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