The 7 Layers of the OSI Model
Last updated: March 03, 2008
The OSI, or Open
System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Application
(Layer 7) |
This layer supports
application
and end-user processes. Communication partners are identified,
quality of service is identified, user authentication and
privacy are considered, and any constraints on data
syntax are
identified. Everything at this layer is application-specific.
This layer provides application services for
file transfers,
e-mail, and other
network
software services.
Telnet and
FTP are
applications that exist entirely in the application level.
Tiered application architectures are part of this layer.
|
Presentation
(Layer 6) |
This layer provides independence
from differences in data representation (e.g.,
encryption) by
translating from application to network format, and vice versa.
The presentation layer works to transform data into the form
that the application layer can accept. This layer formats and
encrypts data to be sent across a
network, providing freedom
from compatibility problems. It is sometimes called the syntax
layer.
|
Session
(Layer 5) |
This layer establishes, manages
and terminates connections between
applications. The session
layer sets up, coordinates, and terminates conversations,
exchanges, and dialogues between the applications at each end.
It deals with session and connection coordination.
|
Transport
(Layer 4) |
This layer provides transparent
transfer of data between end systems, or
hosts, and is
responsible for end-to-end error recovery and
flow control. It
ensures complete data transfer.
|
Network
(Layer 3) |
This layer provides
switching
and routing technologies, creating logical paths, known as
virtual circuits, for transmitting data from
node to node.
Routing and forwarding are functions of this layer, as well as
addressing,
internetworking, error handling,
congestion control
and packet sequencing.
|
Data Link
(Layer 2) |
At this layer, data packets are
encoded and decoded into bits. It furnishes
transmission
protocol knowledge and management and handles errors in the
physical layer, flow control and frame synchronization. The data
link layer is divided into two sub layers: The Media Access
Control (MAC) layer and the
Logical Link Control (LLC) layer.
The MAC sub layer controls how a computer on the network gains
access to the data and permission to transmit it. The LLC layer
controls frame
synchronization, flow control and error checking.
|
Physical
(Layer 1) |
This layer conveys the bit
stream - electrical impulse, light or radio signal -- through
the network at the electrical and mechanical level. It provides
the hardware means of sending and receiving data on a carrier,
including defining cables, cards and physical aspects.
Fast
Ethernet,
RS232, and
ATM are
protocols with physical layer
components.
|
RECOMMENDED READING:
Image courtesy of
The Abdus Salam International Centre for Theoretical Physics.
Last updated: March 03, 2008
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