How the OSI Model Works Network Fundamentals Part 3

In this article all devices on the network have Network hardware and software they need to speak a common language that is one host need to communicate in a way that another host can understand these languages are called protocols which we discussed a little in the last two videos, but there isnt just one protocol that makes the magic happen several protocols need to work together to get the job done but trying to track how they all interact can get complicated, and that’s why we have Network models they help us to understand how everything fits together we’re going to go off on a tangent for a little while and use an analogy to describe the layered model think about sending a letter or a package, there’s a lot of parts to this process some of which you dont even see we’re going to use this process to explain how models work to start with you write a letter you then take that letter and put it in and envelop you write the sender address on the back and the receivers address on the front, you put a stamp on the letter now you take it to the local post office the staff there will work out where it needs to go and put it on the right truck anything gets delivered to the destination post office a postman takes the letter and delivers it to your friend’s house what we’ve done here is we’ve built a model this describes how each of these parts interact but the details underneath may change for example, maybe we’re not writing a letter maybe we’re sending a package the address maybe local or may be international this of course changes the number of steps we’ll need when we’re
ready to send the letter we could take it to the post office or drop another post box maybe a private courier will come and pick it up or perhaps a bike messenger will be involved, it can then be taken to assassination by truck plane
ship or some other combination of transport and finally it may be delivered to a house business or to a p.o box the important part here is that we can take a model, and we can use it to understand the overall process we dont need to know any specific details to start with but the model can help us to
understand them in the end the OSI model can help us to understand the network it breaks the network up into seven layers these layers are physical Data Link Network transport session presentation and application, and if that’s a bit tricky to remember think of this mnemonic please do not throw sausage pizza away I know it’s a bit silly, but it helps us to remember the layers and their order notice that these are all generic layers there’s no Ethernet layer or email layer the OSI model is not about specific technologies, but rather how they fit into the network stack imagine that we have an application on one host that needs a send data to an application on another host let’s now see how this data moves through the OSI model the
data as the application sees it starts at the application layer this is where Network API’s and apps that access the network live, for example, this includes FTP and web browsing the data needs to be in a format that can be been easily understood that’s what the presentation layer is for anything related to data formats lives here of such as image and video files the session layer tracks application processes this includes remote procedure calls and service requests think of this layer as building a session between a local application and a remote one, you can probably see that the session presentation and application layers work with very large pieces of information but that doesnt work well for lower layers for instance what if the data you’re transferring is a very large maybe this doesnt seem like a problem on the surface but what happens if you get right to the end of the transfer and the connection is interrupted you would have to start the whole transfer again also any other application that wants to use the network would have to wait until his transfer is complete fortunately there’s a way around this when the data reaches
the transport layer is broken into manageable chunks now if there’s a problem with the data only one chunk of data needs to be resent not the entire
the file also apps can take turns at sending chunks of data rather than one app hogging the host’s resources this is called multiplexing sometimes we need to
add more information adding the destination address is an example of this any information we add to the front of our data is called the header any
information we add to the back is called a trailer as you can see each piece of data gets bigger and bigger as it moves through the network stack eventually the data reaches the physical layer where it is transmitted over cable or Wireless to a remote host the remote hosts receive the data at the physical layer the process is then reversed the data flows back up through the layers each layer does its job removing headers and trailers, and hurting the data until it is in a form that the application can understand can you see here that each layer will only communicate with the layer above, and the layer below each layer has its job to do and doesnt get involved with
other layers are Then, to pass and receive data you can probably see how this makes it easy to combine different protocols to achieve different tasks now
I’m going to pose you a scenario you’re working in the network team and users are complaining that a new high bandwidth application is causing the network to slow down which layers do you think needs to be addressed well I’m going to leave that for you to think about I’ll put the answer on the website if you’re curious will now take a closer look at what each layer does afterward we’ll run through an example of how this might work developers and application
specialists spend a lot of time in the upper layers this starts with the application layer this is not strictly the application itself but rather it’s how the application accesses the network some examples of this include web browsing accessing emails and transferring files it also includes management sessions like ssh telnet and RTP the application may contains a lot of data this data may not always make sense to the rest of the networks so the presentation layer helps by converting it if needed this conversion may also include services like encryption and compression file formats also live here including images and video files an application may need to talk to several other endpoints so it is important to track where these conversations are occurring each of these conversations are called a session it’s not surprising that this is handled at the session layer traffic you would see here includes a request to remote services you may have heard of session control protocol or SCP it leaves at this layer to the transport layer is used to transport traffic between processors on to endpoints you’ve probably heard of TCP and UDP these are the most common protocols used at this layer Earlier, we were talking about how to data large enough needs to be taken and broken into manageable blocks that’s part of what this layer does generally we call this block of data a segment technically if you’re using TCP
each block is called a segment but if you’re using UDP each block is called a Datagram so now we have many blocks of data probably going to different hosts for different applications so how can we track what goes where the answer is through port numbers each flow of data has a port number associated with the source and destination host these values are added as a part of a header to each block of data this may be easier through an example a web server will use port 80 to listen for web traffic a workstation will send a request to the webserver with port 80 in the header, it will also be select a source port and send that along as well the web server will send a response back to the workstation using its port as the destination but to send information from one host to another we needs some form of addressing in addition to that we need a way to route through the network, that’s what the network layer is for one of the common protocols we see here is the Internet Protocol or IP this layer adds yet another header in the case of IP, this includes the source and destination addresses once this is done the block of data is called a packet now we need to think about.


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