An OSI layer is a seven layer model in which layers provide services to its upper layers and take services from lower layers. Each layer interacts with its peer layer via a protocol.It transmits PDU called as protocol data unit.The seven layers are given below:
Layer7: Application layer
In this layer, communication partners and quality of service are identified and user authentication and privacy are considered. This layer supports application and end-user processes.The layer provides application services for file transfers,e-mail and other network software services.
Layer6: Presentation layer
This layer provides independence from differences
in data representation (e.g., encryption) by translating from application
to network format, and vice versa. This layer formats and encrypts
data to be sent across a network, providing freedom from compatibility
problems. It is sometimes called the syntax layer.
Layer5: Session layer
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.
Layer4: Transport layer
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.
Layer3: Network layer
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.
Layer2:Data link layer
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 sublayers: The Media Access Control (MAC) layer
and the Logical Link Control (LLC) layer. The MAC sublayer 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.
Layer1:Physical layer
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.
Some important things to remember:
Hubs/Repeaters
are found in the Physical Layer
Switches /Bridges/ are found
in the Data Link Layer
Routers are found in the Network Layer
Gateway are found in All 7 of the OSI Layers