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The OSI, or Open System Interconnection, model defines a networking framework to
implement protocols in seven layers. This article explains the 7 Layers of the OSI Model.
The OSI, or Open System Interconnection, model defines a networking framework to implement protocols in
seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and
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
Data Link (Layer 2)
At this layer, data packets are encoded and decoded into bits.
It manages 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 channel and
permission to transmit data
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.
