Applying the OSI Model to Network Communication
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There is a logical sequence to how these layers are involved in a conversation between devices on a network. Network communications are a lot like telephone conversations between friends. At the highest level, phrases with particular meanings to the participants, such as comments about a seminar both attended, are exchanged between the two parties. At lower levels, things happen that the participants are only partially aware of, such as the area code and prefix dialed determining the geographic region to which the call is being made, or not aware of at all, like selecting which physical long distance lines are used to transmit the conversation. The telephone user doesnt control or even need to know about what goes on in the innards of the telephone system to successfully place a call. Lower layers of the system, freeing the telephone user up to concentrate on what theyd like to say, handle these internal operations effectively.
Similarly, the OSI model gives each layer of the network a job to do, so that higher layers dont have to know what kind of cable is being used to transmit a message, and lower layers dont need to know that the message being sent is Hello, James.
As a message is sent, it passes down through the layers of the OSI model from highest to lowest, with each layer making successive additions or modifications to the message, to facilitate its transfer across the network. The message starts in the Application layer, then Presentation (where the message is encrypted, or compressed, if required), then Session (if applicable), Transport (where the system-level details of establishing the connection are handled), Network (where it is routed to the correct network), Data Link (where its destination on the network is specified) and finally the Physical layer which takes care of translating a message like Hello, James, into the encoded signal that travels across the network.
When the message is received at its destination, it is processed by the levels of the OSI model in the reverse order, from lowest to highest, until the message gets all the way from a series of electrical signals on a cable, back to Hello, James, displayed on a users computer.
The OSI 7-layer model provides a convenient way to label and abstract the different operations involved in sending and receiving data on a network. The concept of a stack of operations for sending and receiving data is also reflected in networking hardware and/or software, which is typically dedicated to performing just the functions at a single layer (or two) well, rather than requiring each device to understand how to process messages at every layer. We will see more about the physical devices on a network in a later chapter.
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