The OSI Model Demystified

hello again as you know I'm Eli the computer guy over here for everyman IT today's class is the OSI model this demystified now if you if you've done networking for any length of time or if you plan to do networking you probably have heard of the OSI model and almost everybody hates the OSI model for some reason they they hear of the OSI model and they all they think is that they're just going to have to do a lot of memorization and it's going to be a big pain in the butt honestly as a technologist been doing this for a long time the OSI model is an incredibly useful tool if you take time to learn the OSI model it makes life easier for you to understand all the OSI model is is it's just a logical model for how network systems are supposed to communicate to each other so basically all the model does is break down the different components of network communication and kind of slot them into layers so you may hear the physical layer so the physical layer is all the wiring all the the physical stuff that connects computers together you then have a layer to the data link layer this is the layer where we're switching occurs so all those switches that you plug your computers into all all of what they do resides on on layer 2 so basically the OSI model is seven logical layers that allows you to think about a networking problem and then and then figure out how to try to try to fix the problem if you understand how these layers work it's much easier to focus in on what the individual problem is of in a network system and then and then fix it programmers they do it the opposite way instead of fixing things they use the OSI model to narrow in and figure out exactly what they have to build for like I say it's not that hard it's not that complicated this will be a short and sweet simple little class this class on the OSI model is focused from the IT from the technologists standpoint so for for troubleshooting and such if you're a programmer well I don't know take a programming class we're not going to deal with it here basically I'm going to explain to you the OSI model the way that I actually use it and show you how it helps you troubleshoot problems and zero in on what the the problem you're having is so so give me a second and we'll get the whiteboard over here and we'll we'll dig into this the OSI model okay so in the OSI model this is just o s I model okay the OSI model is made up of seven layers the layers from top to bottom are for what is closest to the end-user so what the user is interacting with all the way down to actually shooting the information out on the network so the first layer the top layer layer seven is called the application layer now the application layer this is the layer that the user is actually interacting with so things like Firefox or Internet Explorer if you're going on to a a web site the application layer for that whole communication with the website is Firefox Internet Explorer Explorer Safari Chrome etc the this is the layer that the user is actually interacting with the information on so if you're doing email this would be the actual Outlook application underneath that is what's called the presentation layer this layer is basically the layer that the operating system is on so you interact with the application layer that sends information down to the presentation layer which is generally they did like I say the the layer that the operating system works on underneath the presentation layer you have what's called the session layer the session layer is the layer that deals with the communication creating a session between the two computers so if you are going to a website basically your computer at the session layer has to create a session with the web server that you're trying to access to get data from so that's what the session layer does the session layer actually creates the session between your computer and the computer you're trying to get information from underneath that is called the transport layer the transport layer this layer is what decides like how much information should be sent at one time so when you're communicating with a website this is the layer that decides how much information you will communicate to the website how much information we communicated back this deals with the transport of the data back and forth you then have the network layer the network layer is the layer that routers operate out so you know your your router that you have at home or you know if you've been doing you know networking technology you know a router routers operate at the network level so your IP address is at the network level underneath this make sure you're still on the screen yep you have the data link layer the data link layer is the layer that switches operate on so again you know all the computers on a network get plugged into a switch so that they can talk to each other that happens that that layer is called the data link layer and then underneath that final layer layer one is called the physical layer physical the physical layer is literally all the physical stuff that connects the computers together this includes patch panels patch cords the cabling etc so basically if your computer isn't actually connected to the network if you don't have a patch cable running from your computer to the network then it's a layer one problem your your network cable isn't connected if a cable gets cut or if a cable is miswired that would be a physical layer problem that would be a layer one problem so basically I mean this is this is pretty simple you all of you got is seven layers you've got the application layer you've got the presentation layer you've got the session there you've got the transport layer you've got the network layer you've got the data link layer and then you have the physical layer so again how this works out you know slow this down make sure you guys understand is when you're dealing with physical layer problems physical layer again is all the wiring and cabling so again if a pack cable gets unplugged that's a physical layer problem there's really no there's no there's no fancy configurations here there's no dos prompts there's no Linux prompts there's no fancy stuff basically this is this is cable this is cat 5 cable that's what the physical layer is as one senior network administrator told me 95% of all networking problems are layer 1 problems basically somebody unplugged the damn cable pretty simple now above the physical layer so this is this is how things are connected above that is this the data link layer this is where switches reside so we're going to have a class on switches to explain how all this works better but basically the data link layer this is where things called MAC address addresses reside these are called media media access control these addresses every single networking device in the world has an individual MAC address assigned to it that is all part of the data link layer there's something called ARP ARP address resolution protocol this is again how switches work how they actually operate we will have a full class on things like ARP just understand everything about how a switch operates resides on the data link layer level so that's layer 2 so you may hear the term layer 2 switching that that's layer 2 switching above the data link level layer is then the network layer this is where things like IP addresses reside so anything you do with the TCP IP protocol with IP addresses all of that happens at the network layer so if you put in the wrong IP address and then you can't get to wherever you want to get to that is a network of layer problem if the router dies that is a network layer problem so anything that involves your IP address would be a network layer problem above this is the transport level this has to do with things something called windowing so when doing is a process where computers send information back and forth we'll have another class on that one but basically the transport level again it decides how large a block of information should be sent how long should the computer wait before it receives any acknowledgment that information was received sent or received so that's the transport layer level honestly as IT people we normally don't play with a transport level very much it just kind of either works or doesn't work for us above this is the session level again every time a computer communicates with another computer they have to open up a session they have to be you know communicating with each other so again if you're if you're going to a bank's website to look at your your your bank account information when you go to that website you create a session between your computer and their computer that all of that happens in what is called the session layer now above the session layer is again the presentation layer basically the presentation layer all this you should really think about is basically the operating system so it's a lot of configurable information but it's not generally what the user is actually playing with the user is normally up on the application layer again the application so whether they're using outlook whether they're using Firefox Internet Explorer Chrome whatever the application level layer is the actual application that the that the user is interacting with to send information back and forth so again this might be Skype it might be Chrome it might be whatever now like I say in in the real world what most of us deal with with the IT level is a you know you have application layer problems so what does this mean this means Firefox is outlook is corrupted there was a Mis configuration etc so so if you sit down and they say my Firefox isn't working and you click on it and it's corrupted well that's an application layer problem then you have the presentation layer problems generally this is device drivers are messed up something within the operating system isn't allowing the user to go onto the internet so maybe they don't have a the right security protocol etc that would be a presentation layer problem session layer again as IT people yeah we sometimes deal with this we don't deal with this a lot the session layer again is what actually opens up the session between the two computers now if you're dealing with a website if you administer a website you may have settings within your web server that are causing problems with this session layer things like you know PHP config files or the Apache config files if those are not set up properly your sessions may not be able to connect so so basically the user can get to your web server they can open up whatever web page is on that server but when the script runs to open open a session let's say to look at their their their bank account information for some reason that isn't working properly so that would be the session layer again transport layer we almost never deal with this was more important back in the days of dial-up modems for the standard IT person basically again this deals with you know how much data is sent back and forth you don't use it a lot the network layer again is a huge one though this is where routers reside so when you're dealing with like I say IP address is default gateways subnet masks DNS all that crap that resides on the network layer this is where your IP address is so if you hear about net our layer 3 problems layer 3 is the routing layer underneath that is the data link layer again this is this is switches there's a whole bunch of stuff that can happen with switches but again you know anything you plug into the switch the switch isn't working properly information isn't being set properly sometimes the net work is slow that can be a switching problem again address resolution protocol and MAC addresses are at this layer we will have a class on switches I'll explain that more and then the physical layer again you know if somebody unplugs the little patch cord or if somebody didn't punch it down they've cabling properly that is the physical layer again all this is all this is is seven layers all all it's trying to do is what I want you to understand it license a professional tech you know I've done this a lot in the real world what models like this try to do for you as a technician is when you walk in to a problem when you walk into a situation you have no idea where to start and this this tries to give you just a model for how to think about the problem so you walk in an entire server room is down or or nobody can get on to the internet or you know any of the hundreds of problems when you go in as a real professional technician this is not going to be that some twelve-year-old can't get onto the Pokemon website it is going to be that email is not routing properly you know thousands of users cannot get to a web server that they need to get to information is not being sent properly and when you walk in that situation it could be any of a thousand problems you know even after you've done this for a long time when you walk into a situation I mean it could be 10 20 30 different reasons of why the problems that are occurring or a current what having a model like this does is kind of tries to allow you to try to break down what the possible reasons could be and then and then go about fixing it so if you understand this it allows you like I say just to give you a beginning point and then give you a place to start troubleshooting from so so if you go in and you know the first thing you do is if a server you know people can't connect to the server you look at the back of the server and make sure that little little connector connecting light is lit if that's lit that means you probably don't have a physical layer problem you know you go through it that way like I say this is something that it'll take you a little bit to get used to but but it really does work but this this is the OSI model again you start the highest level the top layer is where the user is interacting with it so the application layer again Firefox Chrome presentation layer basically the operating system a session layer the connection between the two computers transport layer deals with all the transport how big packets are that kind of stuff Network layer this is where your IP address is resolved Network layer problem it's their IP addresses that a link layer this is where all the switching are and MAC addresses happen and then physical layer all the physical cable stuff it is really that simple so that that's really all there is to the OSI model the OS my model is is just it's a logical model to allow you to think about problems and try to decide where where the problem may be like I say is you may you may not know which specific router is the problem but if you can narrow down that you're having a problem with IP addresses then you know that it's a layer three problem again you know with layer one you know if you don't have a little a little connector light on the back of the computer you may not know where the wiring got cut but you know if there's no connector like the wiring got cut somewhere you know with the application layer if everybody's screaming because you know they can't get to the website or whatever and you go in and you see the problem Firefox or Outlook or whatever is corrupted you know it gives you an idea of where to start you you don't know the specific problem but you have a better idea of where the problem is is basically using this process to divide everything down again the OSI model it's not a huge issue I know a lot of people pull their hair out when they're trying to learn this because a lot of times you have to learn this for tests again I had to learn this for certification but if you do have to learn it don't just forget it at the end of the day I mean it really is useful I say as a twelve-year technician I use this this is something that allows me to break down where the where the problem is that I'm looking at again I may not know what's which is the problem but I can narrow down that there's something going on with a layer to the the switching so that's basically all the OSI model is I hope you understand it like I say very useful very very very very very very useful as you know I'm Eli the computer guy over here for everyman IT as always I hope you understood this this class again important class very very useful I look forward to see you at the next class

Channel: Eli the Computer Guy

Views: 1,583,704

Rating: 4.9017472 out of 5

Keywords: osi, model, 720pH

Id: HEEnLZV2wGI

Length: 18min 41sec (1121 seconds)

Published: Fri Dec 10 2010