Introduction to OSI
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Thread: Introduction to OSI

  1. #1
    Senior Member cwk9's Avatar
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    Arrow Introduction to OSI

    A tutorial/intro by cwk9

    The OSI Reference Model
    Proudly being rammed down the throats of unsuspecting networking students since 1984

    Definition:
    OSI reference model: Open System Interconnection reference model.


    Short History:

    Back in 1984 many of the networks being built were incompatible with each other. So the ISO (International Organization for Standardization) got together and decided to lay down the law. What they came up with was the OSI reference model. I couldnít find anything on the actual process of creating it but I assume it involved locking 20 nerds in a room with a keg and telling them to have something by the next morning.


    Why Itís Important:

    Basically OSI is the theoretical stuff behind networking. It servers as a foundation for building networks and trouble shooting. But more importantly it helps put those big complicated networks into perspective. If youíve taken any Cisco orientated courses youíll know that they make sure that you know OSI like the back of your hand before you even see a router. Itís dry and boring but it really does change the way you look at computer networks.

    Itís all in the layers:
    Transferring data form one computer to another is divided into 7 layers. Each one performing its own function.

    Layer7: The application layer
    Layer6: The presentation layer
    Layer5: The session layer
    Layer4: The transport layer
    Layer3: The network layer
    Layer2: The data link layer
    Layer1: The physical

    Itís important to note that no layer can work with out the help of the layers below it. If you were to send a picture to another computer the data would start at the application layer and move down every respective layer. The computer receiving would start at layer 1 and build the data backup to the app layer. When sending data flows down the layers and when receiving data flows up the layers. The top three layers deal with the data formatting and use interface while 4,3,2,1 deal with the delivery of the data.


    The Application Layer: The layer that cares.
    This is the only layer that you can see. Think of the application layer as the user interface. Its there so you donít have to worry about all those other layers.

    The Presentation Layer: The application layers little helper
    Data Formatting. Thatís the only thing you have to remember about this layer. Things like compression and encoding happen here.

    The Session Layer:
    Handles the over all connection between two host. It decides what needs to be sent and when to send it. Also keeps data for different programs separated.

    Some examples of stuff you would find at these layers
    Application Layer: Internet explorer, Quake, Kazaa,
    Presentation Layer: ASCII, Audio compression, encryption
    Session Layer: The operating system, Application Access Scheduling.

    The above layers are more programming orientated were as the bottom four are were the networking gurus play. Layers 4,3,2,1 are were things like routers and tcp/ip go.

    The Transport layer: The buffer between the upper layers and the lower ones
    Five basic services happen at this layer
    -Segmenting upper-layer application data
    -Providing flow control
    -sending segments from one end host to another end host
    -ensuring data reliability
    -Establishing end-to-end operations
    If your wondering were the error checking was this is were it happens. The Transport layer determines if data needs to be resent. Itís also were two computers decide who sends what and when.

    Stuff that you would find at the Transport layer: UDP, SPX, TCP(not TCP/IP just tcp), Windowing, Error checking.

    The Network Layer: Were stuff happens
    This is the layer were logical addressing happens. The network layer finds out were to send stuff that isnít directly linked by mac address. Think routing.

    Some things you will find and the network layer. Routers, IPX, IP(not tcp/ip just ip)

    The Data Link Layer: (insert your own clever subtitle here)
    This is the first point of logical organization. It deals with preparing the data to make the transit across the network media. The data link layer is concerned with physical addressing not logical. The data link layer can be summed up as Media access control.

    Stuff you find at the data link layer. HDLC 802.3/802.2, NIC cards and switches, EIA/TIA-232

    The Physical Layer: The Layer you can kick
    The actual sending of the electric signal. Getting those 1ís and 0ís from point A to point B. The general rule is if you can kick it its working in the Physical layer. Just keep in mind just because a device is working in layer 1 doesnít mean it only works there.

    Stuff you can kick that works only in the physical layer: Wire, Hubs, Repeaters and most likely any other networking device without a chip in it.

    Encapsulation:
    Encapsulation is an important term when referring to the OSI model.

    A text book definition: Encapsulation is the wrapping of data in a particular protocol header. For example, upper-layer data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network can simply be placed in the header used by the data link layer protocol of the other network.

    That definition never really helped me much. Basically as data moves down the OSI model each protocol throws in its own extra bit of data, like IP and mac address.

    When de-encapsulation occurs the extra data is striped as the packet/frame/segment Öect moves up the layers.

    A quick run down of Encapsulation
    1. Build the data
    2. Prepare the data for end-to-end transport
    3. Add the network address to the header: contains source and destination IP
    4. Add the MAC address to the data link header:
    5. Convert the frames into 1ís and 0ís and fire them off over the network media

    Data Encapsulation at different layers of the OSI model
    Application =>Data
    Presentation =>Data
    Session =>Data
    Transport =Segments
    Network =>Packets
    Data Link =>Frames
    Physical =>bits

    Now there is another model that important to learn and thatís TCP/IP. The TCP/IP model is arguably more important than OSI because it has protocols were as OSI is very generic and theoretical. A common misconception is that TCP/IP is simpler because it has fewer layers.

    Some Differences between the two models

    -TCP/IP combines a few layers

    TCP/IP Layers:
    Application
    Transport
    Internet
    Network Access

    OSI Layers:
    Application
    Presentation
    Session
    Transport
    Network
    Data Link
    Phycical

    -TCP/IP has well known protocols

    Some similarities between the two
    -You should know both
    -They both have layers

  2. #2
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    Thumbs up

    Thats a really good tutorial cwk9 on the ISO OSI Reference Model very inormative.

  3. #3
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    Geez, this is great. You just saved me a lot of time for my MAC tutorial. Thanks.

  4. #4
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    10X so much, very well writen. I have memorized the phisical layer now I had read a few chapters of a TCP/IP networking book before, I had forgoten much of it, this was a good refresher.
    In snatches, they learn something of the wisdom
    which is of good, and more of the mere knowledge which is of evil. But must I know what must not come, for I shale become those of knowledgedome. Peace~

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    Nicely done.

    A few things I'd like to add:
    If your wondering were the error checking was this is were it happens. The Transport layer determines if data needs to be resent. Itís also were two computers decide who sends what and when.
    Stuff that you would find at the Transport layer: UDP, ...
    Just to make things clear, transport is *usually* the place you would find error correction/re-sending... This is not always true: If you take UDP for example, it does not have any reliability built-in. In the case of protocols using UDP, the reliability has to be done at the application level (if really needed.)

    Now there is another model that important to learn and thatís TCP/IP. The TCP/IP model is arguably more important than OSI because it has protocols were as OSI is very generic and theoretical. A common misconception is that TCP/IP is simpler because it has fewer layers.
    Also, TCP/IP isn't really a "diffrent model", it's just that, by itself, it doesn't implement all the levels described by OSI... However, if you use SSL for example, you just added that session layer between transport and application on the tcp/ip "model"...
    This is subtle...

    Ammo
    Credit travels up, blame travels down -- The Boss

  6. #6
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    yeah man, i did up something like this for school once cause i got bored in computer class...... great job tho

  7. #7
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    good one.. keep it up ..
    Be Cool ..


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