The OSI layers (from the bottom up) are as follows: Physical: Transmits data Data Link: Corrects transmission errors Network: Provides the physical routing information Transport: Verifies that data is correctly transmitted Session: Synchronizes data exchange between upper and lower layers Presentation: Converts network data to application-specific formats Application: End-user interface The application, presentation, and session layers are all application-oriented in that they are responsible for presenting the application interface to the user. All three are independent of the layers below them and are totally oblivious to the means by which data gets to the application. These three layers are called the upper layers. The lower four layers deal with the transmission of data, covering the packaging, routing, verification, and transmission of each data group. The lower layers don’t worry about the type of data they receive or send to the application, but deal simply with the task of sending it. They don’t differentiate between the different applications in any way.
The following sections explain each layer to help you understand the architecture of the OSI-RM (and later contrast it with the architecture of TCP/IP).
The Application LayerThe application layer is the end-user interface to the OSI system. It is where the applications, such as electronic mail, USENET news readers, or database display modules, reside. The application layer’s task is to display received information and send the user’s new data to the lower layers. In distributed applications, such as client / server systems, the application layer is where the client application resides. It communicates through the lower layers to the server.
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The Presentation LayerThe presentation layer’s task is to isolate the lower layers from the application’s data format. It converts the data from the application into a common format, often called the canonic a representation. The presentation layer processes machine-dependent data from the application layer into a machine-independent format for the lower layers. The presentation layer is where file formats and even character formats (ASCII and EBCDIC, for example) are lost.
The conversion from the application data format takes place through a “common network programming language” (as it is called in the OSI Reference Model documents) that has a structured format. The presentation layer does the reverse for incoming data. It is converted from the common format into application-specific formats, based on the type of application the machine has instructions for. If the data comes in without reformatting instructions, the information might not be assembled in the correct manner for the user’s application. The Session LayerThe session layer organizes and synchronizes the exchange of data between application processes.
It works with the application layer to provide simple data sets called synchronization points that let an application know how the transmission and reception of data are progressing. In simplified terms, the session layer can be thought of as a timing and flow control layer. The session layer is involved in coordinating communications between different applications, letting each know the status of the other. An error in one application (whether on the same machine or across the country) is handled by the session layer to let the receiving application know that the error has occurred. The session layer can resynchronize applications that are currently connected to each other.
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This can be necessary when communications are temporarily interrupted, or when an error has occurred that results in loss of data. The Transport LayerThe transport layer, as its name suggests, is designed to provide the “transparent transfer of data from a source end open system to a destination end open system,” according to the OSI Reference Model. The transport layer establishes, maintains, and terminates communications between two machines. The transport layer is responsible for ensuring that data sent matches the data received.
This verification role is important in ensuring that data is correctly sent, with a resend if an error was detected. The transport layer manages the sending of data, determining its order and its priority. The Network LayerThe network layer provides the physical routing of the data, determining the path between the machines. The network layer handles all these routing issues, relieving the higher layers from this issue. The network layer examines the network topology to determine the best route to send a message, as well as figuring out relay systems. It is the only network layer that sends a message from source to target machine, managing other chunks of data that pass through the system on their way to another machine.
The Data Link LayerThe data link layer, according to the OSI reference paper, “provides for the control of the physical layer, and detects and possibly corrects errors that can occur.” In practicality, the data link layer is responsible for correcting transmission errors induced during transmission (as opposed to errors in the application data itself, which are handled in the transport layer).
The data link layer is usually concerned with signal interference on the physical transmission media, whether through copper wire, fiber optic cable, or microwave. Interference is common, resulting from many sources, including cosmic rays and stray magnetic interference from other sources. The Physical LayerThe physical layer is the lowest layer of the OSI model and deals with the “mechanical, electrical, functional, and procedural means” required for transmission of data, according to the OSI definition. This is really the wiring or other transmission form. When the OSI model was being developed, a lot of concern dealt with the lower two layers, because they are, in most cases, inseparable.
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The real world treats the data link layer and the physical layer as one combined layer, but the formal OSI definition stipulates different purposes for each. (TCP/IP includes the data link and physical layers as one layer, recognizing that the division is more academic than practical. ).
