right.So, if you look at the standard Ethernet it was more of a connectionless protocol like youtransmit and as such you do not manage the thing to provide for an error control in a full
duplex Ethernet a new sub layer call MAC control is added between the LLC sub layerand MAC layer sub layer.Now we need to have something what we say flow and error control mechanism withinthe Ethernet right. So, if there is a error of transmission; that means, you send the packetthe packet is not reached you have sending an acknowledgement it is dropped. So, thereis both. So, that hamper that error control error hampers overall flow mechanism of thethings because ethernet what we see layer two is a hop by hop, but end of the day it hasneed to transmit across the thing. And in most in several cases in it requires some sort ofa flow control error control mechanism to ensure a some sort of typical service levelassurance or quality of services and type of things.(Refer Slide Time: 30:05)
So, first Ethernet already we have a seen so was designed to compete with the LANprotocol, FDDI, or fibre channel IEEE created fast Ethernet 802.3u has a rate of 100Mbps. So, goal upgrade the data rate to 100 Mbps make it compatible with standardEthernet keep same 48 byte addressing scheme keep the same frame format. So, there isno incompatibility keep the same minimum and maximum frame length so those arekept.So, there is a there is a feature added to fast Ethernet in is a auto negotiation. Newfeature added to fast Ethernet is call auto negotiation. Auto negotiation allows two deviceto negotiate the mode or data rate of operation right. So, which mode and which data rate
operations to allow compatible devices to connect to incompatible devices to connect toeach other that will allow one device to have multiple capabilities right. So, it canconnect to 10 Mbps 10 slash 100 this type of things are possible to allow a station tocheck the hubs capability like it if is a managing then I can check that how muchcapabilities there. And accordingly, I the station can transmit so hub or switch we canthis is loosely used so it is so allow the station to check the switch capability.And finally, the what we are the next evaluation on the thing is the gigabit Ethernethigher data rate of 1Gbps in full duplex mode gigabit Ethernet there is no collision themaximum length of the capable a cable is determined by the signal attenuation in thecable. So, that is the only things which is there and there are different variation of thethings and these days we are having ten gig Ethernet and so and so forth. So, these aresome of the evolution of the things.So, what we do with this we let us conclude today’s lecture on a overview or the overallthat basic functionality or Basic Concept of Ethernet. So now, will in the subsequentlecture will look at more aspects of layer two or data link layer of this, but overall aoverall network a layering or the OSI or TCP/IP layer.Thank you.