Dip Into the 7 Layers of the OSI Model

have a guest or or a co host. I live stream out on twitch dot tv slash tech stuff, and one of the features that Twitch has is a chat room, so I often engage in chat in said chat room, and occasionally I asked people, Hey, is there anything you would like me to talk about in a future episode of tech Stuff And Nick k a k A dirt burr over at the Twitch chat room suggested the os I model, the seven layers of the O S I model, and I thought, you know what, I have never actually

done an episode about the O SI model, and challenge has been accepted. The gauntlet was thrown picked up, and here we go. The seven layer O SI model is an interesting topic. It's typically one that's taught in computer science and telecommunications classes. It's it's a networking thing. It's not necessarily internet. You would usually talk about t c P i P protocols for Internet models of networking, but

it is for computer networks in general. And before I get into this too far with this whole concept of the seven layers of the O SI model, I gotta make something very very clear. The seven layers are not actual physical layers. Even though there is one layer called the physical layer, they are not physical layers to any sort of system. This is not some sort of seven layer DIP situation, which is too bad because that stuff

is of course delicious. This is a conceptual model, and it's a way to imagine the interactions, the rules, the protocols between various elements within a telecommunications network to make it easier to comprehend what is actually going on. Or, as Eli the Computer Guy said in an incredibly useful video on YouTube, you can actually search for Eli the Computer Guy and look for the us I model. You

can actually watch this video. Uh. The model breaks down the components of network communication into layers to make it easier to understand what is actually going on. So, in a way, the seven layer O SI model is sort of like the analogies I tend to use on this show in order to explain various concepts. It's meant to clarify the purposes of various design elements and different protocols and and tasks. By the way, there will be many

analogies in this episode because I love them. I was an English major, so that's kind of my go too. There are also other models, Like I said, there's the Internet Protocol Suite. That's the other standard model of computer networking. But we're going to focus on the O SI model for this episode. Maybe someday I'll do an episode about the t C P I P model, but that will not be this episode. They are similar in many ways,

but there are distinctions between the two. But it's really just a way of thinking about how these communication networks handle tasks. The OSI model, by the way, did not just spring up out of whole cloth either. It's the product of an organization called the International Standards Organization or ISO, not I S o IO. So let's learn about that

group first before we talk about the model. Is SO is a non governmental international organization that is completely independent of any state authority, so it doesn't fall under the purview of any one nation's government. Uh. The membership of the of ISO includes a hundred sixty two national standards bodies, there's only one is a member per country, so you can't have multiple standards organizations from one country in ISO's roster.

It has to be the foremost standards organization of that country. The United States, for example, has a member. Or there's an organization that is in ISO is the American National Standards Institute or and C. And C actually predates ISO. Has and C was founded in nineteen eighteen. IO began in nineteen More on the founding of ISO in just a second. And c's purpose is to accredit standards that

are created by quote qualified groups in the quote. So, in other words, this organization the United States does not create standards. It's not the one that says, this is what the standard X is from now on, you all have to follow it. This is the X by which all other x is must be measured. They don't do that. What they do is they look at the work of qualified groups and say, yes, this example of whatever that thing is is the standard against which all others shall

be judged. So mote it be at this point it might even be good for us to just consider what the heck a standard is in the first place, because otherwise this just becomes a jumble of words, and I start using phrases that don't really mean anything to you, You're gonna totally tune out. So the definition of standard is according to the Miriam Webster Dictionary. Your definitions may

vary depending on the dictionary of use. It includes the definition of something established by authority, custom or general consent as a model or example, or something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or quality. It could also be a giant flag that you carry into battle. But that definition probably doesn't really apply when it comes to the O S. I'm also

we're not gonna think about it right now. Standards are an agreed upon model upon which all other instances of that concept or physical object should be measured. Uh So, units of measurements are standards that are easy to understand. Right. An inch is an inch no matter where you go. A kilogram is a kilogram no matter where you go. That sort of thing. So organizations like ants and is

so help establish these standards. They don't necessarily come up with them, but they might accredit them, they might endorse them, and thus get widespread acceptance of that as a standard, this becomes sort of a generally agreed upon common concept

across multiple nations. The need for standards was apparent from the early days of trade, because merchants had to create standards so that trade systems could actually work across different currencies and regions, not to mention, create standards of measurements so they could make sure they were talking about the same amount of stuff they were selling from place to play. If you're a if you're a spice merchant, you want to make sure the people you're selling to understand the

quantities you're talking about. So you had to set up these standards that people would generally agree to in order to actually make trade work. It was necessary for that

sort of thing. Well, the same thing is true in technology in general, not just with units of measurement things that are more kind of ultimately tangible, but with general concepts where you're gonna have lots of different people from all over the world working within those sort of concepts, and you want there to be interoperability between all of

those people. You have to create these standards so that everyone is kind of working from the same basic set of rules, and by working from the same basic, basic set of rules. You you reduce the possibility of incompatibilities. There will always be incompatibilities just because technology doesn't always work the way we intended it to. But you can at least remove some of those barriers early on in the des line process by agreeing upon a basic set

of rules from the start. That's the whole purpose of organizations like ISO SO. This extends to telecommunication and networking layers. As ISSO states on its web page, international standards make things work. They give world class specifications for products, services,

and systems to ensure quality, safety, and efficiency. Now, i SO got its start back in nineteen forty six during a meeting of the Institute of Civil Engineers in London, England, and there were representatives from twenty five different countries that attended that meeting, and they concluded that they needed an official but independent organization to help coordinate and unify industrial standards.

I would become an official entity on February twenty third, nineteen forty seven, and according to ISO, the organization has published nearly twenty two thousand international standards since it was founded. Oh and uh ice SO is not an acronym. It is not I s O. It is ISO. It is

all capitalized, but still it's ISO. And it's because if you translate the name of the organization into the different languages of all the different countries that have a an organization that's a member of ISO, you don't end up with words that start with I, S and O with every translation. Right. That's just the English version. So the English version is International Standards Organization. But other languages have

it in different orders or even different words. So you can't just call it an acronym because not every country uses the same the same language, right, So they decided instead they would take their name from a Greek word ISOs i S O s which means equal so is so is always is so no matter what language you are speaking. As for the O S I model, now, that actually emerged from a couple of different projects that

we're trying to ablish a framework for network systems. This would be in the seventies when networks were really starting to become a possibility. You may remember that ARPA net had been in the development stages in the sixties and seventies,

and that was a precursor to the Internet. It was also those early early days of networking different devices together so that they could communicate directly to one another instead of having to work on one machine, save your work on some form of physical media, maybe it's a magnetic disc or real to real tape or whatever, taking that physical media over to another and I guess I should say medium over to another device and then loading it up there. Instead of doing that, you can have them

communicate directly to one another using a network solution. But you had to establish those standards, particularly since there were lots of different computers that were being produced and lots of other devices that you also would want to net computers too, and they didn't all use the same sort of operating systems, they didn't communicate in the same way, So you had to create a set of standards that would work across multiple instances of networked machines so that

you didn't just end up with different specific network protocols for very specific instances. Let's say that one place has all the same sort of mainframe style computers, so they design a network that only works with those. That's not any use to somebody who's trying to create a network for a totally different type of machines. So they wanted

to create this basic set standards. In the seventies, you had two different groups doing that, and eventually those two groups they got their work kind of merged together, and in nineteen eighty three they published this, or at least they they submitted this to ISO. UH. It was called the Basic Reference Model for Open Systems Interconnection, and i SO would publish that model the following year, in nineteen eight four. This would be the UH the ISO publication

number seven thousand. So next time you're at pub trivia and someone says, what's the ISO publication number of the os I model, you can just pipe up seven and you will win great acclaim. I don't know why you're going to I T professional pub trivia, but that's you know, I'm not judging. So I SO created the O s I seven layers model. This again was to create a sort of common lexicon across the world for the purposes of discussing telecommunication systems. And let's take a high level

look at what this means. Those seven layers to this model, AH, they represent different tasks sort of or or different layers of responsibility within a communications network. The bottom most layer, layer one is what we call the physical layer, the top layer. Lay year seven is the application layer, and if you'd like to visualize this in a practical way, layer seven is the one that the end user comes into contact with. Layer seven is the part of an

application or program that that you can see. Now it's not Layer seven is not an application in of itself, although some applications can exist entirely in layer seven. Layer seven is the group of services within a communications network that interact directly with whatever is going on with the application and send that further down the stack. So while a web browser is not in itself a layer seven,

it uses elements that do tie into layer seven. In other words, uh, it's it's kind of like the the just under the surface layer for whatever application you have opened that has this network communication aspect to it, whether that's with a phone or a computer or a printer, really any device that's connected to a network. This is the one that's closest to you. So it's the one that's easiest for us to kind of understand because it's

the stuff that we interact with. You interact with this interface, you send commands. Those commands get pushed down the stack and depending it depends on what the command is about where that ends up going on the other end of the model, and we'll go through all the layers in this episode, but right now I'm just looking at the top. In the very bottom on the other end, the bottom of the stack is layer one. That's the physical layer of the networking process. This is the basic bit stream.

This is the zeros and ones that are being carried by some means. You could even argue that it's not even zeros and ones, it's the stuff that represents those zeros and ones. It's the actual electrical impulses or rays of light or radio signals that are carrying information. So this is the conveyance of data. So that depends upon actual physical properties like physical components like cables or Wi Fi cards or things of that nature. That is why

it's called the physical layer. It is completely dependent upon that. It does not care about protocols. None of that stuff matters. This is just carrying the signal. Uh So, between layers seven, the one that we interact with directly, and layer one, that physical layer, are all the intervening levels that handle

protocols for network communication. Again, this these are not physical, literal layers, but rather a way to divide up those protocols for the purposes of understanding which layers hand tasks off to other layers or accept tasks from other layers. So I'm gonna explain this conceptual model with an analogy. So we're gonna get even more abstract because that's what I do, and you can't stop me. It's my show. The O S I seven layer model is like a

metaphorical bucket brigade fighting a fire. So let's say that layer one, that physical layer, that's the person in the bucket brigade who is closest to the water spigot. Water is pouring out of the water spigot. Now, the person who's representing layer one can do a couple of different things.

They can fill up a bucket with water. They can hand a full bucket of water over to the person who is representing layer two, or they can accept an empty bucket from layer two so that they can then fill up the bucket of to make it a bucket of water. Again, Layer two can really only do a

couple of things in this particular analogy. They can accept an empty bucket from layer three, or they can accept a full bucket from layer one and they pass it on in continuing in the direction that it was traveling. When you get all the way down to layer seven, they can accept a full bucket of water from layer six, can splash that water onto a fire, and they can

send a empty bucket down to layer six. That's kind of what's going on here from a conceptual level with network communication, it's instead of water communication tasks and different layers are responsible for different parts of the task. So it gets a little more complicated because it's not just accept something and pass it on. That's it's not as simple as that. There's a little bit more to it.

But I wanted to start big so you can kind of understand that these layers are all dependent upon one another. So I hope that analogy makes some sense to you. I really liked it because while I was trying to kind of digest the OSI model, that's kind of how I imagined it was this process of passing down a task so that it goes down the chain until it hits where it needs to go, comes back up the chain with whatever the response is. Now, the reason the

whole model is important. Why are we even talking about the OSI model in the first place, beyond just giving us an idea of what's going on behind the scenes when you are using devices on a networked system. It's because it provides a guide for developers. For one thing, if you're developing programs or applications that are meant to be networked, it is really good to have an understanding of the OSI model so that you can focus on

the proper way to implement your your program. Uh. And it's also important for you to adhere to the standard that's been established. That's what standards are for, after all. So you might even wonder why is this even important,

why are we even talking about OSI models? Well, I mean beyond just giving us an idea of what's going on behind the scenes of devices that are running on a network and how that communication is supposed to work from a conceptual level, it provides a guide for developers who are creating applications and programs that will run on networked devices. It's really important that their processes adhere to

the standard. I mean, that's why we of standards. It's to make sure that things are falling in line so that the everything is as smooth as it possibly can be, especially when you get into interoperability where you have programs operating with other programs. If you have that happening, they need to agree upon the same basic set of rules. Otherwise it's like getting two people together who speak completely different languages that have no basis, like there's no shared

origin for the two languages. They both arose completely independently, and then try to have them have a conversation without using gestures or anything else to kind of get their meeting across. It's it's practically impossible. So this is very important for programmers in order to make sure that their

approach will be interoperable for other types of processes and programs. Uh. This could be as simple as making sure that your program is not going to make an entire framework on stay will and cause potential crashes or other issues, to something more complicated such as apps being able to share the same data across each other or access each other's

functionality in some way. And it's only because we have these standards in place at such an approach is really possible without having different teams worked very tightly with one

another from the first place. It's also really useful if you're a network specialist like an I T network guy or a lady because you've got a problem happening in a network that you manage, you want to be able to identify where that problem is and how you would go about solving it, and being able to figure out conceptually where in the OSI model that problem may be arising, tells you what approach you may need to take in

order to fix the problem. I've heard that problems fall on the physical layer, which essentially means that someone has not plugged something in. It sounds like an I T. Crowd kind of joke, but apparently it's absolutely true that you know, there's folks who just there's a cable that's not fully plugged into a port, or someone forgot to turn a switch John or something, and a lot of

the problems do stem from that. And then once you get beyond the physical layer, things get real tricky because you got to figure out where is the breakdown happening

and what can you potentially do about it. So it does have a practical application outside of of just a general information The model is also obviously used as a teaching tool to explain computer network architecture and communication protocols, and it gives computer science students a foundation and the logic they'll need to understand to work within computer network architecture. So that's the overview. Now we're gonna take a quick break, and then when we come back, we're gonna start waiting

into what all these layers actually mean. So let's take a quick break to thank our sponsor. All right, let's look at these layers. Now, we're gonna start at that highest layer and work downwards since I think that makes the most sense for the average person. Here's a quick overview, and then we're gonna dive further into detail. All right. Layer seven is the application layer. That's where the end user applications live, or at least where they touch. That's

where they make contact with this OSI model. This would be the stuff that we interact with, like whether you're playing Pokemon, or you're using a web browser to surf the web, or you're checking a fitness app or anything that's along those lines where it's tapping into this network. You are interfacing with something that is touching on this layer. There's a lot more going down below the surface, but we'll jump into that in a minute. Next is layer six.

That's the presentation layer. That layer handles syntax processing. Don't worry. I'll explain that in a little bit. It's sort of like a translator in many ways, so hold on to that idea. Layer five is the session layer. This layer manages the flow of events that establish and terminate network connections, communication channels, if you will, between applications or devices. Layers five through seven, so the ones we just talked about,

are considered application level data layers. Everything below those layers gets into more fundamental aspects of moving data around, where it's not so much concerned with it being identified as an application. It's more basic than that. It reaches progressively more abstract concepts and moving further away from anything we would think of as an application. Level four is the transport layer that provides transfer of data between end systems

and is responsible for end to end recovery. Again, we'll go into this in more detail in a minute. Level three is the network layer that's the domain of things like switches and routers. Level two is the data link layer, in which all information we've been talking about the packets of information getting coded into bits. This layer technically as two sub layers. There's the Media Access control or MAC layer. If you've heard of MAC addresses, that's what this refers to.

And there's the logical Link Control or l C layers, so it's not a limited liability company. It is the logical link control. More on all of that again just a minute. And then finally we get to layer one, that physical layer I mentioned earlier. That is the bitstream conveyance layer. So all that sounds really technical, but we'll break it down layer by layer, and we'll start with

layer seven, that application layer. It identifies the protocols and interface that hosts in a communications network have that the end user uses. A host, by the way, is any network device. So it could be a computer, it could be a smartphone, it could be printer, it could be really any networked device that is connected to a computer network. Uh. Really, it's anything. It's not just servers. You might think a host has to be a server because the name host

and server seem really related. Um, a server is a device that accepts network connections from other devices, and those other devices we call clients. So if you use a web browser to connect to a web page, the computer that has that web page saved on it, because that web web page actually exists somewhere. The computer that has that web page on it is a server. Your browser is acting as the client. It is asking for that that web page to be served to it. So that

is a case of a server. It is allowing the web browser to access it. So all servers are hosts, but not all hosts are servers. Host is a larger term. It includes servers, but includes all the other devices connected to the network as well, or at least all the computer style devices. So this is the stuff we as users encounter when we're working with an application or a program, and as long as that program is accessing the network, if it's a self contained program, it's kind of separate

from this OSI model. It's really considered concerned with network communications. So let's take a real world example and talk about web browsers. The application layer would be the interface ace that we rely upon, uh or at least it would be touching upon the interface we rely upon when we're interacting with our web browser. These are the rules that dictate how we can select links, how it instructs the app on how to display information to the user in

response to commands. This is all front of the curtain stuff as far as users are concerned. For the os I model, this bit also includes aspects like user authentication and quality of service. Everything on this layer is application specific, so uh, very specific to whatever instance you're looking at. It also provides the application services for email, file transfers and other network software services. So some services that exist at this level are FTP, which is filed Transfer protocol

and tell net. That's the service that allows you to use your computer as if it were a terminal for remote server. So, in other words, tell net lets you log into a virtual terminal for another machine and execute commands as if you were working on that computer directly. So there's some computer that's out there in the wild, you use tell neet to log into that computer, and suddenly your computer, the one you're typing on, it's acting as if it is directly connected, like it is part

of the machine that you are tapping into. When you type a command in through your virtual terminal, it's executed on the other machine, not on your computer. Telling that is bi directional. It's an eight bit bite oriented communications specification, and it exists at this layer seven that is the domain of tell neet. Some other examples of stuff you'd find in layer seven, apart from telling neet, an FTP and web browsers are the HTTP protocol and Simple Network

Management Protocol or s n MP. That's a standard for organizing information about devices on IP networks and also the rules for modifying that data in order to change device behavior. Now, one thing the layer is not is the application itself. Right, Applications are separate from the OSI model. They touch on it, but they are their own thing. So this is this would really be the services that applications can make use

of that are universal for communications networks. So you can't get to applications specific and layer seven because then you you depart from the concept of standard. You have to have a list of different services that layer seven handles that applications can tap into. Right, So it'd be like walking into a business that offers certain services. You can't order anything off of that list. You can only accept the ones that are on that list. Same sort of

thing for this os I model. H Anything that would require the application to reference information on the network, or send data to the network, or otherwise interact with the network is part of this layer. Layer six. That presentation layer handles format conversions and encryption and decrypt shin. So, like I said earlier, This is sort of like a translator between different forms of data, and it's usually part

of an operating system. So an example of a layer six task is to take clear text that is sent from an application and encrypt it before moving it further down the layers, or doing the reverse of that and taking encrypted information from further down the chain and translating it into clear text for the use of the application in the layer above. Because of this feature, the presentation layer is sometimes referred to as the syntax layer. Syntax refers to the set of rules for the arrangement of

words and phrases to create something of meaning. Syntax allows for clear communication, whether in human languages or machine ones. So if I tell you later today, I'm going to go to the store so that i can buy some salted caramel pringles because they taste like golden grams, you'll understand what I'm saying. You might not understand why I'm saying it, or why I would bother to do such a thing. By the way, they really do taste like

golden grams, and it's not necessarily a good thing. But let's say I've said that you would understand what I meant. But let's say I rewarded that sentence. I'm using the same words, but I've completely changed the order. So it's now to the store so that i can buy because they taste like golden grams some salted caramel pringles. I'm going to go later today. You might have to take some time to suss out what the heck I'm saying because I'm not using the correct syntax. I'm not following

the rules of English language to communicate clearly. Syntax is really important so that we can be understood. And for machines it's extra important because we typically programmed them to accept very specific formats and anything that departs from that format is essentially nonsense to a machine, so syntax is key now. Examples of concepts and protocols that belong in layer six include encryption methods of encoding such as JPEG's impegs, MIDI, or as key tag. All of that belongs in level six.

Moving on down to layer five, the session layer. This encompasses all the ways that the system enables or terminates connections between applications. At this layer, the network allows conversations, exchanges, and other transmissions between applications to occur, and it's all about session and connection coordination. Another way to think about this is the layer is the one that allows one

machine to chat with another machine across the network. It creates that communication channel, also known as a session, So it creates the session between the two machines, It maintains the channel, and it most importantly, you could argue, terminates the channel once the session has ended, so you don't have a continuation of data exchange once you have indicated

that that communication is over. Otherwise you would have a situation where it's kind of like listening in on somebody when they thought they hung up the phone, but they didn't. It would be a end to that from a data standpoint. So that concludes all the layers that are on the application side, the application layers of the os I model. It's all about moving that data around at the application level. Below that are the layers where we descend into the

real guts of the OSI model. And before I do that, I need to take a rest. I gotta regain some spell slots because I burned a couple in that last section. Also, I've been listening to The Adventure Zone a lot. It's a show that if you haven't listened to it. You should check it out. It's kind of neat, has nothing to do with how stuff works. That's just a free plug for the adventure zone. Let's take a quick break and thank our sponsor. All right, we're down to layer four.

We've been digging down. We've hut the topmost layer in the group of layers that are all about just moving data around in general. That it's kind of more abstract than the application level. Layer four is the transport layer. This is the layer that handles packet ization of data and the delivery of packets, as well as checking incoming packets for errors when the packets arrive. Packets, packets, packets. But wait, I hear you cry out, what the heck

is a packet? All right? So a packet is a basic unit of communication across the network, and there are lots of different kinds of packets. There are the kinds that are most people I think are familiar with. If they've heard the term packet, they're thinking of the um Internet Protocol version of packets. But packets can take other forms as well. It's really structures within which data can exist.

Think of it like an envelope. It's very similar to that From a conceptual level, an envelope is something that you put letters into, or pieces of paper into, and then you can send that through the mail. That's kind of what packets are, except there for data, like raw data, or at least for information that you're going to send across a network, not a sequal letter. But let's say that you have an envelope you have you bought a box of envelopes, and you discover that each envelope has

a certain capacity. It can hold ten pages of paper. But no matter what, you cannot shove eleven or more pages into a single envelope. You just can't. You can only shove up to ten pages in. So if you had a longer document than ten pages, you would need to divide it up into smaller groups so as you

could fit it into several envelopes. So let's say you've just completed a two page manuscript during Nano Rimo, that's a national novel writing month, and you're going to send your physical copy of this manuscript off to a friend for proof reading. But your friend lives very far away. You're not in a real rush, so you're just gonna send it through post. You don't happen to have a digital copy because you typed it out on an old typewriter. It doesn't really matter. So all you have are these

normal envelopes that can hold ten pages each. How you get your document to your distant friend. Well, if you're a machine on a network system, you would divide up the job into manageable chunks and pack each chunk of information into its own packet. So for our real world example, you would have to divide your two page manuscript into twenty ten page chunks and put each ten pages into a different envelope. Now you would realize, because you were a bit of a doof you sealed all the envelopes,

but you forgot to include page numbers on the individual pages. Well, fortunately you still have all the envelopes in the right order, So you include information on the envelope itself to indicate to the recipient where those pages fit within the entire manuscript. So the first envelope says pages one through ten, for example, and envelope two says pages eleven through twenty. You dump all twenty envelopes in the mail and the postal service

picks them up. Now it's a busy time at the post office, and the destination for these envelopes is quite a far way away, and the various envelopes get a little divided up. They get put into different bags, and they get distributed to different trucks and different planes. As

they travel to their destination. They're all going to the same place, they're each taking a slightly different pathway there, your friends starts to get the envelopes in the mail and is able to put the pages in order because you included information on those envelopes themselves, And eventually your friend has the whole manuscript. Well, that's sort of how data transmission across a network happens, except there are a few extra considerations. One is that devices will typically send

more than one of each packet. Sometimes packets will encounter problems as they travel across the network. Maybe a node in the network goes offline without warning, or something else happens. Maybe there's some traffic congestion at that section of the network. So if you send multiple copies of each packet, there's a better chance that the whole thing will get to

the destination. Even if you don't send multiple copies, what will happen is there will be an acknowledgement on the recipient side of receiving a packet, and if there's a packet missing, that message gets sent across and then the sender can send that specific packet again and it can travel across the network to the recipient. On the receiving end, the transport layer takes in these packets and consults the data in the header of that packet to assemble the

data properly. That's kind of like those page numbers on the envelopes I was talking about. It's the equivalent of that saying, here's where this data fits in context with all the other information that's coming in that is related to this particular file. Uh, there's a bit more to it than that, but the nature of the packets are dependent upon the type of packet it is and the protocol in question. And I think you get the general

idea here now. As Eli the computer guy explains it, the session layer opens up communication channels, but the transport layer is the one that decides how much data is actually exchanged. Betwe lean those different devices that are in communication with each other, so they're very tightly bound together, so much so that some people put them in the

same layer. They say that session and transport a kind of related closely enough where you can think of them as a single layer as opposed to to separate ones. Let's move down to layer three. That's the network layer. This is the logical pathway layer or the virtual circuit layer. This is the layer of switching and routing where data

transmits from one note to the network to another. And at this layer a network performs all of the internetworking, all the congestion control, all the addressing features, as well as packet sequencing. So another way to think of this is that this is the layer in the network where network protocols that are in charge of making sure data gets from the sender to the recipient does so. Like it's all the rules that will guide data along the

network to get to their destination. That means that it's kind of like a road system with all the road signs and traffic instructions included. Right, the IP address of a machine is at the network level. Now we get down to layer two, that is the data link layer, and this is where data packets getting coded into bits or decoded from bits into packets. At this layer you also have transmission protocols and management as well as error detection and response from errors in the physical layer below.

So if something goes wrong in the physical layer below, it is the data link layer that is important. Is the data link layer that actually detects those errors and sends the information up the chain so that someone can do something about it. So if there's a switch that's not on or a cable that's not plugged in, that ends up being detected at layer two and then sent up the stack. Uh. There are two sub layers in

the data link layer. The Media Access control or MAC layer is the first one, and you probably have heard the term MAC address. You might wonder what the heck that means. It's a hardware addresses unique to every node within a network. The MAC layer has a direct interface with a network medium, and every device that is connected to a network has its own MAC address, and it's

actually burned or recorded on every devices network card. It is unique to that And of course the purpose of an address is so that a network has to know where to send information. Otherwise the whole system wouldn't work because if you had a network that didn't use addresses, it would be like shouting into the void and getting nothing back, or worse, it would be like getting everything back, including the stuff you wanted, but also everything else, so it would just be a big jumbled mess and you

wouldn't be able to make sense out of anything. So you've got to have the addresses for this to work properly. The other sub level of this layer, or sub layer I guess of this layer is the logical link control layer. Uh. That is the sub layer that controls frame synchronization and flow control as well as error checking. So that error checking was talking about with the physical air that belongs to the I'll see. Yeah, those are some new terms, and I guess we needed to find some of them.

So in telecommunications, which is different from video because there's frame synchronization and video as well, but in telecommunications it means something slightly different. Frame synchronization is the process in which incoming framed data gets extracted for decoding with the

help of frame alignment signals. Frame synchronization is necessary because sometimes data sent to a receiver encounters issues, and frame synchronization can identify and correct for those issues, synchronizing between the sender and receiver so that you get the full amount of information that you expect. Flow control is the management of data flow between nodes on a network, and

it's important so that data can be handled efficiently. So you don't want to send more data than a recipient can accept at any given time, because you'll hit data overflow, and that means that not all the information you're sending is going to go through. You're gonna have to re send it anyway. So it's better to go ahead and make the system more efficient and control that flow of data in order to avoid that problem in the first place.

In this case, the analogy I always think of is that classic Christmas episode of I Love Lucy in which Lucile Ball is trying to wrap candies along an assembly line and the candies start coming faster and faster, so she's frantically shoving pieces down her shirt and in her hat and in her mouth so that they don't get past her. You want to make sure that the data like sweet Sweet candy gets sent at just the right pace so that it can be handled properly instead of

shoved into Lucile Ball's mouth or something. And too much data is sent to any particular node in the network, you get that data overflow situation, and finally we get down to layer one. That physical layer. This is the layer that handles the actual method of conveyance of data. Now, earlier we were talking about data in various formats such as clear text or encrypted data, or ultimately as bits. But even as we break all that down to the basic units of information, there has to be some way

to actually send that information somewhere. You have to get beyond this concept of information. How do you get that information somewhere else? It has to travel through some sort of medium. Typically it would take the form of something like radio signals or light pulses of light, or electrical impulses. So this would be the electrical or mechanical part of the system that allows for that transmission. That includes physical

components like cables and computer cards. It also includes information about the nature of that conveyance, such as the radio frequency you're going to use for wireless transmission, or the actual electrical characteristics of various components, like how many pens are in any particular connector, or the voltages that you need to use for physical components. Layer one. It's so basic that's beneath the realm of protocols. It's completely unconcerned

with protocols. It's literally us the layer that creates the actual connection, whether it's physical or wireless. That's all the connections between the different devices on a network, and that's it. Those are the layers in the OSI model and what they mean now, I know I got a little fuzzy in there, largely because so many of those layers depend upon an understanding of other topics and computer science and telecommunications.

But really, when you break it down, you're just talking about the combination of all the components of network communication and then breaking those down into logical layers to make it less overwhelming, right, because you think about all the things that have to happen for network communication to actually work. Then you say, well, that's too big of a picture. What if I divide this up into slices and I group stuff together in ways that just kind of logically

are related to one another. Even if when you break down physical components it may be a different story. Complementary components tend to be in the same layer in this model, and some of these layers are in practice often combine and like I said, layers four and five, those transport and session layers can sometimes be combined with one another. Thank you so much, Nick for the suggestion of the

OSI model. I hope I didn't gum it up too much because I wanted to keep it fairly high level so that people who are completely unfamiliar when network communications could follow it without getting so high leveled that it was not a useful conversation. If you guys have suggestions for future topics of tech Stuff, there are a lot of ways you can get in touch with me. One of those is through something called email, which, as you recall, is handled by layers seven of the O SI model.

Send me an email message the addresses tech Stuff at house stuffworks dot com. Let me know what you would like me to cover, or what sort of guests you would like on the show, or maybe a guest co host you would love to hear on the show. Let me know that. You can also get in touch with me via Twitter or Facebook. The handle of both of those is tech Stuff h s W or like Nick, you can join us over at twitch dot tv e slash text stuff. On Wednesdays and Fridays, we record new episodes.

You can watch live as I mess things up and chat with the chat room, and you can even suggest future topics right then, and they're just as Nick did. And that's it for today's episode. I'll talk to you guys again, really say. For more on this and thousands of other topics, is how staff Works dot com