Speaker 1: Get in text with technology with tech Stuff from how
Speaker 1: stuff works dot com. Hey there, and welcome to tech Stuff.
Speaker 1: I am your host, Jonathan Strickland. I'm an executive producer
Speaker 1: at how stuff Works and I love technology. You guys,
Speaker 1: you may or may not be aware that I live
Speaker 1: stream text stuff whenever it's just me and I don't
Speaker 1: have a guest or or a co host. I live
Speaker 1: stream out on twitch dot tv slash tech stuff, and
Speaker 1: one of the features that Twitch has is a chat room,
Speaker 1: so I often engage in chat in said chat room,
Speaker 1: and occasionally I asked people, Hey, is there anything you
Speaker 1: would like me to talk about in a future episode
Speaker 1: of tech Stuff And Nick k a k A dirt
Speaker 1: burr over at the Twitch chat room suggested the os
Speaker 1: I model, the seven layers of the O S I model,
Speaker 1: and I thought, you know what, I have never actually
Speaker 1: done an episode about the O SI model, and challenge
Speaker 1: has been accepted. The gauntlet was thrown picked up, and
Speaker 1: here we go. The seven layer O SI model is
Speaker 1: an interesting topic. It's typically one that's taught in computer
Speaker 1: science and telecommunications classes. It's it's a networking thing. It's
Speaker 1: not necessarily internet. You would usually talk about t c
Speaker 1: P i P protocols for Internet models of networking, but
Speaker 1: it is for computer networks in general. And before I
Speaker 1: get into this too far with this whole concept of
Speaker 1: the seven layers of the O SI model, I gotta
Speaker 1: make something very very clear. The seven layers are not
Speaker 1: actual physical layers. Even though there is one layer called
Speaker 1: the physical layer, they are not physical layers to any
Speaker 1: sort of system. This is not some sort of seven
Speaker 1: layer DIP situation, which is too bad because that stuff
Speaker 1: is of course delicious. This is a conceptual model, and
Speaker 1: it's a way to imagine the interactions, the rules, the
Speaker 1: protocols between various elements within a telecommunications network to make
Speaker 1: it easier to comprehend what is actually going on. Or,
Speaker 1: as Eli the Computer Guy said in an incredibly useful
Speaker 1: video on YouTube, you can actually search for Eli the
Speaker 1: Computer Guy and look for the us I model. You
Speaker 1: can actually watch this video. Uh. The model breaks down
Speaker 1: the components of network communication into layers to make it
Speaker 1: easier to understand what is actually going on. So, in
Speaker 1: a way, the seven layer O SI model is sort
Speaker 1: of like the analogies I tend to use on this
Speaker 1: show in order to explain various concepts. It's meant to
Speaker 1: clarify the purposes of various design elements and different protocols
Speaker 1: and and tasks. By the way, there will be many
Speaker 1: analogies in this episode because I love them. I was
Speaker 1: an English major, so that's kind of my go too.
Speaker 1: There are also other models, Like I said, there's the
Speaker 1: Internet Protocol Suite. That's the other standard model of computer networking.
Speaker 1: But we're going to focus on the O SI model
Speaker 1: for this episode. Maybe someday I'll do an episode about
Speaker 1: the t C P I P model, but that will
Speaker 1: not be this episode. They are similar in many ways,
Speaker 1: but there are distinctions between the two. But it's really
Speaker 1: just a way of thinking about how these communication networks
Speaker 1: handle tasks. The OSI model, by the way, did not
Speaker 1: just spring up out of whole cloth either. It's the
Speaker 1: product of an organization called the International Standards Organization or ISO,
Speaker 1: not I S o IO. So let's learn about that
Speaker 1: group first before we talk about the model. Is SO
Speaker 1: is a non governmental international organization that is completely independent
Speaker 1: of any state authority, so it doesn't fall under the
Speaker 1: purview of any one nation's government. Uh. The membership of
Speaker 1: the of ISO includes a hundred sixty two national standards bodies,
Speaker 1: there's only one is a member per country, so you
Speaker 1: can't have multiple standards organizations from one country in ISO's roster.
Speaker 1: It has to be the foremost standards organization of that country.
Speaker 1: The United States, for example, has a member. Or there's
Speaker 1: an organization that is in ISO is the American National
Speaker 1: Standards Institute or and C. And C actually predates ISO.
Speaker 1: Has and C was founded in nineteen eighteen. IO began
Speaker 1: in nineteen More on the founding of ISO in just
Speaker 1: a second. And c's purpose is to accredit standards that
Speaker 1: are created by quote qualified groups in the quote. So,
Speaker 1: in other words, this organization the United States does not
Speaker 1: create standards. It's not the one that says, this is
Speaker 1: what the standard X is from now on, you all
Speaker 1: have to follow it. This is the X by which
Speaker 1: all other x is must be measured. They don't do that.
Speaker 1: What they do is they look at the work of
Speaker 1: qualified groups and say, yes, this example of whatever that
Speaker 1: thing is is the standard against which all others shall
Speaker 1: be judged. So mote it be at this point it
Speaker 1: might even be good for us to just consider what
Speaker 1: the heck a standard is in the first place, because
Speaker 1: otherwise this just becomes a jumble of words, and I
Speaker 1: start using phrases that don't really mean anything to you,
Speaker 1: You're gonna totally tune out. So the definition of standard
Speaker 1: is according to the Miriam Webster Dictionary. Your definitions may
Speaker 1: vary depending on the dictionary of use. It includes the
Speaker 1: definition of something established by authority, custom or general consent
Speaker 1: as a model or example, or something set up and
Speaker 1: established by authority as a rule for the measure of quantity, weight, extent, value,
Speaker 1: or quality. It could also be a giant flag that
Speaker 1: you carry into battle. But that definition probably doesn't really
Speaker 1: apply when it comes to the O S. I'm also
Speaker 1: we're not gonna think about it right now. Standards are
Speaker 1: an agreed upon model upon which all other instances of
Speaker 1: that concept or physical object should be measured. Uh So,
Speaker 1: units of measurements are standards that are easy to understand. Right.
Speaker 1: An inch is an inch no matter where you go.
Speaker 1: A kilogram is a kilogram no matter where you go.
Speaker 1: That sort of thing. So organizations like ants and is
Speaker 1: so help establish these standards. They don't necessarily come up
Speaker 1: with them, but they might accredit them, they might endorse them,
Speaker 1: and thus get widespread acceptance of that as a standard,
Speaker 1: this becomes sort of a generally agreed upon common concept
Speaker 1: across multiple nations. The need for standards was apparent from
Speaker 1: the early days of trade, because merchants had to create
Speaker 1: standards so that trade systems could actually work across different
Speaker 1: currencies and regions, not to mention, create standards of measurements
Speaker 1: so they could make sure they were talking about the
Speaker 1: same amount of stuff they were selling from place to play.
Speaker 1: If you're a if you're a spice merchant, you want
Speaker 1: to make sure the people you're selling to understand the
Speaker 1: quantities you're talking about. So you had to set up
Speaker 1: these standards that people would generally agree to in order
Speaker 1: to actually make trade work. It was necessary for that
Speaker 1: sort of thing. Well, the same thing is true in
Speaker 1: technology in general, not just with units of measurement things
Speaker 1: that are more kind of ultimately tangible, but with general
Speaker 1: concepts where you're gonna have lots of different people from
Speaker 1: all over the world working within those sort of concepts,
Speaker 1: and you want there to be interoperability between all of
Speaker 1: those people. You have to create these standards so that
Speaker 1: everyone is kind of working from the same basic set
Speaker 1: of rules, and by working from the same basic, basic
Speaker 1: set of rules. You you reduce the possibility of incompatibilities.
Speaker 1: There will always be incompatibilities just because technology doesn't always
Speaker 1: work the way we intended it to. But you can
Speaker 1: at least remove some of those barriers early on in
Speaker 1: the des line process by agreeing upon a basic set
Speaker 1: of rules from the start. That's the whole purpose of
Speaker 1: organizations like ISO SO. This extends to telecommunication and networking layers.
Speaker 1: As ISSO states on its web page, international standards make
Speaker 1: things work. They give world class specifications for products, services,
Speaker 1: and systems to ensure quality, safety, and efficiency. Now, i
Speaker 1: SO got its start back in nineteen forty six during
Speaker 1: a meeting of the Institute of Civil Engineers in London, England,
Speaker 1: and there were representatives from twenty five different countries that
Speaker 1: attended that meeting, and they concluded that they needed an
Speaker 1: official but independent organization to help coordinate and unify industrial standards.
Speaker 1: I would become an official entity on February twenty third,
Speaker 1: nineteen forty seven, and according to ISO, the organization has
Speaker 1: published nearly twenty two thousand international standards since it was founded.
Speaker 1: Oh and uh ice SO is not an acronym. It
Speaker 1: is not I s O. It is ISO. It is
Speaker 1: all capitalized, but still it's ISO. And it's because if
Speaker 1: you translate the name of the organization into the different
Speaker 1: languages of all the different countries that have a an
Speaker 1: organization that's a member of ISO, you don't end up
Speaker 1: with words that start with I, S and O with
Speaker 1: every translation. Right. That's just the English version. So the
Speaker 1: English version is International Standards Organization. But other languages have
Speaker 1: it in different orders or even different words. So you
Speaker 1: can't just call it an acronym because not every country
Speaker 1: uses the same the same language, right, So they decided
Speaker 1: instead they would take their name from a Greek word
Speaker 1: ISOs i S O s which means equal so is
Speaker 1: so is always is so no matter what language you
Speaker 1: are speaking. As for the O S I model, now,
Speaker 1: that actually emerged from a couple of different projects that
Speaker 1: we're trying to ablish a framework for network systems. This
Speaker 1: would be in the seventies when networks were really starting
Speaker 1: to become a possibility. You may remember that ARPA net
Speaker 1: had been in the development stages in the sixties and seventies,
Speaker 1: and that was a precursor to the Internet. It was
Speaker 1: also those early early days of networking different devices together
Speaker 1: so that they could communicate directly to one another instead
Speaker 1: of having to work on one machine, save your work
Speaker 1: on some form of physical media, maybe it's a magnetic
Speaker 1: disc or real to real tape or whatever, taking that
Speaker 1: physical media over to another and I guess I should
Speaker 1: say medium over to another device and then loading it
Speaker 1: up there. Instead of doing that, you can have them
Speaker 1: communicate directly to one another using a network solution. But
Speaker 1: you had to establish those standards, particularly since there were
Speaker 1: lots of different computers that were being produced and lots
Speaker 1: of other devices that you also would want to net
Speaker 1: computers too, and they didn't all use the same sort
Speaker 1: of operating systems, they didn't communicate in the same way,
Speaker 1: So you had to create a set of standards that
Speaker 1: would work across multiple instances of networked machines so that
Speaker 1: you didn't just end up with different specific network protocols
Speaker 1: for very specific instances. Let's say that one place has
Speaker 1: all the same sort of mainframe style computers, so they
Speaker 1: design a network that only works with those. That's not
Speaker 1: any use to somebody who's trying to create a network
Speaker 1: for a totally different type of machines. So they wanted
Speaker 1: to create this basic set standards. In the seventies, you
Speaker 1: had two different groups doing that, and eventually those two
Speaker 1: groups they got their work kind of merged together, and
Speaker 1: in nineteen eighty three they published this, or at least
Speaker 1: they they submitted this to ISO. UH. It was called
Speaker 1: the Basic Reference Model for Open Systems Interconnection, and i
Speaker 1: SO would publish that model the following year, in nineteen
Speaker 1: eight four. This would be the UH the ISO publication
Speaker 1: number seven thousand. So next time you're at pub trivia
Speaker 1: and someone says, what's the ISO publication number of the
Speaker 1: os I model, you can just pipe up seven and
Speaker 1: you will win great acclaim. I don't know why you're
Speaker 1: going to I T professional pub trivia, but that's you know,
Speaker 1: I'm not judging. So I SO created the O s
Speaker 1: I seven layers model. This again was to create a
Speaker 1: sort of common lexicon across the world for the purposes
Speaker 1: of discussing telecommunication systems. And let's take a high level
Speaker 1: look at what this means. Those seven layers to this model, AH,
Speaker 1: they represent different tasks sort of or or different layers
Speaker 1: of responsibility within a communications network. The bottom most layer,
Speaker 1: layer one is what we call the physical layer, the
Speaker 1: top layer. Lay year seven is the application layer, and
Speaker 1: if you'd like to visualize this in a practical way,
Speaker 1: layer seven is the one that the end user comes
Speaker 1: into contact with. Layer seven is the part of an
Speaker 1: application or program that that you can see. Now it's
Speaker 1: not Layer seven is not an application in of itself,
Speaker 1: although some applications can exist entirely in layer seven. Layer
Speaker 1: seven is the group of services within a communications network
Speaker 1: that interact directly with whatever is going on with the
Speaker 1: application and send that further down the stack. So while
Speaker 1: a web browser is not in itself a layer seven,
Speaker 1: it uses elements that do tie into layer seven. In
Speaker 1: other words, uh, it's it's kind of like the the
Speaker 1: just under the surface layer for whatever application you have
Speaker 1: opened that has this network communication aspect to it, whether
Speaker 1: that's with a phone or a computer or a printer,
Speaker 1: really any device that's connected to a network. This is
Speaker 1: the one that's closest to you. So it's the one
Speaker 1: that's easiest for us to kind of understand because it's
Speaker 1: the stuff that we interact with. You interact with this interface,
Speaker 1: you send commands. Those commands get pushed down the stack
Speaker 1: and depending it depends on what the command is about
Speaker 1: where that ends up going on the other end of
Speaker 1: the model, and we'll go through all the layers in
Speaker 1: this episode, but right now I'm just looking at the top.
Speaker 1: In the very bottom on the other end, the bottom
Speaker 1: of the stack is layer one. That's the physical layer
Speaker 1: of the networking process. This is the basic bit stream.
Speaker 1: This is the zeros and ones that are being carried
Speaker 1: by some means. You could even argue that it's not
Speaker 1: even zeros and ones, it's the stuff that represents those
Speaker 1: zeros and ones. It's the actual electrical impulses or rays
Speaker 1: of light or radio signals that are carrying information. So
Speaker 1: this is the conveyance of data. So that depends upon
Speaker 1: actual physical properties like physical components like cables or Wi
Speaker 1: Fi cards or things of that nature. That is why
Speaker 1: it's called the physical layer. It is completely dependent upon that.
Speaker 1: It does not care about protocols. None of that stuff matters.
Speaker 1: This is just carrying the signal. Uh So, between layers seven,
Speaker 1: the one that we interact with directly, and layer one,
Speaker 1: that physical layer, are all the intervening levels that handle
Speaker 1: protocols for network communication. Again, this these are not physical,
Speaker 1: literal layers, but rather a way to divide up those
Speaker 1: protocols for the purposes of understanding which layers hand tasks
Speaker 1: off to other layers or accept tasks from other layers.
Speaker 1: So I'm gonna explain this conceptual model with an analogy.
Speaker 1: So we're gonna get even more abstract because that's what
Speaker 1: I do, and you can't stop me. It's my show.
Speaker 1: The O S I seven layer model is like a
Speaker 1: metaphorical bucket brigade fighting a fire. So let's say that
Speaker 1: layer one, that physical layer, that's the person in the
Speaker 1: bucket brigade who is closest to the water spigot. Water
Speaker 1: is pouring out of the water spigot. Now, the person
Speaker 1: who's representing layer one can do a couple of different things.
Speaker 1: They can fill up a bucket with water. They can
Speaker 1: hand a full bucket of water over to the person
Speaker 1: who is representing layer two, or they can accept an
Speaker 1: empty bucket from layer two so that they can then
Speaker 1: fill up the bucket of to make it a bucket
Speaker 1: of water. Again, Layer two can really only do a
Speaker 1: couple of things in this particular analogy. They can accept
Speaker 1: an empty bucket from layer three, or they can accept
Speaker 1: a full bucket from layer one and they pass it
Speaker 1: on in continuing in the direction that it was traveling.
Speaker 1: When you get all the way down to layer seven,
Speaker 1: they can accept a full bucket of water from layer six,
Speaker 1: can splash that water onto a fire, and they can
Speaker 1: send a empty bucket down to layer six. That's kind
Speaker 1: of what's going on here from a conceptual level with
Speaker 1: network communication, it's instead of water communication tasks and different
Speaker 1: layers are responsible for different parts of the task. So
Speaker 1: it gets a little more complicated because it's not just
Speaker 1: accept something and pass it on. That's it's not as
Speaker 1: simple as that. There's a little bit more to it.
Speaker 1: But I wanted to start big so you can kind
Speaker 1: of understand that these layers are all dependent upon one another.
Speaker 1: So I hope that analogy makes some sense to you.
Speaker 1: I really liked it because while I was trying to
Speaker 1: kind of digest the OSI model, that's kind of how
Speaker 1: I imagined it was this process of passing down a
Speaker 1: task so that it goes down the chain until it
Speaker 1: hits where it needs to go, comes back up the
Speaker 1: chain with whatever the response is. Now, the reason the
Speaker 1: whole model is important. Why are we even talking about
Speaker 1: the OSI model in the first place, beyond just giving
Speaker 1: us an idea of what's going on behind the scenes
Speaker 1: when you are using devices on a networked system. It's
Speaker 1: because it provides a guide for developers. For one thing,
Speaker 1: if you're developing programs or applications that are meant to
Speaker 1: be networked, it is really good to have an understanding
Speaker 1: of the OSI model so that you can focus on
Speaker 1: the proper way to implement your your program. Uh. And
Speaker 1: it's also important for you to adhere to the standard
Speaker 1: that's been established. That's what standards are for, after all.
Speaker 1: So you might even wonder why is this even important,
Speaker 1: why are we even talking about OSI models? Well, I
Speaker 1: mean beyond just giving us an idea of what's going
Speaker 1: on behind the scenes of devices that are running on
Speaker 1: a network and how that communication is supposed to work
Speaker 1: from a conceptual level, it provides a guide for developers
Speaker 1: who are creating applications and programs that will run on
Speaker 1: networked devices. It's really important that their processes adhere to
Speaker 1: the standard. I mean, that's why we of standards. It's
Speaker 1: to make sure that things are falling in line so
Speaker 1: that the everything is as smooth as it possibly can be,
Speaker 1: especially when you get into interoperability where you have programs
Speaker 1: operating with other programs. If you have that happening, they
Speaker 1: need to agree upon the same basic set of rules.
Speaker 1: Otherwise it's like getting two people together who speak completely
Speaker 1: different languages that have no basis, like there's no shared
Speaker 1: origin for the two languages. They both arose completely independently,
Speaker 1: and then try to have them have a conversation without
Speaker 1: using gestures or anything else to kind of get their
Speaker 1: meeting across. It's it's practically impossible. So this is very
Speaker 1: important for programmers in order to make sure that their
Speaker 1: approach will be interoperable for other types of processes and programs. Uh.
Speaker 1: This could be as simple as making sure that your
Speaker 1: program is not going to make an entire framework on
Speaker 1: stay will and cause potential crashes or other issues, to
Speaker 1: something more complicated such as apps being able to share
Speaker 1: the same data across each other or access each other's
Speaker 1: functionality in some way. And it's only because we have
Speaker 1: these standards in place at such an approach is really
Speaker 1: possible without having different teams worked very tightly with one
Speaker 1: another from the first place. It's also really useful if
Speaker 1: you're a network specialist like an I T network guy
Speaker 1: or a lady because you've got a problem happening in
Speaker 1: a network that you manage, you want to be able
Speaker 1: to identify where that problem is and how you would
Speaker 1: go about solving it, and being able to figure out
Speaker 1: conceptually where in the OSI model that problem may be arising,
Speaker 1: tells you what approach you may need to take in
Speaker 1: order to fix the problem. I've heard that problems fall
Speaker 1: on the physical layer, which essentially means that someone has
Speaker 1: not plugged something in. It sounds like an I T.
Speaker 1: Crowd kind of joke, but apparently it's absolutely true that
Speaker 1: you know, there's folks who just there's a cable that's
Speaker 1: not fully plugged into a port, or someone forgot to
Speaker 1: turn a switch John or something, and a lot of
Speaker 1: the problems do stem from that. And then once you
Speaker 1: get beyond the physical layer, things get real tricky because
Speaker 1: you got to figure out where is the breakdown happening
Speaker 1: and what can you potentially do about it. So it
Speaker 1: does have a practical application outside of of just a
Speaker 1: general information The model is also obviously used as a
Speaker 1: teaching tool to explain computer network architecture and communication protocols,
Speaker 1: and it gives computer science students a foundation and the
Speaker 1: logic they'll need to understand to work within computer network architecture.
Speaker 1: So that's the overview. Now we're gonna take a quick break,
Speaker 1: and then when we come back, we're gonna start waiting
Speaker 1: into what all these layers actually mean. So let's take
Speaker 1: a quick break to thank our sponsor. All right, let's
Speaker 1: look at these layers. Now, we're gonna start at that
Speaker 1: highest layer and work downwards since I think that makes
Speaker 1: the most sense for the average person. Here's a quick overview,
Speaker 1: and then we're gonna dive further into detail. All right.
Speaker 1: Layer seven is the application layer. That's where the end
Speaker 1: user applications live, or at least where they touch. That's
Speaker 1: where they make contact with this OSI model. This would
Speaker 1: be the stuff that we interact with, like whether you're
Speaker 1: playing Pokemon, or you're using a web browser to surf
Speaker 1: the web, or you're checking a fitness app or anything
Speaker 1: that's along those lines where it's tapping into this network.
Speaker 1: You are interfacing with something that is touching on this layer.
Speaker 1: There's a lot more going down below the surface, but
Speaker 1: we'll jump into that in a minute. Next is layer six.
Speaker 1: That's the presentation layer. That layer handles syntax processing. Don't worry.
Speaker 1: I'll explain that in a little bit. It's sort of
Speaker 1: like a translator in many ways, so hold on to
Speaker 1: that idea. Layer five is the session layer. This layer
Speaker 1: manages the flow of events that establish and terminate network connections,
Speaker 1: communication channels, if you will, between applications or devices. Layers
Speaker 1: five through seven, so the ones we just talked about,
Speaker 1: are considered application level data layers. Everything below those layers
Speaker 1: gets into more fundamental aspects of moving data around, where
Speaker 1: it's not so much concerned with it being identified as
Speaker 1: an application. It's more basic than that. It reaches progressively
Speaker 1: more abstract concepts and moving further away from anything we
Speaker 1: would think of as an application. Level four is the
Speaker 1: transport layer that provides transfer of data between end systems
Speaker 1: and is responsible for end to end recovery. Again, we'll
Speaker 1: go into this in more detail in a minute. Level
Speaker 1: three is the network layer that's the domain of things
Speaker 1: like switches and routers. Level two is the data link layer,
Speaker 1: in which all information we've been talking about the packets
Speaker 1: of information getting coded into bits. This layer technically as
Speaker 1: two sub layers. There's the Media Access control or MAC layer.
Speaker 1: If you've heard of MAC addresses, that's what this refers to.
Speaker 1: And there's the logical Link Control or l C layers,
Speaker 1: so it's not a limited liability company. It is the
Speaker 1: logical link control. More on all of that again just
Speaker 1: a minute. And then finally we get to layer one,
Speaker 1: that physical layer I mentioned earlier. That is the bitstream
Speaker 1: conveyance layer. So all that sounds really technical, but we'll
Speaker 1: break it down layer by layer, and we'll start with
Speaker 1: layer seven, that application layer. It identifies the protocols and
Speaker 1: interface that hosts in a communications network have that the
Speaker 1: end user uses. A host, by the way, is any
Speaker 1: network device. So it could be a computer, it could
Speaker 1: be a smartphone, it could be printer, it could be
Speaker 1: really any networked device that is connected to a computer network. Uh. Really,
Speaker 1: it's anything. It's not just servers. You might think a
Speaker 1: host has to be a server because the name host
Speaker 1: and server seem really related. Um, a server is a
Speaker 1: device that accepts network connections from other devices, and those
Speaker 1: other devices we call clients. So if you use a
Speaker 1: web browser to connect to a web page, the computer
Speaker 1: that has that web page saved on it, because that
Speaker 1: web web page actually exists somewhere. The computer that has
Speaker 1: that web page on it is a server. Your browser
Speaker 1: is acting as the client. It is asking for that
Speaker 1: that web page to be served to it. So that
Speaker 1: is a case of a server. It is allowing the
Speaker 1: web browser to access it. So all servers are hosts,
Speaker 1: but not all hosts are servers. Host is a larger term.
Speaker 1: It includes servers, but includes all the other devices connected
Speaker 1: to the network as well, or at least all the
Speaker 1: computer style devices. So this is the stuff we as
Speaker 1: users encounter when we're working with an application or a program,
Speaker 1: and as long as that program is accessing the network,
Speaker 1: if it's a self contained program, it's kind of separate
Speaker 1: from this OSI model. It's really considered concerned with network communications.
Speaker 1: So let's take a real world example and talk about
Speaker 1: web browsers. The application layer would be the interface ace
Speaker 1: that we rely upon, uh or at least it would
Speaker 1: be touching upon the interface we rely upon when we're
Speaker 1: interacting with our web browser. These are the rules that
Speaker 1: dictate how we can select links, how it instructs the
Speaker 1: app on how to display information to the user in
Speaker 1: response to commands. This is all front of the curtain
Speaker 1: stuff as far as users are concerned. For the os
Speaker 1: I model, this bit also includes aspects like user authentication
Speaker 1: and quality of service. Everything on this layer is application specific,
Speaker 1: so uh, very specific to whatever instance you're looking at.
Speaker 1: It also provides the application services for email, file transfers
Speaker 1: and other network software services. So some services that exist
Speaker 1: at this level are FTP, which is filed Transfer protocol
Speaker 1: and tell net. That's the service that allows you to
Speaker 1: use your computer as if it were a terminal for
Speaker 1: remote server. So, in other words, tell net lets you
Speaker 1: log into a virtual terminal for another machine and execute
Speaker 1: commands as if you were working on that computer directly.
Speaker 1: So there's some computer that's out there in the wild,
Speaker 1: you use tell neet to log into that computer, and
Speaker 1: suddenly your computer, the one you're typing on, it's acting
Speaker 1: as if it is directly connected, like it is part
Speaker 1: of the machine that you are tapping into. When you
Speaker 1: type a command in through your virtual terminal, it's executed
Speaker 1: on the other machine, not on your computer. Telling that
Speaker 1: is bi directional. It's an eight bit bite oriented communications specification,
Speaker 1: and it exists at this layer seven that is the
Speaker 1: domain of tell neet. Some other examples of stuff you'd
Speaker 1: find in layer seven, apart from telling neet, an FTP
Speaker 1: and web browsers are the HTTP protocol and Simple Network
Speaker 1: Management Protocol or s n MP. That's a standard for
Speaker 1: organizing information about devices on IP networks and also the
Speaker 1: rules for modifying that data in order to change device behavior. Now,
Speaker 1: one thing the layer is not is the application itself. Right,
Speaker 1: Applications are separate from the OSI model. They touch on it,
Speaker 1: but they are their own thing. So this is this
Speaker 1: would really be the services that applications can make use
Speaker 1: of that are universal for communications networks. So you can't
Speaker 1: get to applications specific and layer seven because then you
Speaker 1: you depart from the concept of standard. You have to
Speaker 1: have a list of different services that layer seven handles
Speaker 1: that applications can tap into. Right, So it'd be like
Speaker 1: walking into a business that offers certain services. You can't
Speaker 1: order anything off of that list. You can only accept
Speaker 1: the ones that are on that list. Same sort of
Speaker 1: thing for this os I model. H Anything that would
Speaker 1: require the application to reference information on the network, or
Speaker 1: send data to the network, or otherwise interact with the
Speaker 1: network is part of this layer. Layer six. That presentation
Speaker 1: layer handles format conversions and encryption and decrypt shin. So,
Speaker 1: like I said earlier, This is sort of like a
Speaker 1: translator between different forms of data, and it's usually part
Speaker 1: of an operating system. So an example of a layer
Speaker 1: six task is to take clear text that is sent
Speaker 1: from an application and encrypt it before moving it further
Speaker 1: down the layers, or doing the reverse of that and
Speaker 1: taking encrypted information from further down the chain and translating
Speaker 1: it into clear text for the use of the application
Speaker 1: in the layer above. Because of this feature, the presentation
Speaker 1: layer is sometimes referred to as the syntax layer. Syntax
Speaker 1: refers to the set of rules for the arrangement of
Speaker 1: words and phrases to create something of meaning. Syntax allows
Speaker 1: for clear communication, whether in human languages or machine ones.
Speaker 1: So if I tell you later today, I'm going to
Speaker 1: go to the store so that i can buy some
Speaker 1: salted caramel pringles because they taste like golden grams, you'll
Speaker 1: understand what I'm saying. You might not understand why I'm
Speaker 1: saying it, or why I would bother to do such
Speaker 1: a thing. By the way, they really do taste like
Speaker 1: golden grams, and it's not necessarily a good thing. But
Speaker 1: let's say I've said that you would understand what I meant.
Speaker 1: But let's say I rewarded that sentence. I'm using the
Speaker 1: same words, but I've completely changed the order. So it's
Speaker 1: now to the store so that i can buy because
Speaker 1: they taste like golden grams some salted caramel pringles. I'm
Speaker 1: going to go later today. You might have to take
Speaker 1: some time to suss out what the heck I'm saying
Speaker 1: because I'm not using the correct syntax. I'm not following
Speaker 1: the rules of English language to communicate clearly. Syntax is
Speaker 1: really important so that we can be understood. And for
Speaker 1: machines it's extra important because we typically programmed them to
Speaker 1: accept very specific formats and anything that departs from that
Speaker 1: format is essentially nonsense to a machine, so syntax is
Speaker 1: key now. Examples of concepts and protocols that belong in
Speaker 1: layer six include encryption methods of encoding such as JPEG's impegs, MIDI,
Speaker 1: or as key tag. All of that belongs in level six.
Speaker 1: Moving on down to layer five, the session layer. This
Speaker 1: encompasses all the ways that the system enables or terminates
Speaker 1: connections between applications. At this layer, the network allows conversations, exchanges,
Speaker 1: and other transmissions between applications to occur, and it's all
Speaker 1: about session and connection coordination. Another way to think about
Speaker 1: this is the layer is the one that allows one
Speaker 1: machine to chat with another machine across the network. It
Speaker 1: creates that communication channel, also known as a session, So
Speaker 1: it creates the session between the two machines, It maintains
Speaker 1: the channel, and it most importantly, you could argue, terminates
Speaker 1: the channel once the session has ended, so you don't
Speaker 1: have a continuation of data exchange once you have indicated
Speaker 1: that that communication is over. Otherwise you would have a
Speaker 1: situation where it's kind of like listening in on somebody
Speaker 1: when they thought they hung up the phone, but they didn't.
Speaker 1: It would be a end to that from a data standpoint.
Speaker 1: So that concludes all the layers that are on the
Speaker 1: application side, the application layers of the os I model.
Speaker 1: It's all about moving that data around at the application level.
Speaker 1: Below that are the layers where we descend into the
Speaker 1: real guts of the OSI model. And before I do that,
Speaker 1: I need to take a rest. I gotta regain some
Speaker 1: spell slots because I burned a couple in that last section. Also,
Speaker 1: I've been listening to The Adventure Zone a lot. It's
Speaker 1: a show that if you haven't listened to it. You
Speaker 1: should check it out. It's kind of neat, has nothing
Speaker 1: to do with how stuff works. That's just a free
Speaker 1: plug for the adventure zone. Let's take a quick break
Speaker 1: and thank our sponsor. All right, we're down to layer four.
Speaker 1: We've been digging down. We've hut the topmost layer in
Speaker 1: the group of layers that are all about just moving
Speaker 1: data around in general. That it's kind of more abstract
Speaker 1: than the application level. Layer four is the transport layer.
Speaker 1: This is the layer that handles packet ization of data
Speaker 1: and the delivery of packets, as well as checking incoming
Speaker 1: packets for errors when the packets arrive. Packets, packets, packets.
Speaker 1: But wait, I hear you cry out, what the heck
Speaker 1: is a packet? All right? So a packet is a
Speaker 1: basic unit of communication across the network, and there are
Speaker 1: lots of different kinds of packets. There are the kinds
Speaker 1: that are most people I think are familiar with. If
Speaker 1: they've heard the term packet, they're thinking of the um
Speaker 1: Internet Protocol version of packets. But packets can take other
Speaker 1: forms as well. It's really structures within which data can exist.
Speaker 1: Think of it like an envelope. It's very similar to
Speaker 1: that From a conceptual level, an envelope is something that
Speaker 1: you put letters into, or pieces of paper into, and
Speaker 1: then you can send that through the mail. That's kind
Speaker 1: of what packets are, except there for data, like raw data,
Speaker 1: or at least for information that you're going to send
Speaker 1: across a network, not a sequal letter. But let's say
Speaker 1: that you have an envelope you have you bought a
Speaker 1: box of envelopes, and you discover that each envelope has
Speaker 1: a certain capacity. It can hold ten pages of paper.
Speaker 1: But no matter what, you cannot shove eleven or more
Speaker 1: pages into a single envelope. You just can't. You can
Speaker 1: only shove up to ten pages in. So if you
Speaker 1: had a longer document than ten pages, you would need
Speaker 1: to divide it up into smaller groups so as you
Speaker 1: could fit it into several envelopes. So let's say you've
Speaker 1: just completed a two page manuscript during Nano Rimo, that's
Speaker 1: a national novel writing month, and you're going to send
Speaker 1: your physical copy of this manuscript off to a friend
Speaker 1: for proof reading. But your friend lives very far away.
Speaker 1: You're not in a real rush, so you're just gonna
Speaker 1: send it through post. You don't happen to have a
Speaker 1: digital copy because you typed it out on an old typewriter.
Speaker 1: It doesn't really matter. So all you have are these
Speaker 1: normal envelopes that can hold ten pages each. How you
Speaker 1: get your document to your distant friend. Well, if you're
Speaker 1: a machine on a network system, you would divide up
Speaker 1: the job into manageable chunks and pack each chunk of
Speaker 1: information into its own packet. So for our real world example,
Speaker 1: you would have to divide your two page manuscript into
Speaker 1: twenty ten page chunks and put each ten pages into
Speaker 1: a different envelope. Now you would realize, because you were
Speaker 1: a bit of a doof you sealed all the envelopes,
Speaker 1: but you forgot to include page numbers on the individual pages. Well,
Speaker 1: fortunately you still have all the envelopes in the right order,
Speaker 1: So you include information on the envelope itself to indicate
Speaker 1: to the recipient where those pages fit within the entire manuscript.
Speaker 1: So the first envelope says pages one through ten, for example,
Speaker 1: and envelope two says pages eleven through twenty. You dump
Speaker 1: all twenty envelopes in the mail and the postal service
Speaker 1: picks them up. Now it's a busy time at the
Speaker 1: post office, and the destination for these envelopes is quite
Speaker 1: a far way away, and the various envelopes get a
Speaker 1: little divided up. They get put into different bags, and
Speaker 1: they get distributed to different trucks and different planes. As
Speaker 1: they travel to their destination. They're all going to the
Speaker 1: same place, they're each taking a slightly different pathway there,
Speaker 1: your friends starts to get the envelopes in the mail
Speaker 1: and is able to put the pages in order because
Speaker 1: you included information on those envelopes themselves, And eventually your
Speaker 1: friend has the whole manuscript. Well, that's sort of how
Speaker 1: data transmission across a network happens, except there are a
Speaker 1: few extra considerations. One is that devices will typically send
Speaker 1: more than one of each packet. Sometimes packets will encounter
Speaker 1: problems as they travel across the network. Maybe a node
Speaker 1: in the network goes offline without warning, or something else happens.
Speaker 1: Maybe there's some traffic congestion at that section of the network.
Speaker 1: So if you send multiple copies of each packet, there's
Speaker 1: a better chance that the whole thing will get to
Speaker 1: the destination. Even if you don't send multiple copies, what
Speaker 1: will happen is there will be an acknowledgement on the
Speaker 1: recipient side of receiving a packet, and if there's a
Speaker 1: packet missing, that message gets sent across and then the
Speaker 1: sender can send that specific packet again and it can
Speaker 1: travel across the network to the recipient. On the receiving end,
Speaker 1: the transport layer takes in these packets and consults the
Speaker 1: data in the header of that packet to assemble the
Speaker 1: data properly. That's kind of like those page numbers on
Speaker 1: the envelopes I was talking about. It's the equivalent of
Speaker 1: that saying, here's where this data fits in context with
Speaker 1: all the other information that's coming in that is related
Speaker 1: to this particular file. Uh, there's a bit more to
Speaker 1: it than that, but the nature of the packets are
Speaker 1: dependent upon the type of packet it is and the
Speaker 1: protocol in question. And I think you get the general
Speaker 1: idea here now. As Eli the computer guy explains it,
Speaker 1: the session layer opens up communication channels, but the transport
Speaker 1: layer is the one that decides how much data is
Speaker 1: actually exchanged. Betwe lean those different devices that are in
Speaker 1: communication with each other, so they're very tightly bound together,
Speaker 1: so much so that some people put them in the
Speaker 1: same layer. They say that session and transport a kind
Speaker 1: of related closely enough where you can think of them
Speaker 1: as a single layer as opposed to to separate ones.
Speaker 1: Let's move down to layer three. That's the network layer.
Speaker 1: This is the logical pathway layer or the virtual circuit layer.
Speaker 1: This is the layer of switching and routing where data
Speaker 1: transmits from one note to the network to another. And
Speaker 1: at this layer a network performs all of the internetworking,
Speaker 1: all the congestion control, all the addressing features, as well
Speaker 1: as packet sequencing. So another way to think of this
Speaker 1: is that this is the layer in the network where
Speaker 1: network protocols that are in charge of making sure data
Speaker 1: gets from the sender to the recipient does so. Like
Speaker 1: it's all the rules that will guide data along the
Speaker 1: network to get to their destination. That means that it's
Speaker 1: kind of like a road system with all the road
Speaker 1: signs and traffic instructions included. Right, the IP address of
Speaker 1: a machine is at the network level. Now we get
Speaker 1: down to layer two, that is the data link layer,
Speaker 1: and this is where data packets getting coded into bits
Speaker 1: or decoded from bits into packets. At this layer you
Speaker 1: also have transmission protocols and management as well as error
Speaker 1: detection and response from errors in the physical layer below.
Speaker 1: So if something goes wrong in the physical layer below,
Speaker 1: it is the data link layer that is important. Is
Speaker 1: the data link layer that actually detects those errors and
Speaker 1: sends the information up the chain so that someone can
Speaker 1: do something about it. So if there's a switch that's
Speaker 1: not on or a cable that's not plugged in, that
Speaker 1: ends up being detected at layer two and then sent
Speaker 1: up the stack. Uh. There are two sub layers in
Speaker 1: the data link layer. The Media Access control or MAC
Speaker 1: layer is the first one, and you probably have heard
Speaker 1: the term MAC address. You might wonder what the heck
Speaker 1: that means. It's a hardware addresses unique to every node
Speaker 1: within a network. The MAC layer has a direct interface
Speaker 1: with a network medium, and every device that is connected
Speaker 1: to a network has its own MAC address, and it's
Speaker 1: actually burned or recorded on every devices network card. It
Speaker 1: is unique to that And of course the purpose of
Speaker 1: an address is so that a network has to know
Speaker 1: where to send information. Otherwise the whole system wouldn't work
Speaker 1: because if you had a network that didn't use addresses,
Speaker 1: it would be like shouting into the void and getting
Speaker 1: nothing back, or worse, it would be like getting everything back,
Speaker 1: including the stuff you wanted, but also everything else, so
Speaker 1: it would just be a big jumbled mess and you
Speaker 1: wouldn't be able to make sense out of anything. So
Speaker 1: you've got to have the addresses for this to work properly.
Speaker 1: The other sub level of this layer, or sub layer
Speaker 1: I guess of this layer is the logical link control layer. Uh.
Speaker 1: That is the sub layer that controls frame synchronization and
Speaker 1: flow control as well as error checking. So that error
Speaker 1: checking was talking about with the physical air that belongs
Speaker 1: to the I'll see. Yeah, those are some new terms,
Speaker 1: and I guess we needed to find some of them.
Speaker 1: So in telecommunications, which is different from video because there's
Speaker 1: frame synchronization and video as well, but in telecommunications it
Speaker 1: means something slightly different. Frame synchronization is the process in
Speaker 1: which incoming framed data gets extracted for decoding with the
Speaker 1: help of frame alignment signals. Frame synchronization is necessary because
Speaker 1: sometimes data sent to a receiver encounters issues, and frame
Speaker 1: synchronization can identify and correct for those issues, synchronizing between
Speaker 1: the sender and receiver so that you get the full
Speaker 1: amount of information that you expect. Flow control is the
Speaker 1: management of data flow between nodes on a network, and
Speaker 1: it's important so that data can be handled efficiently. So
Speaker 1: you don't want to send more data than a recipient
Speaker 1: can accept at any given time, because you'll hit data overflow,
Speaker 1: and that means that not all the information you're sending
Speaker 1: is going to go through. You're gonna have to re
Speaker 1: send it anyway. So it's better to go ahead and
Speaker 1: make the system more efficient and control that flow of
Speaker 1: data in order to avoid that problem in the first place.
Speaker 1: In this case, the analogy I always think of is
Speaker 1: that classic Christmas episode of I Love Lucy in which
Speaker 1: Lucile Ball is trying to wrap candies along an assembly
Speaker 1: line and the candies start coming faster and faster, so
Speaker 1: she's frantically shoving pieces down her shirt and in her
Speaker 1: hat and in her mouth so that they don't get
Speaker 1: past her. You want to make sure that the data
Speaker 1: like sweet Sweet candy gets sent at just the right
Speaker 1: pace so that it can be handled properly instead of
Speaker 1: shoved into Lucile Ball's mouth or something. And too much
Speaker 1: data is sent to any particular node in the network,
Speaker 1: you get that data overflow situation, and finally we get
Speaker 1: down to layer one. That physical layer. This is the
Speaker 1: layer that handles the actual method of conveyance of data. Now,
Speaker 1: earlier we were talking about data in various formats such
Speaker 1: as clear text or encrypted data, or ultimately as bits.
Speaker 1: But even as we break all that down to the
Speaker 1: basic units of information, there has to be some way
Speaker 1: to actually send that information somewhere. You have to get
Speaker 1: beyond this concept of information. How do you get that
Speaker 1: information somewhere else? It has to travel through some sort
Speaker 1: of medium. Typically it would take the form of something
Speaker 1: like radio signals or light pulses of light, or electrical impulses.
Speaker 1: So this would be the electrical or mechanical part of
Speaker 1: the system that allows for that transmission. That includes physical
Speaker 1: components like cables and computer cards. It also includes information
Speaker 1: about the nature of that conveyance, such as the radio
Speaker 1: frequency you're going to use for wireless transmission, or the
Speaker 1: actual electrical characteristics of various components, like how many pens
Speaker 1: are in any particular connector, or the voltages that you
Speaker 1: need to use for physical components. Layer one. It's so
Speaker 1: basic that's beneath the realm of protocols. It's completely unconcerned
Speaker 1: with protocols. It's literally us the layer that creates the
Speaker 1: actual connection, whether it's physical or wireless. That's all the
Speaker 1: connections between the different devices on a network, and that's it.
Speaker 1: Those are the layers in the OSI model and what
Speaker 1: they mean now, I know I got a little fuzzy
Speaker 1: in there, largely because so many of those layers depend
Speaker 1: upon an understanding of other topics and computer science and telecommunications.
Speaker 1: But really, when you break it down, you're just talking
Speaker 1: about the combination of all the components of network communication
Speaker 1: and then breaking those down into logical layers to make
Speaker 1: it less overwhelming, right, because you think about all the
Speaker 1: things that have to happen for network communication to actually work.
Speaker 1: Then you say, well, that's too big of a picture.
Speaker 1: What if I divide this up into slices and I
Speaker 1: group stuff together in ways that just kind of logically
Speaker 1: are related to one another. Even if when you break
Speaker 1: down physical components it may be a different story. Complementary
Speaker 1: components tend to be in the same layer in this model,
Speaker 1: and some of these layers are in practice often combine
Speaker 1: and like I said, layers four and five, those transport
Speaker 1: and session layers can sometimes be combined with one another.
Speaker 1: Thank you so much, Nick for the suggestion of the
Speaker 1: OSI model. I hope I didn't gum it up too
Speaker 1: much because I wanted to keep it fairly high level
Speaker 1: so that people who are completely unfamiliar when network communications
Speaker 1: could follow it without getting so high leveled that it
Speaker 1: was not a useful conversation. If you guys have suggestions
Speaker 1: for future topics of tech Stuff, there are a lot
Speaker 1: of ways you can get in touch with me. One
Speaker 1: of those is through something called email, which, as you recall,
Speaker 1: is handled by layers seven of the O SI model.
Speaker 1: Send me an email message the addresses tech Stuff at
Speaker 1: house stuffworks dot com. Let me know what you would
Speaker 1: like me to cover, or what sort of guests you
Speaker 1: would like on the show, or maybe a guest co
Speaker 1: host you would love to hear on the show. Let
Speaker 1: me know that. You can also get in touch with
Speaker 1: me via Twitter or Facebook. The handle of both of
Speaker 1: those is tech Stuff h s W or like Nick,
Speaker 1: you can join us over at twitch dot tv e
Speaker 1: slash text stuff. On Wednesdays and Fridays, we record new episodes.
Speaker 1: You can watch live as I mess things up and
Speaker 1: chat with the chat room, and you can even suggest
Speaker 1: future topics right then, and they're just as Nick did.
Speaker 1: And that's it for today's episode. I'll talk to you
Speaker 1: guys again, really say. For more on this and thousands
Speaker 1: of other topics, is how staff Works dot com