OSI Model

Posted on 2009-12-18
Last Modified: 2012-06-22
I learning about networking and would like to better understand the flow of data within the Server only using the OSI model,  Application Layer  to the NIC. Whare I'm a bit confused is as follows,
The flow of data will start from: 1. Application Layer 2. Prsentation Layer 3. Session Layer 4. Transport Layer 5. Is the Network Layer skipped here since this is a routing ip function? 6. Datalink Layer (NIC) 7. Physcal data on the wire. To simply the questions, what layers does the server or pc use from the application to the NIC only?

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Question by:8AVega
    LVL 10

    Expert Comment

    Application Layer
    Presentation Layer
    Session Layer
    Transport Layer
    Network Layer

    PC/Server uses the above layers depending on request and response.

    LVL 21

    Accepted Solution

    The network layer is not skipped but is where your layer 3 address, usually IP, will be applied. Routers use this address in their decisions of how to forward a packet but all the devices in the network have one.
    LVL 25

    Assisted Solution

    First off the OSI model is ONLY a conceptual model.  It is not strictly followed in actual implementations.  For example, the TCP/IP implementation maps to the OSI like so:

    Application -> Application, Presentation, Session
    Transport -> Transport
    Internet -> Network
    Link -> Data-link, Physical

    So from application to NIC it works like this (kind of, because like I said functions don't map strictly to OSI layers perfectly)

    Application - the application has a user interface that interfaces with the network stack and is in raw format
    Presentation - here the raw data is ensured to be in the appropriate format; e.g. EBCDIC, ASCII, etc.
    Session - used if multiple connections are to be done.  think ftp'ing multiple files.  the application handles it all in the same session
    Transport - application maps to a port.  this is how return packets end up getting routed back to the correct application
    Network - add routing information
    Data Link - add local link connectivity information as well as convert into proper format to be put onto the wire
    Physical - specifies how the data is encoded on the wire (what defines a 1 or 0) as well as the physical attributes of the wire and connectors

    This is why the tcp/ip model collapsed the layers it did.  The application writer mainly takes care of the user interface, data encoding, and session control (from the application side of things), the OS takes care of the transport application-to-port mapping and routing functions, and the nic manufacturer takes care of the physical attributes and nic driver writing of the lowest layers.

    every layer within the same system can ONLY talk to the layer above or below.  every layer between different systems can ONLY talk with the same layer on the other system.

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