The 7 Layers of the OSI Model
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.
|This layer supports application and end-user processes. Communication
partners are identified, quality of service is identified, user
authentication and any constraints on data completed. Everything at
this layer relies on layers 1 to 6 working efficiently.
|The presentation layer works to transform data into the form that the
application layer can accept. This layer formats data sent across a network,
providing freedom from compatibility problems. Everything at this layer
relies on layers 1 to 5 working efficiently.
|This layer establishes, manages and terminates connections ie data
conversations / exchanges between applications. Everything at this layer
relies on layers 1 to 4 working efficiently.
|This layer is where the TCP/IP protocol and other communications level
protocols exist. Without TCP, the application will fail, without
layers 1 to 3, TCP will fail. Data validation is now performed -
When data gets corrupted at layer 1, TCP will request retransmission and
simply wait until a clear packet is received - applications further up the
OSI stack are simply put on hold until this is complete. Everything at
this layer relies on layers 1 to 3 working efficiently.
|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.
Everything at this layer relies on layers 1 and 2 working efficiently.
|At this layer, data packets are encoded and decoded into bits. In
practice, layer 2 is the network interface hardware - ie the NIC card.
Everything at this layer relies on layer 1 below working efficiently.
|This ls the layer we install. Everthing else depends on our workmanship, which conveys the data stream. It provides the hardware and the means of sending and receiving data by the carrier protocols . Ethernet, Fast
Ethernet, RS232, and
ATM are protocols
with physical layer components.
This graphic is taken from
The Abdus Salam International Centre for Theoretical Physics.