Understand the OSI Model

0:00Hey friends and welcome to the HPE Aruba Networking Certified Associate course here on CBT Nuggets. So in this opening skill what we're going to be talking about is some foundational topics related to computer networking. More specifically we're going to be looking at something known as the OSI model. So let's discuss the seven layers within the OSI model. We'll discuss what each one actually does and the first thing we'll begin talking about is something known as the application presentation and session layers. So with that said let's just get to it.

0:00So friends to kick off this skill, what I want to do is to first talk to you about the concept of modeling and And no, I'm not talking about a catwalk or a runway type of modeling. What I'm talking about here is the ability to model How data is actually transported across a network? So say for example you on perhaps this machine here wanted to talk to this server over here. How does the data actually get from Your machine all the way through all of these different nodes end up in the server whereby the server can understand your request And ultimately relay it back with the correct response directly to your machine without any particular issues Well as you can imagine for this to actually happen There is a lot of complexity going on and in order to understand the complexity We want to break down each component into individual steps and this is what we 're modeling We're modeling the actual steps taken when we send data from point a all the way over to point b Now the most well-known model that was ultimately developed way back in the 1970s as the OSI model Now all OSI stands for is open systems interconnect And basically what we do describe the connection from say for example

1:15This device here to another device within a network It has been logically broken down into seven distinct steps Now let's plant a flag before we go any further. So we have the application layer The presentation layer followed by the session layer the transport layer the network layer Then the data link layer and then finally the physical layer So what we're going to do in this particular skill is we're going to talk about each of these individual layers And describe what they actually bring to the table how they actually play a part in facilitating this connection from here All the way over to here now very very important point to note These seven layers they actually are not how things really operate in reality So upon hearing that you might think so if this is not how things are actually done Then why even bother doing this at all but quite honestly this is more of a helpful utility Whereby we can easily identify layers in isolation So if you want to troubleshoot a very particular issue say for example, you say hey I'm going to troubleshoot a transport layer issue That refers to a very precise portion of the communication very very different from say for example examining a data link issue Now the way things work in reality is actually more closely modeled via something known as the tcpip model

2:32Now the tcpip model is really quite similar but it happens to combine some of the layers within the osi model together So in the osi model we have the application layer the presentation layer as well as the session layer And the osi they're all subdivided very very clearly whereas in the tcpip model all of these different layers are simply just Aggregated and referred to as the one application layer. Okay. So what does this layer actually cover? Well, again We'll talk about all of these individual components as we progress throughout this particular skill Now the next layer in the osi model happens to be the transport layer and the good news is is in the tcpip model This one is still the exact same basically the model the exact same thing similarly With respect to the osi model we happen to have the network layer And this is kind of the same thing in the tcpip layer But it's known as a different name It's referred to as the internet layer and then finally when it comes to the last two layers in the osi model This is the data link layer as well as the physical layer These last two in the osi model are ultimately combined and just referenced as the network access layer in tcpip So ultimately in the tcpip model what we get is this the application transport

3:47internet and network access layer and this much more closely models How things are done in reality? But like I said the osi model is still very very important It's used all the time to reference particular problems within troubleshooting scenarios It gives us a model to much more precisely diagnose particular issues So it honestly is very important that we understand how each layer actually operates So to get things kicked off let's go and begin first talking about the application presentation and session layers

0:00Alright friends, so let's begin talking about the application layer within the OSI model. So the first thing to state about the application layer is that it's referenced as layer 7. This is the highest layer within the OSI model and you can kind of conceptual ise this as the way the end user, such as you or I, actually interacts with the network itself. So what exactly do I mean then? Well imagine you happen to be over here, let's say this is you on your little laptop or computer and you want to go and access a web server out on the internet. How on earth would you actually do that? Well what you could do is you could open up your web browser and quite simply you just type in the page that you want to go to and suddenly you happen to reach it. Now there is a lot going on in that single transaction there but the way we interacted with the service itself is via something known as HTTP which is the hypertext transfer protocol. More accurately though, you probably will use in modern terms HTTPS, this is the secure version of HTTP with encryption. Alright start the bus there , start the bus.

1:06This particular protocol, HTTPS, this is actually operating at layer 7 of the OSI model. So whether you happen to be using Firefox or you happen to be using Chrome or Edge, the same application layer protocols are ultimately in play, HTTP and HTTPS. But quite simply hyper text transfer protocol is not the only layer 7 protocol that we can use. Say that we as the end user want to access email services, okay? So what do we do? Well we can use things like Outlook or you can use things like Gmail and ultimately under the hoods they happen to use a whole bunch of different protocols. Now we don't really have to talk about these different protocols, certainly not quite just yet but just understand that these protocols are actually in place. So say for example we have SMTP which is the simple mail transfer protocol. This is how emails are actually sent between servers and the way we can actually receive our emails is going to be using things like IMAP4 or maybe even POP3.

2:04Now once again, despite being very very different from hypertext transfer protocol, all of these protocols right here they are also operating at layer 7 and in a similar way if you want to maybe transfer files from a particular server to another server over the network once again. Well there are even more protocols we can use such as FTP which is file transfer protocol or we could use SFTP, the secure version of that protocol or similarly we could use SCP for secure copy. All of these protocols allow us to do just that copy files from one location to another and again these particular protocols are once again acting at the application layer.

2:45All of them essentially are our bridge as the end user to the underlying network that we want to use. Let's talk about the next layer. The next layer as you may imagine is going to be layer number six. Now this one here is referred to as the presentation layer. So what does this one do? Well let me tell you. So again let's say we are over here on our laptop here and we want to reach a web server over here. Let's imagine that this is google.com or something okay. We know via the application layer we can use HTTP or rather HTTPS as we would in the modern day and we can ultimately establish that connection and get the data back but the data itself has got to be formatted in a particular way and we've also got other considerations about how we want the data presented to us.

3:31Would we like say for example the data to come back in an encrypted form? Well if that is the case the data is going to be encrypted on the wire and then decrypted at the end points so that ensures that as the data is actually traversing the network it's not readable to any type of eavesdropper but it also means that the data is readable to you on your end station when you get it back on your browser. The handling of encryption and decryption well this is going to be handled via the presentation layer. It's how the data is going to be presented in a similar way. Perhaps we want to have the data compressed. How on earth is this going to be handled once again? This is just yet another task of the presentation layer in a similar way. What about the data actually being formatted? Say for example is the data going to come back in ASCII or maybe the data is coming back as a JPEG maybe the data is going to be presented as an mp3 or maybe even the data is coming back in some type of data format like JSON or maybe even XML. Now it doesn't matter if you know what JSON and XML are absolutely not but not focused on that at all. These are just file formats that are very popular when machines want to talk to other machines. That detail isn't actually what is important.

4:45The important part is that whether or not we're using JSON or XML or we're dealing with jpegs and images or ASCII and regular text. All of that stuff along with encryption and compression this is once again all handled via the presentation layer number six which then leads us to the next layer that I just want to talk to you about and you already know the number this is going to be layer number five. So this one here is known as the session layer and the name itself really is quite descriptive. It's going to allow us to manage connections between the two endpoints. So this device here talking to this device here. So in fact you know what let's maybe say you are here and I am over here okay. We want to connect over the network and have some type of video conferencing call okay. Now in order for this connection to actually begin we 've got to do a few things. We've got to first establish the connection with one another using things like authentication. So we're actually talking to the correct person when we happen to be talking we want to ensure that the audio and the video is actually correctly in sync with one another and if we happen to get some type of glitch in the network and the connection temporarily drops we do not want to have to start the meeting all over again instead we would much rather just have the application itself just kind of figure that out. Everything I've just said

6:00here is ultimately controlled via the session layer layer number five. So that is our introduction into the application presentation and session layers layers five six and seven. Let's now move on and talk about the transport layer layer number four.

0:00So friends, the next layer of the OSI protocol, OSI protocol, OSI model, this one is known as Layer 4, otherwise referred to as the transport layer. The whole purpose of the transport layer is really about facilitating what is known as end-to-end communication between our devices. So really we're talking about how connections between devices are established, how they actually are maintained throughout their life cycle, as well as how we can tear down that connection once the data transfer is over.

0:31Now just for clarity and posterity, the data at this particular layer is referred to as segments. Alright, first things first, very, very important. Two well-known protocols happen to operate at this layer. The first one is known as TCP. The other one, this one is referred to as UDP. So the name for TCP or rather the acronym for TCP actually stands for the name Transmissions Control Protocol, really, really important protocol, very, very useful.

1:03The UDP protocol, this one is referred to or rather stands for User Datagram Protocol. Alright, so apart from having different names, what actually is the use case or the purpose of having two distinct protocols to work at the same layer? Well, as a high-level overview, this one here is the one we want to use when we want to establish a reliable connection. So when the data between sender and receiver, we want to ensure that the receiver gets all of that data, and if any data happens to be lost or corrupted, we want the protocol to notice this and to re-transmit that data so that ultimately it can be rebuilt at the end point and nothing is lost.

1:46When we want to do this, we're going to be using our friend, Transmission Control Protocol. Now you can maybe imagine that this is a little bit involved to ensure that all the data is always sent and if it's not, we re-transmit. The drawback of doing this is that ultimately we get a slower connection and this also actually adds overhead to the connection itself. So more data slows things down. This is the price we pay for reliable transmission. So you might be able to imagine what the benefits and drawbacks are for UDP.

2:19UDP is not reliable in the sense that it's not going to re-communicate any type of lost or corrupted data. UDP is kind of like a fire hose just going blaaaand just sending all the data all at once it doesn't care if any gets lost. It's just fire-hosing this data towards the end point destination. Now whilst this might sound a little bit reckless, it might sound sub-optimal, the use case here is that because UDP doesn't concern itself with the re-transmission of data, it 's also going to be much more lightweight.

2:50So now we have a faster connection with less overhead. Now for clarity here, you as the end user, you're not going to be concerned with picking a particular protocol. You're not going to say hey I want to use TCP, hey I want to use UDP instead. This is going to be handled via the upper layers within the OSI model. The application layer is going to handle this for you transparently. Now okay, very very important. This part here really lets you know that you're working with the transport layer. Remember we're talking about utilising particular ports, we're talking about operating at the transport layer.

3:27Now just to be clear here, we are not talking about physical ports, you know like taking a cable and plugging it into the port of a computer. This is not the same thing at all. We are talking about logical ports, these are just end software and used to ultimately differentiate between different services. So let's imagine this, okay? Here I am over here, this can be my little personal PC within my local network. And let's go with all the way over here, this server, let's give it the IP address of let's maybe go with, so I don't know 192.168, let's go with 2.10.

4:02So I over here want to talk to this server over there. Now this server actually has multiple different services that it actually supports, so it's got web services. So it's going to allow connections over HTTP. It also allows for secure web access, so it also supports HTTPS, the secure version with encryption. Let's maybe say it also supports the ability to handle SSH connections which is often used for secure management of a server, as well as a whole bunch of others.

4:35We're not too concerned about the details of what it might be actually running. The thing I want to highlight here is that I over here want to connect to this server and I'm going to connect over this IP address, what is going to differentiate between if my connection to this server, if say for example I'm using a web browser to manage the connection, how do I differentiate whether I'm going to be connecting over HTTP or I'm going to wrap the connection and encryption and use HTTPS. Well, drum roll please, the way we're going to do this is via the use of our logical ports.

5:08The connection is not just specified with the IP address, we also add on a little colon, let me just remove this, and then specify the port number, so if you want an un encrypted connection, we specify port 80. And now, we get the connection to the server, it's unencrypted and anyone can read it. If we're not happy with this, what do we do? Specify the logical port of 443 over the transport layer once again, now the connection between these end devices is going to be fully encrypted and protected from any potential eavesdroppers.

5:41Now, one thing to state here is that when I'm specifying these port numbers, that is not the full story. In the case of say for example HTTP, whilst it does use port 80, it also uses TCP port 80. Okay, this is very, very important. This is going to be a reliable connection. Any errors or any type of loss data is going to be retransmitted. This is the standard here. Similarly for using HTTPS, you can imagine it's going to be port 443, but also wanting that reliable connection, it's going to be TCP port 443.

6:15In fact, even the other example I was giving to you, SSH, this is going to be port 22. Once again, though, it also uses TCP. So you might be thinking, John, does everyone just use TCP? Is there ever a use case for UDP? And the answer for that is absolutely yes, very, very often. If you happen to be dealing with say, for example, real time data, you can imagine if you are, let's go with an a zoom meeting, okay, some type of teleconference. In this case here, we are going to be utilizing UDP for the transport.

6:47Here's the reason why. For a start, UDP is lightweight. It's fast. We want that for real time communication, but also think about it. In the event that we lose data, let's just draw it out. Once again, let's say here I am over here. Here you are over there. Let's say we are connecting over, let's say, Skype to our devices. If along the way, as we are streaming data to one another, let's say some of that data happens to be temporarily lost or corrupted. You know, something happens in the network. There's a little blip. Some data doesn't actually make it from you to me.

7:17So the net effect of that is I, as the end user, I am watching my screen, I see your face. I can hear you talking and maybe in the actual audio, I get a little bit of distortion in the sound. But you've all heard it before. The picture resolution maybe dips a little bit temporarily. The audio becomes a little less clear. But then the connection resolves itself. The HD comes back. The actual audio comes back, nice and clear. Nobody actually cares. What use therefore do you really have for retransmitting that lost data?

7:48Think about it. You and I are having a conversation in real time. Let's maybe say half of a word that you said gets cut out or clipped. The conversation is continuing on. Does it really make sense to retransmit that data and get the missing word five seconds later and then just insert into the communication. Doesn't make much sense at all. So in this case here UDP is actually the way we're going to handle this end to end communication between these two end devices. Speed and low overhead is all we're actually after.

8:20Now here's another example. Let's maybe say that I want to access this particular server. Okay, let's maybe say it's a web server. Now this web server is let's maybe say we just know the name of the actual address. So let's say it's called server one dot CBT nuggets dot com. Now the way we actually communicate is via IP addressing. But the problem is is that I don't know the IP address of this particular server. The way we actually get this is by taking the domain name we see right here and we resolve that to the servers actual IP address.

8:53The way we resolve it is via the main name system, a DNS request. Now we will talk about DNS in more detail later on. But for now just assume that let's go with this server right here. Okay, this one on my local network is my DNS server. It's just there to resolve these types of names to IP addresses. So effectively I make a connection to my own server here, the DNS and I ask you the question hey, what is IP address associated with this particular name? Now you see that little request I just made right there.

9:25This was also made over UDP and it's been made over UDP port 53. This is the well known port for DNS requests. So what the server ultimately does it responds back over the UDP connection gives my device the IP address information. Now I know the IP address I can therefore make the connection over TCP this time to the web server. Over let's go with port 443 so it's nice and encrypted and fully secure. Now just notice all the nuance here.

9:55Sometimes we're asking for a very particular request such as DNS. How we doing that? We're using port numbers, we're using the transport layer when we want to build the connection to the web server whilst we are also using other layers such as the network layer but also relying on that transport layer. In this case here utilizing port 443. So really when we're talking about the transport layer we're talking about port numbers we're talking about two very particular protocols UDP and TCP we know their advantages and disadvantages. One is reliable, one is very fast but that is something very very important about this reliability that we get with TCP.

10:32The way this is actually instantiated is via something known as a three way handshake. I know that sounds pretty pretty cool. So how about we talk about that in the very next video.

0:00Alrighty then, so let's talk about the three-way handshake. Now, first things first, to be clear, the three-way handshake is a function of TCP. This is not something we're going to be doing when establishing a connection over UDP. So let's talk about how the actual mechanics work then. So let's imagine we happen to have a machine on the right here, on the right, on the left here. Let's just call this PC1. Okay, super creative name, I know. And on the right hand side, we have PC number two.

0:32So here's the deal. PC1 wants to talk to PC2. We want to have a reliable connection over TCP. So we're going to have to establish that everyone is a okay with this connection coming through. The way we do this is by establishing the three-way handshake. So it consists of three particular parts, no real surprise there. The very first part is something known as a SIN message. And as a SIN with AY, it's basically a message for synchronization.

1:03So the first thing is we do is we send off the SIN message to what will be a receiver. And the receiver, if everything is AOK, we'll send their own response back. And this response is going to be something known as SIN. Now, the SIN portion might be self-explanatory. It's the exact same type of message as we just saw above. The difference though is we have this additional acknowledgments. Now, before we go on, any further, let's just close off this three-way handshake. Once the initiator receives the SIN ACK, what is then sent is the third and final component of the three-way handshake.

1:40And that's just simply an ACK message, basically an acknowledgement. So in a regular conversational style way, what is happening is PC1 says, "Hey, Mr. PC2, I would like to talk to you. Are you ready for this?" PC2 is up for a good old chat and PC2 says, "Yup, I hear your request and I'm ready for it." Would you also like to talk to me, hence its own SIN message? And PC1 goes, "You betcha. Let's begin this conversation. Here is your acknowledgement." So once this has happened, both sides are waiting and ready to participate.

2:14PC1 can now begin sending data to PC2 as they expect. Now, these are the three main components of the actual handshake, but there actually is something else that we're missing out here that is something known as a sequence number. And this is going to kind of play into the fact that TCP has reliability. Remember, if we happen to lose a packet or have a packet corrupted, the TCP protocol is going to be built in to handle that type of issue. So let me just describe it a little further with a little bit more detail.

2:46Once again, PC1 on the left, that's the sender, PC2 on the right, the receiver. So we already know the first message is going to be a SIN packet, but this packet is also going to include an initial sequence number. And this is just in numerical value chosen by PC1. Now, the value itself that happens to be chosen isn't really that important. Whether the number happens to be 1000 or the number happens to be 5 or the number happens to be 3520, the initial number is not what is important.

3:18What happens to that number though is, so let me describe it. So let's imagine in this packet here, we send the sender quest and we also have this sequence number. So let's just say this is sequence number. Let's go with 100. Nice simple number, okay. Now we know PC2 is going to respond with its own SIN Act. So let's just draw out the packet right here. This is SIN Act coming back. Now, similar, this particular device here is also going to choose a randomized numerical value for its sequence number.

3:49So let's just imagine that this one happens to use the sequence number of let's go with 5000. So we'll say sequence is 5000. But as a part of the Act message is going to send an numerical value associated with that Act and that is going to be the value of the initial sequence number that was sent and be simply incre mented by one. Here's what's happening. PC1 over here chooses a number at random. This is 100, no case here. That is then communicated to PC2 in the form of the initial SIN request.

4:21And in the response, we take that number and add one to it and the acknowledgement. Pretty much what we're saying here is that we are ready to receive the packet with sequence number 101 next, which is the next one in line that PC1 would send. We just incremented by one. But remember the acknowledgement is only one portion of the SIN Act. We've also sent our own sequence number right here. So can you predict what the response from PC1 and its acknowledgement message is actually going to be?

4:52Well, like we say, we know we're going to get the Act back right here. So in fact, let me just draw this here as the packet coming back at the Act message. Now, that actually is two numbers that are going to be associated here. There is going to be a sequence number. And what do you think that sequence number is going to be? It's going to be the exact same value as the previous Act right here because remember in this message, PC2 is saying, hey, the next thing I'm expecting, not actually have received, but I'm expecting will be sequence number 101.

5:22And this is exactly what we're getting here. Sequence number 101 is indeed the one that's been sent. Now, we also have to remember for sending an acknowledgement and what is it? But actually acknowledging we're saying, hey, I saw your sequence number of in this case, you're 5000. The next one I would expect would be 5000 and one. And then from this point on, once the connection has been established, we can then begin sending our data with these sequence numbers intact. All right, so let me just try to simplify this a little bit more so we can actually just identify how these sequence numbers help detect errors.

5:58OK, so PC1 on the left PC2 over on the right. PC1 begins by sending over a message. And let's for now just really focus on the sequence numbers. OK, so we send the packet over with sequence number of let's go with, I don't know, let's say number 12. OK, what is that act coming back from that then? Well, the act message to acknowledge the previous sequence number is going to take the number plus one. So we're going to send an act back of 13 pretty much saying, hey, I got your message with sequence number 12.

6:29The next one I expect from you will be 13. Now let's just say this particular device, PC1 sends a bunch of messages and for some reason they don't get through. OK, so PC1 then sends another message which finally does get through. However, it's got a sequence number of let's go with 16. Now this device here is going to notice, hey, you just sent me a message of sequence number 16. The number I was actually expecting was 13. Therefore we have a mismatch here and I can interpret that as at some point between this point here and that point there, there was some type of loss data.

7:06Hey, can you resend the information back because I'm expecting the packet with sequence number 13. And in that case here, this device here can begin resending these packets and everything is OK once again. This is the beauty and the reliability of how TCP actually operates.

0:00So the next layer we are going to be talking about is, you guessed it, it's going to be layer number three. This is the network layer and it's so, so important. We're really going to do a lot of focus on this layer as we progress throughout this particular course. So what is it all about? Well, you might already have a little bit of an inclination as to what we're trying to do here. The network layer, this is where we're going to do things such as routing. All right, but what does that even mean? Well, as you happen to be watching this, no doubt within your home you happen to have a router and you are probably familiar that this particular device let's you go to the internet. Now to be clear here, I'm not saying that routing only involves your visiting the internet, more specifically when we're talking about routing, we're talking about moving data across different networks. Now first thing to point out right here, see this term I happened to use, I said data this is not really the terminology we would use when we're talking about layer three, the data layer three is actually referred to as packets. So more accurately what we're doing when we're routing, we're moving packets across from one network over to another.

1:08Now when you happen to visit the internet, all the internet is is a whole bunch of connected networks, you know, all over the world, all over the globe. So let's imagine this is now your little home, okay? And this is your home router. So let's imagine this is your computer here. And this one can be your sister's computer, you know, you've got one computer in each bedroom maybe . Let's pretend that this here is not a server but rather it's a printer. So because this printer here is in the same network as you over here, if you want to print something on that printer.

1:42So in this case here, there actually is no routing involved. And the reason why there's no routing involved is because all of this communication you to the printer is all within that same local network. Whereas if you want to go and let's maybe say this is google.com, this is any separate network entirely, you ultimately have to send that packet to your router. And the router will figure out what to do with it. Most likely what it's going to do is just send it to your ISP, the people who you pay for your internet and then their routers will ultimately find the correct path to get you to the server that you want. Thus facilitating the connection that you're after . So that is the big big distinction when we want to talk to a different network. We're going to be operating at layer three with an OSI model. We're going to be routing and very, very important the way this is actually handled or the way this is actually facilitated is via the use of logical addressing.

2:36Now these logical addresses, but it might sound a little bit complex right now, you no doubt have heard of these at least in passing the logical addresses we use at layer three are known as IP addresses. This is how the connection actually happens. So your computer here will have an IP address. And if you want to talk to your printer in this case here that will have its own IP address, and as we'll get to learn because in the same network right here, the IP addresses of both the printer and your router are going to be within the same network range. What does that mean?

3:10Again, do not worry about that quite just yet. We'll talk about the details very, very soon. However, when we want to speak to a server out in Internet such as let's go with google.com once again, it is also going to have an IP address. But because it's not in this network, it's going to have an IP address and a different network range. And thus the routers here are going to go figuring out the best path how to reach that particular IP address. This is what we're talking about this particular layer. So the address portion of the name might be quite self- explanatory.

3:40The same way the mailman can deliver your mail because you have a particular house address within a particular region. This is the same type of concept that we're talking about here, but this portion might not be quite so obvious. IP, what does this actually mean? Well, we're actually referring to the Internet protocol. Now it is worth just mentioning that IP is not the only protocol you can get in layer three, but it is by far and away the most popular one. The one you're going to see all the time at this particular layer. Now once we dive into IP addressing in a little bit more detail, we'll get to learn that there actually are two different versions of IP.

4:14And one is known as IP V4. This is version four of the IP protocol. And we also have IP V6 version six. This is the older version. And this one is the newer one. This one is still really quite common. And it is a little bit easier to use. This one here though is more modern and also provides some distinct advantages. Again, we'll talk about the details very, very soon. But really at its core, when we're talking about layer three, the data is referred to as packets. When we're moving these packets across different networks, we are routing using routers. The protocol on play is going to be the IP protocol, which can be in version four or version six. And these protocols ultimately allow us to address each individual node on our networks uniquely so that it can be found such that if this device wants to talk to this device, we can do so. And in a similar way, even if we happen to be trying to connect to a different network entirely because of IP addressing that connection can be facilitated. Now another thing that is managed at layer three and something we will talk about is something known as quality of service. So what does this actually mean? So let's say that this is a corporate network, we can imagine maybe there are a lot more computers than just two.

5:29And let's say for this particular company, video calling and video conferencing is very, very important. Now let's imagine that the users within this particular network are kind of slacking off on the job and spending a lot more time watching YouTube so there is visiting the internet, they're consuming lots of video, full HD, maybe it's 4K, they're just consuming lots and lots and lots of data. What this means though, is that they're ultimately putting a lot of demands on their own network. So the actual bandwidth availability is going to be reduced for other users because some users are well taking more than their fair share of the network. And because of this, users who really are doing work such as doing video conferencing, they are noticing a lot of lag on their systems. The network really seems to be congested and slowed down. So what can also happen at layer three is we can actually identify very particular traffic traversing a network such as traffic that is used for video calling or voice calling. And we can just say, hey, we want to give that traffic a very particular priority. So even if all of the other users on the network are wasting time watching YouTube, what we can ultimately do is prioritize any of the traffic that is actually used for video calling so that the people who are watching YouTube, they are

6:44the ones who suffer congestion, they are the ones who start noticing lag. So if this person here is actually doing a video call, they're actually going to be able to make that connection without much lag at all. Whereas the person on YouTube, they might start noticing that all of their videos are buffering and not really watchable. Now again, we'll talk about how we can actually implement quality of service much later on within this very course, but understand though that this particular concept, once again, all of this is happening at layer three within the OSI model.

7:15So routing forwarding packets between different networks, marking particular packets for a very particular quality of service, all of this made possible, via layer three within the OSI model.

0:00Alright friends, so let's round this scale off by talking about the final two layers. This is going to be layers number two, as well as layer number one. So let's focus on layer two first. This one here is referred to as the data link layer. So what is this particular layer actually responsible for? Well, its primary purpose is to reliably transfer data over a physical medium. Quite simply, this is how all of our local connections happen. Now what I mean when I say local connections, I'm talking about connections between the devices within the same network.

0:34So remember the way routing was allowing us to facilitate communication across different networks right here, we're talking about devices in the same network. So if I or my little machine here want to talk to my printer on this same local network, the big thing that's going to allow us to do this is the data link layer. Now remember, we have things such as segments, layer four, we have packets at layer three. When we're talking about layer two, the actual data is going to be encapsulated into what is known as frames. Now the data link layer is responsible for quite a lot of different things.

1:05We're going to talk about this in more detail in an upcoming scale, but as a high level overview, some of the things it's going to do are, well of course it's going to manage the actual framing of the packets. It's going to allow us to detect errors within these frames so error detection is also covered. It'll manage things such as flow control to reduce congestion across the physical lengths, but one of the most important things, maybe something you've already heard of, is it actually is responsible for physical addressing. So remember we talked about IP addressing, how that was logical addressing. This time here we're talking about the physical connections and I really mean physical. So again, let's imagine I am over here.

1:42If I want to talk to this printer over here, I have an actual interface right here. This can be my local ethernet interface as the diagram shows. And this interface here, this physical interface, this is the interface from which I can reach the server. Now each of these interfaces, this one here and this one there, they actually have physical addresses. And in a similar way, this device here in the middle is switched, which is something we'll look at in great detail very, very shortly in an upcoming scale. This is going to manage the layer to communication utilizing these physical addresses. In fact, all of the ports here, this one here, this one, this one, and this one, each one of these interfaces all have their own physical address. Now this physical address I'm referring to, you may have heard it known as a MAC addresses, is what we're talking about here, okay? But it also has other names such as simply just a layer to address. And sometimes it's just referred to as a burned in address. Now with respect to IP addressing, at least in IP before addressing, the IP addresses look something like this, 192.168.1.1.1.

2:44This is a valid IP for address. When we're talking about addresses for MAC addresses, the format looks a little bit different. There are going to be 12 characters long, and they're actually going to use hexadecimal characters. Again, we're not going to struggle and worry about what this actually looks like quite just now. But to give you a rough idea, this would be a type of MAC address. Now I've just made this MAC address up completely off the top of my head, but you can see here it does look really quite different. If you count the characters, we're going to see we actually have 12 right here. And like I said, each of these characters are going to be hexadecimal. So in the example of this local network, when I want to talk to the printer right here, my interface here is going to have a MAC address in a similar format to what we just saw there before, as well this interface right here. Now the switch itself actually is quite intelligent with respect to this matter, because what happens when a frame is created, we actually specify a source MAC address within the frame, basically who originated the frame itself.

3:42And we also include a destination MAC address, much like with respect to your packets, we have a source IP address and a destination IP address. Same type of thing is true here. So if I specify that, hey, the source is here, and the destination is this one right here. Once we pass that information, this frame over to the switch itself, the switch is going to have a little table that maps all of the MAC addresses to a particular port. So when this switch here receives the frame coming in, it's going to look at the destination. And it's going to say, hey, whenever I see this MAC address destination, I know what I do is I actually forward the traffic out of this particular interface right here. So I don't forward it out of this, I don't forward it back down here or this one, I know that the destination MAC goes out of this interface right here. And ultimately, it ends up at the correct destination. Now again, the details of how the switch knows how to do this. This is something we'll talk about very, very shortly in the next scale. But effectively, this is what is happening. So the data length layer is all about dealing with frames. We use source and destination MAC addresses. This is how we communicate over a local network. And then the data before it's transmitted is encapsulated one more time. This time at layer one, you guessed it. This one is known as the physical layer. Now this might sound really quite similar to what we're

4:57just discussing. But in this case here, we are being a little bit more literal. We're talking about the physical transfer of the data, the actual binary bits, you know, as in the actual binary bits, the ones and zeros that are being transmitted. So whether or not the data has been transmitted as voltage over a copper wire ones and zeros, or perhaps maybe it's been transmitted over a fiber optic cable and pulses of light. So when the light is on, we have a one and the light is off, we have a zero, or maybe even the data has been transmitted over radio waves, which is what you would see with respect to Wi-Fi or maybe even Bluetooth. So cables, network interface cards, ethernet ports, all of this information relates to the physical layer within the OSI model. So really, when we have a device here wanting to talk to a server over there, the data that we're going to be transmitting is going to be encapsulated all the way from layer seven to layer six and layer five.

5:52And then we're going to wrap it any segment at layer four, we'll wrap that in a packet and then wrap that once again in a frame before ultimately transmitting all this data over the physical media as ones and zeros utilizing layer one. And then eventually when we hit our end points, the process of decapsulation is ultimately going to do the same thing in reverse this time stripping away the physical layer to reveal the data link layer to reveal the packets, the segments, all the way up to layer seven, whereby the application can actually read the initial data itself.

6:25All of the complexity we've just talked about, the binary bits, the physical medium, whether it's radio waves or pulses of light, the routers routing packets with layer three addresses, the switches forwarding frames with layer two addresses, all of this complexity , each individual component, all of it can be represented theoretically in this nice clean way utilizing the seven layers of the OSI model.