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Open Systems Interconnection (OSI) Reference Model
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Layer Number |
Layer |
Function |
|
7 |
Application |
Provides the interface and services that support user applications
and provides general access to the network |
|
6 |
Presentation |
Serves as the translator layer of the OSI model and is responsible
for data conversion and encryption |
|
5 |
Session |
Establishes and maintains the communication link between the
sending and receiving nodes |
|
4 |
Transport |
Responsible for end-to-end data transmission, flow control, error
checking, and recovery |
|
3 |
Network |
Provides the logical addressing system used to route data on the
network |
|
2 |
Data link |
Responsible for the framing of data packets and the movement of the
data across the physical link |
|
1 |
Physical |
Manages the process of sending and receiving bits over the physical
network media (the wire and other physical devices) |
When data is sent by a network node, that data moves down through
the OSI stack and then is transmitted over the network media. When the data
is received by a node, such as another computer on the network, it moves up
through the OSI stack until it is again in a form that can be accessed by a
user on that computer.
Each of the layers in the OSI model is responsible for
certain aspects of getting user data into a format that can be transmitted
on the network. Some layers are also establishing and maintaining the
connection between the communicating nodes, and other layers are responsible
for the addressing of the data so that it can be determined where the data
originated and where the data’s destination is.
An important aspect of the OSI model is that each layer is
in the stack to provide services to the layer directly above it. Only the
Application layer, which is at the top of the stack, would not provide
services to a higher-level layer.
The process of moving user data down the OSI stack on a
sending node is called encapsulation. The process of moving raw data
received by a node up the OSI is referred to as de-encapsulation.
To encapsulate means to enclose or surround, and this is
what happens to data that is created at the Application layer and then moves
down through the other layers of the OSI model. A header, which is a segment
of information affixed to the beginning of the data, is generated at each
layer of the OSI model, except for the Physical layer. This means that the
data is encapsulated in a succession of headers- first the Application layer
header, then the Presentation layer header, and so on. When the data reached
the Physical layer, it is like a candy bar that has been enclosed in several
different wrappers.
When the data is transmitted to a receiving node, such as a
computer, the data travels up the OSI stack and each header is stripped off
of the data. First, the Data Link layer header is removed, then the Network
layer header, and so on. Also, the headers are not just removed by the
receiving computer; the header information is read and used to determine
what the receiving computer should do with the received data at each layer
of the OSI model.
It is with these headers that the sending computer is able
to communicate with the receiving computer and either provide the receiving
computer with information or actual instructions related to the disposition
of the data at the various levels of the OSI model. Using the candy bar
analogy again, the situation on the receiving computer would be like opening
a candy bar enclosed in many layers of wrapping, with each individual
wrapper providing important instructions on how to unwrap the candy further
and eventually eat the candy bar.
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