OSI Model (OSI Reference Model) : The 7 Layers Explained

OSI Model (OSI Reference Model) : The 7 Layers Explained


This is Ram here from EngineeringMentor.com In this video, we will discuss the OSI Reference
Model. We’ll first try to understand what is the OSI
model and then discuss the 7 layers of the
model. so let’s start. As we discussed in our previous video, the main idea with computer networks is
communication between different systems. The purpose of communication can be
Information sharing. Suppose we have two systems A and B which
want to communicate with each other. Let us say these two systems are connected to
each other through a computer network. The distance between A and B may only be a
few metres or they might be across different continents. If A wants to send some data to B, then the data
has to travel through the network to reach B. During this transfer, the data will have to
navigate through many intermediate nodes. To ensure that the data reaches from source
machine A to destination machine B Securely and correctly we need to have some rules which guide the
communication between A and B. These rules are called as Protocols. Thus for every communication, we need to have
some protocols which guide the communication These protocols are grouped together and called
as a Communication Model. The OSI Reference model is one such
Communication Model OSI stands for “Open Systems Interconnection” which means that every system participating in
this model is open for communication with other systems. This model was first defined by an organization
called as ISO The ISO is a standards organisation which
works on standardisation of protocols The OSI reference model defines the
functionality of network communication The OSI model divides the communication into
“7 layers” Let us now look at each of these layers in detail This is how the OSI model looks like At first glance, you might feel this is
complicated and hard to remember But hold on to that thought.
The OSI model is very easy to remember All you have to do is remember these 7 layers. In our previous example we said that system A
wants to communicate with B over the network This model depicts the same process These are the end systems A and B
connected by a network The intermediate nodes implement only the
bottom 3 layers of communication since their task is to just pass on the data
along the navigation path The top 4 layers are implemented only by end
systems, but bottom three are implemented by all nodes
in the path. Every layer has some protocols using which it
communicates with corresponding layer in the other system So these dotted lines indicate “Protocols” We call these as Application layer protocols, presentation layer protocols, session layer
protocols and so on Every layer also communicates with the layer
above and below it We say that the every layer provides some
“Service” to the layer above it So Protocols work between same layers of
different machines, where as Services work with different layers
within same machine We will discuss about “Services and Protocols”
in detail in a different video Now let us see what each of these layers do At the top is the Application Layer This is the layer at which users communicate
with the systems This layer provides some protocols using which
applications can communicate with each other and example of Application layer protocols is the
FTP protocol Next is the presentation layer This layer is concerned with the format of data
exchanged between the end systems For example, if the integer format in system A is
32 bits long and in system B is 64 bits long then appropriate conversions have to de done This is done by presentation layer Session layer allows users on different
machines to create sessions between If an application creates different transport
streams session layer can bind all these different
streams belonging to same application For example, if you are doing a video chat then session layer combines the audio stream
with the video stream Transport layer recieves the data from session
layer and divides it into smaller units called as “Messages” These messages are passed on to network
layer On the recieving end, it makes sure these
messages are accepted and arranged in correct order The messages merged and passed on upper
layers Network layer breaks a message into “Packets” and transmits them across the network It is responsibility of the network layer to make
sure the packets reach the correct destination This feature is called as Routing This is implemented on every node in the
network A Node here can be computers, switches,
routers etc The data link layer is concerned with
transmission of error free data It breaks the packets into smaller units called
as Frames These frames are passed on to physical layer
for transmission On receiving end it collects raw bytes from physical layer and
aggregates it into frames Data Encoding, Framing, Error
Detection and Correction techniques are all applied here The bottom layer is the physical layer which is concerned with transmission of raw bits
over the communication link So this how the OSI Reference Model works There is another reference model called as the
TCP/IP Reference Model which we can discuss in another tutorial So now lets do a quick recap Application Layer is where the users interact
with applications to provide data Presentation Layer is concerned with the format
of data exchanged between the end systems Session Layer allows users on different
machines to create sessions between them Transport Layer is concerned with end to end
communication of messages Network Layer is concerned with routing of
packets to correct destination Data Link Layer is concerned with transmission
of error free data in the form of Frames Physical Layer is concerned about transmission
of raw bits over the communication link

100 thoughts to “OSI Model (OSI Reference Model) : The 7 Layers Explained”

  1. Thank you so much for explaining the OSI model in such a short time. It was very easy to learn the way you have explained.

  2. I have a question: when the application of host A wants to establish a connection with a host B it has to have the address right, which e.g. can be an IP address. But how does this work? The layer model tells us that the NETWORK-layer deals with IP addresses and attaches them. But how does the IP-address get kind of passed down from the application to the network layer? Thank you!

  3. OSI MODEL
    ===========
    pls Physical Layer
    do Data link layer
    not Network layer
    tell Transport Layer
    secret Session Layer
    password Presentation
    anyone Application

  4. Awesome explaination, really I have understood a lot, d vedio lecture was shot n understandable. Thanks a lot sir for dis wonderful explaination.

  5. Tq.nice. please make it more easy by explaining in non technical language and with examples we are fed up with those technical jargons as we are COMMERCE STUDENTS SO MAKE IT MORE USER FRIENDLY.
    BTW I SUBBED.

  6. Its a great explanation sir. I wish i can find TCP/IP model too in ur channel i searched for it but failed to get it

  7. The OSI model flows from physical layer to Application layer not reversed. Apart from this the concept was nicely presented.
    Thxx

  8. Good explanation of the OSI model. Many get the ALP wrong by quoting the application itself responsible, but its the underlying protocols.

  9. well explained

    i went through 5-6 b4… fr OSI model explaination

    bt this was the best…exactly wht i was looking

  10. Thank you for your great and clear explanation! I've one question. Is the OSI model a protocol or not? Starting from 1:31, if i understood you rightly, you say that the OSI model is a protocol (and so does the internet), but in the (ISC)2 SSCP official study guide, they emphasise that the OSI model is not a protocol. Now i'm not sure anymore.

    Quote from Ch 8, page 320: "The OSI model is not a protocol, it is a model used for understanding and designing a communication architecture that allows any two systems to communicate regardless of the underlying hardware or software infrastructure."

  11. nice video

    vrf lite===== https://www.youtube.com/watch?v=_4LaalpnfZg&t=147s
    basic packet flow====https://www.youtube.com/watch?v=5kCGv9mfuDQ

  12. Very well done sir. I am currently writing a training on networking for new hires to the software company that I work for. This is a great explanation and helps me out alot. Thanks for taking the time to put this together.

  13. 1.Please (Physical Layer)

    2.Do (Data Link Layer)

    3.Not (Network Layer)

    4.Throw (Transport Layer)

    5.Sausage (Session Layer)

    6.Pizza (Presentation Layer)

    7.Away (Application Layer)

  14. Transport's layer Protocol Data Unit is not a message, it is Segment, be careful with it. Plus, a switch is used on Data Link Layer, just like a hub.

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