OSI Reference Model
As many networking tutorials do, this one begins with an
introduction to the Open Systems Interconnection (OSI)
Reference Model (OSI Model). The OSI model is a layered,
abstract description for communication and computer network
protocol design, developed as part of the Open Systems
Interconnection initiative. It is also called the OSI 7-layer model.
The OSI model divides the functions of a protocol into a series of
layers. Each layer has the property that it only uses the functions
of the layer directly below, and only exports functionality to the
layer directly above. A system that implements protocol behavior
consisting of a series of these layers is known as a protocol stack
or simply stack. Protocol stacks can be implemented either in
hardware or software, or a mixture of both. Typically, only the
lower layers are implemented in hardware, with the higher layers
being implemented in software.
The application layer provides a means for the user to access
information on the network through an application. This layer is
the main interface for users to interact with the application and
therefore the network.
The presentation layer transforms data to provide a standard
interface for the application layer. Encoding, data compression,
data encryption and similar manipulation of the presentation is
done at this layer to present the data as a service or protocol
developer sees fit.
The session layer controls the connections (sessions) between
computers. It establishes, manages and terminates the
connections between the local and remote application.
The transport layer provides transparent transfer of data between
end users, thus relieving the upper layers from transfer concerns
while providing reliable data transfer. The transport layer controls
the reliability of a given link through flow control, segmentation/
desegmentation, and error control.
The network layer provides the means of transferring data
sequences from a source to a destination by using one or more
networks while maintaining the quality of service requested by
the Transport layer. The Network layer performs network routing
functions, and might also perform segmentation/desegmentation,
and report delivery errors.
Data Link Layer
The data link layer provides the means to transfer data between
network entities and to detect and possibly correct errors that
may occur in the Physical layer. It arranges bits from the physical
layer into logical chunks of data, known as frames.
The physical layer defines all the electrical and physical
specifications for devices. This includes the layout of pins,
voltages, and cable specifications.