Lecture - Media Access Control

before we get into today's material just going to step back and go over we did on Monday obviously we missed lab yesterday I'll be posting a little video on what I was going to have you guys do in lab there will be another lab assignment posted it's going to be almost identical to the last lap you guys did it's just going to be changing some vlsm stuff around the idea that we're trying to drive out to you for labs is to be comfortable with setting up a dressing blocks how to assign them and assign the correct once they get a little anal to tell you what numbers to be putting at certain locations follow directions obviously if you have a whole range of them it wouldn't matter if you get any number in there in the real world but for package traits are to able to assess your ability it's going to be this address for this device and that address for that device I'm sure you sell that in the last lab on Monday we got into the data link layer and we're talking about logical topologies vers physical topologies and basically the three logical apologies I got into I believe I only did two was the ring topology the multi-access which I called the bus and then the apology I'm going to get into today is going to be point-to-point start scholarship yeah well and that's what I'm going to get into this with point-to-point because I did mention something about star ax star topologies and I said well that's multi access but it's with a little flavor because you can get really carried away with logical topology especially we start getting the hybrids when you take multiple topologies and join them together to get the benefit the idea of topologies is they're giving you the concept of how the data signals will behave as they travel on a higher level they're not telling you how to physically lay them out and map them together so I simply drew a circle and I called that the ring topology right that's the way it would look like on a sheet of paper that's the way I'm going to draw it because to me logically it tells me that data is going to flow from this device to this device to that device by the way the letter ends short for node right we're not saying Network and we're just going to keep on going around in circles like this why would we implement this technique for the same reason why do we implement roundabouts when it comes to our highway infrastructure so why why this logically what does this give me and what is the whole purpose of the data link layer what's this yeah it controls the flow of data I mean the whole purpose of the data link layer is to put frames on the medium and to take them off well sometimes the medium is going to be shared look at this situation the medium is shared this is the common medium and remember the definition of a network two or more devices connected together over a common medium governed by the same set of rules we clear about that so if these are connected together through a common medium there's going to be a chance of collisions when it comes to ring topology we eliminate that how do we eliminate that by using a controlling method which I told you is going to be called deterministic we're going to control this we're going to say okay for the person who has the token you're allowed to talk only one device on this topology or network is allowed to talk if you have the microphone say what you want to say then let everybody else respond if you will this eliminates collisions but what does it cause additional overhead because we got to deal with timing and who has what now performance is always questionable because if you have a lot of users on this network then what's going to happen the collisions well collisions don't exist performance would it go up or down if you increase the number of users hmm interesting welcome back to that at the end of class but I just want to let you guys know that we typically use these ring topologies when we connect cities together okay locally we move away from ring topologies and we create we call multi access it gets its name because this common medium this backbone if you will is accessed by more than one device correct okay collisions can occur in fact this typical local sorry this typical topology is implemented locally like in a network like this why would this the Polish be implemented on a network like this verse this typical topology and by the way when you look at a network like the way I'm looking at this network do not think that in a ring network you must arrange your desks around in a circle like they do in English class what's that typically the physical diagram for a ring topology might look something like this three nodes or even four doesn't bother me it gets goes on and on this is connected to that one disconnected that one that one's connected to that one and then the end or the last node is then connected back to the first node correct so you guys really have no way of looking at something physically and determining the logical topology when you look at the logic topology or thinking how it behaves all these devices in this classroom come to one central device you might not know what that central device is and that's such a device that intermediary device is going to dictate the logical behavior if that central device was a hub then the behavior that it's going to dictate is going to be something like this and that is when somebody puts a frame on the medium all the other devices are going to get that right so in this bus topology we need to concern ourselves with collisions we need to develop a mechanism that can detect collisions and recuperate recover from them so what are we going to do if this node wants to communicate with that node remember that any node is connected to that medium that common medium they're all going to get it correct and so I said that the data link layer had to handle some addressing remember the two sub layers of the data link layer the logical link control which is going to do all the orchestrating between the software and the physical layer it's going to do the framing if you will and then the mac layer the media access control is going to be responsible for telling the physical layer what bits to put on the medium and also detecting the framing address the media access control addressing I don't care what type of topology we are driving to we need to try to get to the point that we understand that each of these devices are going to have a local address a physical address that one they're connected to a common medium we'll help them determine whether or not that frame is destined for them I remember a frame is a PD u for the data link layer correct just like a packet is a PD u for the native sorry the network layer so when I look at this each of these devices are going to have some kind of physical address if I'm using a hub in this case I'm representing that when the frame that gets put on the medium it's going to be shared through all the other devices these ends are called the terminators and that means the signal is going to stop its propagation okay now we could hook this up to a router and it would stop its propagation at the router because what does a router do breaks the network up into broadcast domains correct so here we are putting a message on their node one wants to talk to node three no to sauce can receive that but the question might come up is what happens if node 1 and node 2 both want to talk to node 3 in a ring topology it's controlled it's deterministic I know the data is flowing in one direction that means as this gets passed through there this computer is going to check that token if you will the carrier and say hey if there's nothing on it let me put something on it it's no different if I pass around the cart and if you see something empty just add to it if it's full just keep on passing it around then if your name's on that package in the car you're going to take it off correct and then you're going to pass it next person sees it's empty and if they have something to put in there they're going to do that no collisions here we're dealing with a non deterministic method why do we call it non-deterministic there's no control why isn't there any control can you guys control the electricity that flows from one outlet to another if they're both part of the same string of outlets so for instance if you plug in a device right here in this outlet and then plug in another device into this outlet up here electricity is flung from those outlets all the way down through is the same electricity correct so there's really no ability to determine what devices a lot of access to electricity when they're all plugged into the same circuit does that make sense the same this is really critical here when I want to deliver a message to another device I need to use electricity that's my carrier if I'm using copper wires and all these copper wires are touching each other when I put electricity on one copper wire what's going to happen to all the other copper wires they're gonna come alive they're going to receive the voltage correct voltage will represent our bits hence I have no way of controlling that so what am I going to do why would I implement something like this when I have no control over collisions who's talking when like over here I do have control i hook up a wire from here to here that'll stop the circuit that will stop the electricity then I'll hook it from here to there and I'll continue doing this alright folks I have been really good this semester about laying down on you this is your time that you pay for me if you are doing anything other than paying attention what I have going up here then I'm going to ask you to leave you're disrupting other students I'm not going to single anybody out but this is your only chance your last warning I've warned you at the beginning this semester I'm being very delicate okay so from here on out if I start seeing other things happening and by the way the camera is running and I do notice this I do not feel like being a parent I don't have any kids I don't want you guys to be my kids you are my students I am your instructor let me do my job as teaching and don't make me a disciplinary figure okay because it's real simple the doors there if I am boring you you do not need to be here if you find this stuff below you if you will find engage me if you're not catching it ask questions okay I know from both of you this is not your major and I'm fine with that indulge me only asking fifty-five minutes of your life the class will go a lot better if those 55 minutes are spent with you being engaged enjoying by actively learning raise your hands engage you will have a better experience with all this okay guys let's get back to this here we are we have a bus topology I put a signal on the wire every other device connected that main wire is going to get the signal collisions are going to happen because this device also wants to put voltage on the wire what happens when you guys take two batteries and put them in series what happens the level voltage if you will it's going to go up right and each time you add voltage on the wires going to go up and up and up and we're going to have we won't have the ability to determine what's a one or what's a zero in fact if we're using fast ethernet and Manchester encoding we're going to do with varying levels as voltage changes from 10 volts to 12 volts to 12 volts to 15 volts I'm going to say that's a 1 so as I'm increasing my voltage I'm getting once as I decrease my voltage what do you think I'm going to get I'm going to get zeroes right I only have ones or zeroes so as I increase voltage I get one followed by another one followed by another one as I'm changing the phase as I'm increasing as I'm decreasing I'm going to get zeros which makes it really nice the problem is if this device is adding voltage to the wire and this device is adding voltage then all it's going to look like as a bunch of once nothing's ever transmitted with a bunch of ones we're talking about pioneers we're going to zeros and ones we're going to have a combination so what's going to happen before this device goes to talk to any other device it's going to do this sees the carrier right the electrical signal carrying the bits s is the ability to sense if there is no voltage on the wire and by the way there's always going to be voltage on the water why is that interference we're talking about copper wires copper wires will pick up radio frequencies is it's called the induction effect so there's always going to be voltage depending on how sensitive your network interface card is and the faster the bandwidth gets like one gigabit 10 gigabits the more sensitive the card becomes hence the distance of these technologies might be limited this is why local area networks have a larger bandwidth but their distance geographically it's smaller less interference alright so we look at this we know there's going to be voltage in there but we have to have a threshold we're going to ignore after like 5 volts anything andra 5 volts we're just going to ignore it so when this thing tests the wire to see if there's voltage on there it says oh it's less than 5 volts it must be clear if it's clear it's going to transmit correct all these other devices are going to get it washed you it's obviously going to take time for the message to go from here as well as to go to there from this device correct how long does it side to wait if it detects that there's no signal could it just be because it hasn't gotten it yet I mean at this moment I'm sending it out and this device says hey there's no signal and this all happens in milliseconds but a millisecond maybe eternity for a computer for us it's a blink of an eye but a lot can be done in a millisecond when it comes to our computer power so obviously they both detected there was no carrier they go to both transmit and now we're going to have a collision correct that leads to the next part by the way the MA after that si si sorry si s ma carrier sense multiple access right multiple access is already given to us in bus topology at least it's to the next part collision detection so this is the technology that we implement when it comes to multi access networks we say get rid of control on networks with smaller users what are the chances of collisions actually occurring verse networks with a lot of users the more users you have the greater chance of collisions correct seems to make a lot of sense I mean this is why your insurance rates are higher in the cities than they are in the country correct so here we are larger areas many users we have to be more controlling this is also why cities have more traffic lights than we do in the country so fewer people we say get rid of the controlling mechanisms why because controlling mechanism slow us down because they add all this overhead does it affect performance that depends on the number of users you're doing this balancing act the more control you have the more overheads going to be there the more users you better have otherwise you're going to be sacrificing performance so the deciding factor to implement this deterministic approach verse non-deterministic really depends on the number of users in fact remind me what was the definition of a local area network how do we define a local area network there's three conditions geographical location or the size or the number of users if you will the purpose and then it's being managed by one by single organization or by administrator that makes sense guys these networks are usually implemented or so these topologies are usually implemented a local area network so now we're dealing with the collision detection who's going to detect the collision if this goes out remember the sequence is also going out this way is also going up here and it's going to carry all the way out to the end we're going to detect it how do we know the collision has occurred we don't deal anything nas with yet that's the transport layer all we're doing is frames and all all my interface cards are getting these signals and they're jotting down ones and zeros they're all jotting this down so the question becomes is how does the computer know that there's a collision occurring one they get all these zeros and ones that aren't adding up to anything and then have this increase unruhe Dix the amount of voltage going on because remember a collision occurs when two or more devices are talking at the same time right we add voltage what happens when you add voltage it's going to increase because your normal operating range for frequencies should be like between five and twelve volts depending on the type of mediums I'm just giving you guys a hypothetical so if they're a peering between five and twelve volts anything beyond the twelve volt range must be a collision who gets detected who's responsible for detecting that any device that first to text it will be the one responsible for a collision detection how does it inform everybody else that there was a collision that occurred it sends out a jamming signal to everybody the jamming signals going to be the highest voltage possibly imagined and it's basically you telling your brother and sister that you're not listening them to anymore by closing your ears and doing what after you do that yeah exactly exactly so you guys did that too right I'm listening what are you doing to the airwaves you're jamming them up so they can't talk so they're all going to get that and then they're going to reset their timer they're going to call this to back off timer because it'd be like okay wait a minute let's let this clear again and everybody's going to have a random timer then they're going to go back and recheck it different intervals correct the person with the lowest random timer is going to be the first one to transmit if they have something to transmit clear about that so this one might be the first one at the lowest timer but has nothing to transmit so the what happens to the pathway it's clear correct there's no signals this might be the second person it has nothing to transmit so now we're back to us we do have something to transmit before we actually go and do that we're going to check again to see the signals available if it is we put it on there and away it goes this whole concept the back off timers jamming signals are all involved in Ethernet standard the important thing is when you have a collision it really screws things up it affects performance because now you have to re transmit but not only you have to retransmit but you have to wait and you're idling and nothing's occurring on your network so what can we do to try to avoid that all together we implement a point-to-point network so this is a topology called PPP it's like me having a conversation with you do I have to worry about anybody else interrupting me if it's just the two of us in this classroom the doors are shut no think about this conversation if I'm talking what are you going to be doing alright we spoke about this on Monday's class and that is we call this half duplex right it gave you my driveway as an example it's only wide enough to fit one car down through it so if another person comes in the opposite direction they won't be able to leave so we can overcome this because it always depends on the type of medium we have if I add one pair in there then I'm just going to have the ability to do half duplex but if I add two pairs in there then I'll have the ability to do what that means I can talk while I'm listening correct will I have any collisions to worry about hence the media access control methods are negated I don't have to worry about instituting control or dealing with csma/cd right yes Michael carrier sense multiple access the CD at the end of it stands for collision detection that's where that jamming signal comes in and the back off timers are available whereas in point-to-point I eliminate that concept now here I'm showing you guys a physical point-to-point connection and it only makes sense it's one node connected to another node when do we typically see this why did you guys ever see me draw a little Z yeah wide area networks we're connecting routers together correct and that's critical because most the time were access the Internet not only among actresses on the internet but another device is accessing the Internet as well or another network outside of my local area network so this is going to concentrate into our gateway correct so there should be theoretically a lot of traffic amongst these routers and we want to try to do minimize the chances of collisions now can we take this concept and join it with this concept and this we're going to call a hybrid or a star topology I want multi access why do I want multi access because it allows me to scale my networks real easily I come into it I plug in a device away I go an appoint appoint network I can't do that in fact remember we have turned out turn off our routers at a special interface card like a serial connection and boot them back up I don't have to do that for every time I add a device on my network in fact that was one of the biggest issues about multi access topologies I have to shut down the network if I want to add a new device on it because I'm going to literally cut this coax add a T to it because in order for me to use this network I have terminators at the end of if I don't have these terminators then the signal will bleed off and will propagate back there for my mechanisms will fail so in order for me to add a new device on this network I've got to shut it down and I have to tap into the clocks the main feed to hook up another device to the network and in a busy Network like Wall Street or time is money you can't shut down a network the other problem with this network is if there was a crack even right there I might have problems with it and it becomes hard to diagnose hard to troubleshoot so I want the flexibility of being able to add many users to it with the benefit of negating or minimizing the ability of collisions and last time I showed you something called a star a star topology obviously is going to look something like this with nodes on the outside but what's going to be in the center and you'll be able to remember this because the letter s in star ass which is going to be intermediary device you see how the intermediary device is really affecting the behavior of your logical topology between each of these nodes and the switch I have a private connection a point-to-point network we call a point-to-point protocol this is the logical point-to-point okay because in essence it's a multi access because any of these devices can communicate at the same time on deterministic without knowing it so it will require that we use CS MA / CD however if each of these nodes were having a private conversation with the port of the switch when they go to check their wire it should be clean the carrier should be available why because the switch acts like a buffer each port has its own memory so you when you transmit to your router in order to get to your router you must use your switch to prevent collisions what must the switch understand to prevent collisions what must the switch understand what did the hub not understand remember hub would give us this type of network what did the hub not understand and more importantly what do we use to make up those Mack tables or those are tables the frame address yes since the hub doesn't understand frame addresses they can't identify you that's like going to an English class where they don't know your name and they just address you guys as everybody right you'd have a better experience if they know you by your name than you if they did by a number correct likewise here the hub did not know you by your physical name so it said hey I'm going to say it I'm going to send it to everybody because I really don't know you don't care to know you what does a switch do it assigns or develops a Mack table and that is it takes every one of the physical addresses and associates them with a port a little bit different than an art table we'll talk about Arps later why would you switch want to do that so they can create a logical point-to-point between the device that's communicating with creating zero collisions or minimizing collisions so I transmit my message my message is stored in a buffer area the switch looks at the frame address and says oh it's destined to go out here once that port that comes available I will pipe it in there so you see that you have a hundred percent bandwidth between here and there no collisions as long as your memory on your switch doesn't overfill that make sense the switch will say oh this line became available let me send it out so the switch is basically coordinating events so collisions don't occur will a collision happen on this network when's the only time will all the devices on this type of topology will receive a frame at once when it's doing a broadcast so our RF is broadcasting remember how to develop your op you have to broadcast that out to everybody so that's an example of a broadcasting or what we're broadcasting is using good about that so we do have to implement csma/cd but how often is it used to detect collisions very seldomly this gives you the best of both worlds flexibility because I can add a device to my network without shutting it down can I troubleshoot this a lot easier remember the steps of troubleshooting ping yourself ping your IP address ping another device on your network if I'm having an issue maybe I'll unplug this and plug it into another port maybe it's the port maybe it's the wire but I'm able to narrow it down just to that one segment you see the difference between logical and physical topologies now logical is all about the behavior allows us to understand the media access control what is media access control it depends on what type of topology we are implementing and they're basically telling us about do we have to share the medium if we have to share the medium how we're going to handle collisions am I going to control it adding overhead to eliminate collisions or am I going to be really nearly about it to say anybody talk it's not a big Network a lot of traffic isn't happening so the chances of collisions are going to happen we're going to be very slim do you put a traffic light middle of nowhere and say hey everybody has to stop or do you say hey let's remove it all hopefully these people have common sense and they don't happen and great and where I used to live is this little country between Allentown and writing we actually my grandparents lived out in the country and there was a t-intersection I always wondered this always wasn't very busy you went under a highway and it looked like this woopsie acts like that and our house is like right here and this was a hill if you will where would you put stop sign on the perpendicular street there wasn't any there wasn't any stop signs at all I went home couple I guess a year ago maybe for the holidays and they did put a stop sign put it right here blew right through it for 20 years I was just in a collapse I'm being there blew right through a cop pulled me over what are you talking about stop sign and look back and go well Dan there is one the reason why they didn't bother with putting any stop signs originally is this is very open very flat you can see in all directions and they assumed if you will that if you see a car here and you're here you're going to give the right away right this person here would probably have the right away and go up and maybe instead of putting a stop sign there they should have probably put one a yield do you see the different controlling mechanisms and a stop sign I actually had to stop for four seconds in a yield it was implied before we would look there's nobody there go now the yields telling me the right away that if the yield signs right here I have to give away to that person right I have to yield to that person so maybe they yield should be over there it would only make sense but what if I told you more cars would go that way then this way now does it make sense where they put the stop sign still doesn't make any sense why they didn't put a yield sign in there but nevertheless it's all about trying to control the access here there's a shared medium between these two segments I need to avoid collisions if a collision does happen I need to find a way to manage it okay question why didn't they put it where because there's more traffic here than over there so they figured they put a stop sign here for the very few people that are ever bothered with it and then these people can keep on coming their way I mean would you want a million cars to stop every four seconds or would you only want a thousand cars to stop yeah hey I know and that's that's the whole idea it's all about how you want to implement this so let's get a sheet of paper out I've got a little exercise for you guys to do to show you how this is all occurring by the way the data link layer is filled with a bunch of standards a bunch of protocols to be able to accept all the different types of mediums out there and that's one of the benefits by going through a modeled approach like we are right now is that as newer technologies are being filled like better copper fiber optics microwaves I mean that's the biggest area right now as we go in from like 2g to 3G to 4G you call it LTE or whatever you guys want to call it but nevertheless the area of Technology is and Wireless and that's rapidly changing all the time the only reason why we have to buy new phones it's not because we have to buy install new software on it but because the newer wireless technology requires a different antenna correct and and then 10 is a physical device but naturally did we change the software about it did Apple when they release their iPhone 5 develop a new operating system to handle the LTE now they did it an update but it still works on the old phone correct the really only reason had to buy knife iPhone 5 to get to the new 4G is because the new antenna that was all but the software was all that high-level stuff the IP address is eight phones have in fact the only thing that makes iPhone 5 really neat is that they got rid of the analog signal and all your phone traffic is all digital it's all voice over IP address it's all voice over IP so there is no more analog voice anymore when it comes to the newer LTE phones it's interesting that's another thing for the fire speeds yeah it's a new lightning bolt but that's beyond the scope of this I'd be like a hardware class but I'm just trying to tell you the data link in the physical layer have to work together because the antenna is the interface that actually puts the signal on the wire or in this case the air and takes it off right but the linking between the two allows the data link layer to adapt and evolve without changing the upper layers like layer 3 layer 4 and layer 5 so what I have for you guys is a little a demonstration of how we put mediums on the air sorry bits on the wire so I call this data link in action let's see f5 I think that's going to be a good location and let me give you a breakdown on what's going to happen you guys have a sheet of paper you're going to write down what you see but you're not going to be writing down black white black white every time you see I believe a black screen it's going to be a zero and every time you see a white screen it's going to be a one all right because remember the way we transmit is using binary so the data link layer is taking that binary stuff and propping it up to be delivered to be put on the medium we good about that hence the word link it's linking the two sections software with hardware so here we are ah the data I'm going to be sending going to be text so basically I'd be taking ASCII and what is asking useful or for those that have computer Central's yeah we're going to convert letters to binary write text to binary the transport layer I'm going to be signing guys a port address you'll find out what that is later what a port address is used for to identify the software that's being used for sending and then the service used to receive it we good about that and then the network layer stamps what IP address right the IP address is logical that's the one that will change as you take your device from one network and plug it into another network the data link layer will stamp what for and where is it used for yeah is going to have a physical address if we're using Ethernet we call it the MAC address it's 48 bits and that's used for local delivery what connects us together the medium the medium is going to be the air now instead of using sound for transmission I'm going to use light so when the screen flashes to a blank white screen it's going to be a 1 it's going to stay there for five seconds the only reason why I'm leaving that in five seconds to let you guys know that you have time to look up the screen look down in sheet of paper and draw a one then you can look back up right and black is going to be zero we good about that this is the software so this is me installing the drivers in your operating system up here letting you know when you see white put a one when you put when you see a zero or you see black put a zero all right like I said the meaning that we're using is the air uh there will be csma/cd being implemented in other words if you guys whip out your own powerpoint and start flashing me with your lights that I'm gonna be like wait man I got this going on here don't be screwing up anybody else alright so I'm going to be looking this over making sure that the carrier is available right so there's no lights flipping back and forth all right you guys ready so I'm going to check the medium nobody's flashing me and I'm not flashing you so the medium isn't it I know you guys are thinking something else so alright now here and that would work too I guess close no close one zero I think it's just more appropriate that I use the black white all right so yeah I don't know if I'm the one doing it so you guys ready but now you guys should see why it's important to set a clock rate for a serial connection because without that five counting down when it goes from a white screen to a white screen you wouldn't know like in this particular case we went from black to a black but how many bits that I put in it's like being a fiber card yeah but trust me it happens a lot faster than this you guys when you have time to write down one in fact it happened so factor I could even detect the change in lights when you guys think of 1 million bits per second that's like this flashing back and forth a million times in one second that's what fastethernet okay finally thank you all right before I actually get into how to check and verify that data does was in tact transmit it how did you guys know that it began I had to lay down some rules but if I come on next Monday with this PowerPoint and all I do is go right into the black and whites how would you know that it began there I mean you started writing down did you guys write like a 1 for the first one and then you write a 0 for the second one then a 1 for the third one then a 0 for the fourth one okay do you see that pattern ahead you guys go on off on off alright but what if I just jumped right into that and or you came late into class and you start looking at the screen and you tell your friend well you don't tell them anything you just came late into the program but you know the rules you start writing things down how do you know that you grab the beginning of the frame and not in the middle of the frame because this point this is all that's happening right for all you know the very first 30 minutes of this class the very first 50 minutes of this class was one one one one one one one and then all of a sudden I changed the lights correct how do you know where the beginning is and where the ends at because that's the same thing your interface cards face with all the time exactly see that pattern that I developed for this this is called a preamble or the start of the frame I created some kind of mechanism not Ethernet likes doing this because watch the the tone or the beat here now if you guys have seizures you're going to be like but nevertheless you see I did that at a certain rate I went on off on off on off and II thought I might do that eight bytes so eight times eight would be the number of times I would do that to establish the clock rate so there wouldn't be a clock up there and that's why when you guys configure your Ethernet interfaces for your router you don't have to set up clock rate because it has a preamble but on the serial side you guys do why because we want to take that framing stuff and try to minimize it by reducing the overhead I should have better performance correct I don't want to worry about collisions I know that when I put something on this wire it's going to get to the other end when you start receiving something that's the beginning of it we stopped receiving something that's the end of it the clock rates set to determine when you have a 1 1 1 isn't is it a one now is it a one now no different than I set up the five so let's do this the very end of it you guys should have an inverse so go to the very last four bits and it should end with a one then it should go with 0 then a 1 then a 0 all right if the first 4 bits was a 1 0 1 0 and we call that the start of the frame and the last 4 bits are 0 1 0 1 what do you think that's going to be called end of frame ok the bits before there ok so we have 4 there I got to make sure I count these up correct it's going to be a 0 see a 1 in the 8th sorry 16 and 8 a 4 a 2 and a 1 so what you guys will have is last four is going to be a 1 0 1 sorry 0 1 0 1 those are the last four the next bits 1 2 3 4 5 6 the next 6 bits you guys have that ok this is called the frame check sequence sometimes it's written as crc cyclic redundancy check if you were to convert that binary number into decimal you get 23 depending on the standard and what they implement it my standard the way I transmit it with you guys here as I said you should have 23 ones anywhere to the left of the last ten bits so if you count up your number ones anywhere to the left less 10 bits is you have 23 ones that's my formula if you do not get 23 ones and they don't have to be in a row they're going to be scattered all over the place you screw it up throw away your work and while to start over again because this is the only way that another device that's receiving this frame determines whether or not transmission was successful this is the only way we verify that do you see I did this at the very end of this remember the only time and the OSI that we add a trailer is at the data link layer and the trailer has two major important fields in it the FCS and the EOF frame check sequence and the end of frame the end of frame is designed to tell me to stop recording then run a CRC or FCS the frame checking to see if it was approved now it just tries to check Darren it's not perfect because you could have made a mistake put a one here and then a zero there they could have been switched and you still had 23 ones correct so it doesn't correct the errors and it's not foolproof it just is some kind of mechanism that before you pass it up the stack accept it or not is that clear now that the integrity of the information was verified what do you think the interface card is going to do next it's going to go back to the beginning of the frame and it's going to drop off so you guys should have these types of 1 zeros so this should be the final answer but you guys probably have them strung across it's going to go back to the beginning of the frame in fact let me show you this two series if you guys did this correctly it should look like this this part is the start of the frame right this part right here is the destination MAC address notice it's all once it's a broadcast address I'm sending to everybody if this was like 1 0 1 1 0 then only the person whose physical address matches that binary pattern would accept this that means everybody else in this class would discard this frame you matrix got them writing all those zeros and ones you tell me throw it away that's what your computers do all the time when they're in a multi access topology like bus would that happen when you're in a star topology if there's a switch there no a switch would get it would check there are there Mac table match this address with it and say go out this port but nevertheless the switch is recording everything that comes in in that port all right we will finish this up on Monday and I'll be assigning chapter sevens assessment Monday evening so check this weekend make sure you guys go over the powerpoints finish up data link and I'll wrap it up with this there will be a lab post and make sure you guys check that you guys take care and I will see you next

Channel: Nicholas Andre

Views: 29,118

Rating: 4.9008265 out of 5

Keywords: Data, LInk, Layer

Id: zynndYNic2I

Length: 53min 9sec (3189 seconds)

Published: Thu Nov 01 2012