Logical networking models separate network functionality into modular
layers. These modular layers are applied to the physical network to isolate
network problems and even create divisions of labor. For example, if the
symptoms of a communications problem suggest a physical connection problem, the
telephone company service representative can focus on troubleshooting the T1
circuit that operates at the physical layer. The repair person does not have to
know anything about TCP/IP instead; they can concentrate on the physical
circuit. If it functions properly, then either the repair person or a different
specialist looks at areas in another layer that could be causing the problem.
The OSI model provides a common language for network engineers. Having
looked at using a systematic approach, documentation, and network
architectures, it can be seen that the OSI model is pervasive in
troubleshooting networks. The model allows troubleshooting to be described in a
structured fashion. Problems are typically described in terms of a given OSI
model layer. At this stage it is assumed that there should be an intimate
familiarity with the model.
The OSI reference model describes how
information from a software application in one computer moves through a network
medium to a software application in another computer. The model was developed
by the International Organization for Standardization (ISO) in 1984, and it is
now considered the primary architectural model for intercomputer
communications. The OSI model divides the tasks involved with moving
information between networked computers into seven smaller, more manageable
task groups. A task, or group of tasks, is then assigned to each of the seven
OSI layers. Each layer is reasonably self-contained, so that the tasks assigned
to each layer can be implemented independently. This enables the solutions
offered by one layer to be updated without adversely affecting the other
layers. The figure details the seven layers of the OSI reference model.

The OSI model provides a logical framework and a common language used by
network engineers to articulate network scenarios. The Layer 1 through Layer 7
terminology is so common that most engineers do not think twice about it any
more.
The upper layers (5 – 7) of the OSI model deal with application
issues and generally are implemented only in software. The application layer is
closest to the end user. Both users and application layer processes interact
with software applications that contain a communications component.
The
lower layers (1– 4) of the OSI model handle data-transport issues. The physical
layer and data-link layer are implemented in hardware and software. The other
lower layers generally are implemented only in software. The physical layer is
closest to the physical network medium, such as the network cabling, and is
responsible for actually placing information on the medium.
When sending
data from an application in one host to an application in a second, the network
software on the source host takes data from an application and converts it as
needed for transmission over a physical network.
The
process involves:
- Converting data into segments
- Encapsulating segments with header information that includes logical
network addressing information, also the process of converting segments into
packets
- Encapsulating packets with a header, including physical addressing
information, and converting packets to frames
- Encoding frames into bits
The data is now ready for travel over the physical medium as bits. The
encapsulation process as a whole represents the initial stage in transferring
data between two end systems.
