hello this is IT have you tried turning off and on again no I'm just having a problem with the server I can't even pin it yeah that sounds like a layer three problem I don't do those bye aside from being the worst support call in history what else is interesting about this the support officer mentioned layers he is talking about a layer of the OSI model but what does this mean and why is it important to us that's what we're here to discuss in this video all devices on the network have Network hardware and software
they need to speak a common language that is one host needs 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 isn't 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 don't 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 an envelope 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 may be 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 a 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 don't 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 easily understood that's what the presentation layer is for anything related to data formats lives here 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 doesn't work well for lower layers for instance what if the data you're transferring is very large maybe this doesn't 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 it 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 file also apps can take turns at sending chunks of data rather than one app hogging the hosts 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 own job to do and doesn't 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 layer do you think needs to be addressed well
I'm gonna 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 contain a lot of data this data may not always make sense to the rest of the network 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 is called a session it's not surprising that this is handled at the session layer traffic you would see here includes a requests 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 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 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 web server with port 80 in the header it will also 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 need 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 now called a packet now we need to think about getting data from one device to another this is where the data link layer comes in notice that the goal is to get data from one device to another this layer does not try to get data to the end device that's the responsibility of layers 3 & 4 part of this includes establishing a logical link between the devices on the same network segment we need to have a logical link as devices probably aren't directly connected they're usually connected with a shared medium like a switch
or over Wi-Fi a well-known protocol here is Ethernet which uses MAC addresses yet another header is added which in the case of Ethernet includes the source and destination Mac's a trailer may also be added we error correction information once the header and trailer are added this block is now called a frame as data moves across the network it will likely pass through one or more routers as each of these routers are a separate device the destination layer 2 addresses will change with each hop but the destination layer 3 address will remain the same the data
link layer is special as it contains two sub layers the logical link control sub layer is responsible for translating between the network layer and the data link layer the media access control sub layer is the part that's responsible for adding headers and trailers to the packet creating the frame it's also responsible for error correction and finally the physical layer this concerns itself with the physical components of the network this includes radio frequencies and channels pulsing light pins electrical signals and standards of copper cabling further this means that the physical layer is responsible for encoding information
in a way that can be understood for example what electrical signals represent a 1 or a 0 or what if Wireless is used if you want to dig deeper into this topic the CCENT guide is quite good also I have more information on my website and I'll include a link to both of those in the description so is this making sense to you tell me which layers do each of these items live in I'll put the answers on the website of course and mole around it pause the video and see if you can match the
layer with the description how would you like to see an example of how this works well protocols these days do not fit the OSI model exactly remember it's just a reference model to help us but still let's try looking at a client sending a request to a web server for a web page on the client side the web browser is the application a user opens a web browser and types in a URL the web browser uses a protocol called HTTP to prepare the request this request is the data that needs to be sent to the
server HTTP doesn't simply fit into one layer if you're sitting in exam HTTP is considered an application layer protocol however practically it spans a few layers so we'll say that it's an upper layers protocol it creates the request that we want to send and it makes sure any special characters were sending up properly encoded HTTP doesn't really manage a session in the upper layers that's managed by the transport layer at this layer a protocol called TCP is used one of the first things that TCP will do is break the data into segments we're only talking
about a small request here so TCP shouldn't really need to break up anything in our particular case it does still need to create the session we were just talking about the first part of this is assigning port numbers HTTP usually uses port 80 on the server side this is the destination port the client simply generates a random port number and uses this as the source port these port numbers are added to the data in the form of header TCP then stores all this information in memory this is how it keeps track of this session and
knows which application is waiting for the response now the network will add addressing information in our case we're using IP addressing it firstly needs to know where to send the request which you can get from the URL this becomes the destination address it also needs the address of the client so the web server will know who to respond to this is the source address these addresses are add to the segment as a header they are also stored in memory along with port numbers to track the session but you've probably noticed that the client and server
are not directly connected they're on different networks were to join by a router so the packet can't be sent straight to the server it needs to be sent to the router first in our data link layer we have the Ethernet protocol this creates a logical session with the router Ethernet we'll create a header for the packet containing the routers MAC address as the destination and the clients MAC address as the source notice that we don't need any MAC addresses for the switch a simple switch operates at layer two it's only their two forward frames to
the correct device Ethernet also includes a small piece of error correction information call the CRC at the end of the packet and finally it's time to convert the frame into bits and put them on the wire to be transmitted our data moves through the switches without being altered and arrives at the router the router then takes out the source and destination MAC addresses it now puts the server's mac as the destination and its own MAC as the source and sends the packet on it also generates new error checking information the service NIC receives the electrical
signals from the cable at the data link layer the server will check that there are no errors with the frame by checking the trailer if all is good it strips off the layer two header and trailer and inspects the IP address information at layer 3 it will check that this packet was indeed meant for itself and it will store the source and destination IP addresses in memory so it can reply later it now strips the layer 3 header off and looks at the source and destination ports it will store these ports in memory along with
the IP addresses in this way it has also established a session with the client it sees that the destination port was port 80 and it knows that it has web server software listening on port 80 the data is then given to the correct application and HTTP will take it from there the server will of course want to send a response it will go through the same process to send out over to the client the next video of this series we'll take a look at how IP addressing works I hope to see you there
