Open System
Interconnection (OSI) was created by the International Organization for
Standardization (ISO) in order to help vendors to create interoperable network
devices and software. The OSI model is the primary architectural model for
networks. It describes how data and network information are communicated from
an application on one computer (i.e. mail, voip) through the network to an
application on another computer. The OSI model approaches this communication
though layers.
Application
|
Layer 7
|
Presentation
|
Layer 6
|
Session
|
Layer 5
|
Transport
|
Layer 4
|
Network
|
Layer 3
|
Data Link
|
Layer 2
|
Physical
|
Layer 1
|
Top-Down
Approach of OSI Model
Application Layer
The application layer is the OSI layer closest to the end user. In other
words, it provides a user interface and it comes into play when an application
needs access to the network.
Presentation Layer
The
Presentation Layer as its name states is responsible not only to present data
to the Application layer but for data translation and code formatting.
Session Layer
The Session Layer controls the
connections between computers. It is responsible for setting up, managing and
then tearing down sessions between Presentation layer entities. It provides
full-duplex, half-duplex or simplex operation.
Transport Layer
The Transport Layer provides
transparent transfer of data between end users, providing reliable data
transfer services to the upper layer. In order to achieve this it segment and
reassemble data from upper layer applications and unite it onto the same data
stream. The Transport Layer can be connection-oriented or connectionless. The
two famous protocol of this layer are TCP and UDP.
Network
Layer
The Network Layer is
responsible for packet forwarding including routing through routers. The
connection model that used is connectionless. That means a packet can travel
from a sender to a recipient without the recipient having to send an acknowledgement.
Router is a Layer 3 (Network
Layer) device.
Data
Link Layer
The Data Link Layer provides the physical
transmission of the data and handles error notification, network topology and
flow control. In other words, Data Link Layer will ensure that messages are
delivered to the proper device on a LAN using hardware addresses and translate
messages from the Layer 3 into bits for the Physical Layer to transmit.
Switch is a Layer 2 device.
Physical
Layer
The Physical Layer is the
lowest layer in the OSI Model. It is a fundamental layer the logical data
structures of the higher level functions in a network. Mainly, it defines the
means of transmitting bits over a physical link connecting network devices.