The 7 Layers of the OSI Model
Last updated: March 03, 2008
, or Open
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.
This layer supports
and end-user processes. Communication partners are identified,
quality of service is identified, user authentication and
privacy are considered, and any constraints on data
identified. Everything at this layer is application-specific.
This layer provides application services for
, and other
applications that exist entirely in the application level.
Tiered application architectures are part of this layer.
This layer provides independence
from differences in data representation (e.g.,
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
, providing freedom
from compatibility problems. It is sometimes called the syntax
This layer establishes, manages
and terminates connections between
. 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.
This layer provides transparent
transfer of data between end systems, or
, and is
responsible for end-to-end error recovery and
ensures complete data transfer.
This layer provides
technologies, creating logical paths, known as
, for transmitting data from
Routing and forwarding are functions of this layer, as well as
, error handling,
and packet sequencing.
At this layer, data packets are
and decoded into bits. It furnishes
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
) layer and the
Logical Link Control
The MAC sub layer controls how a computer on the network gains
access to the data and permission to transmit it. The LLC layer
, flow control and error checking.
This layer conveys the bit
stream - electrical impulse, light or radio signal -- through
at the electrical and mechanical level. It provides
means of sending and receiving data on a carrier,
including defining cables, cards and physical aspects.
with physical layer
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