:: OSI (Open Systems Interconnection) Reference Model

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OSI Reference Model -


Open Systems Interconnection (OSI) reference model is a conceptual model that standardizes the internal functions of a communication system by partitioning it into abstraction layers
The model groups communication functions into seven logical layers as mentioned here :

1) Physical Layer :


Physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium.
It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.
Physical layer example includes Ethernet, FDDI, V.35, RJ45.

2) Data Link Layer :


Data Link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.
These layers perform error correction and detection facilities.
Data Link layer examples include PPP, FDDI, ATM, HDLC, Frame Relay.

3) Network Layer :


Network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.
Network layer is responsible for Addressing, routing and delivery of packets on a network.
Network layer examples include IP, IPX.

4) Transport Layer :


Transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications.
It relieves the higher layer protocols form any concern with the transfer of data between them and their peers.
Transport layer examples include TCP, UDP.

5) Session Layer :


Session layer allows session establishment between processes running on different stations.
Session layer perform interhost communication, managing sessions between applications.
Session layer examples includes NFS, RPC, SQL.

6) Presentation Layer :


Presentation layer formats the data to be presented to the application layer.
Data presentation layer perform data representation, encryption and decryption, convert machine dependent data to machine independent data.
Presentation layer examples include ASCII, GIF, JPEG, MPEG.

7) Application Layer :


Application layer serves as the window for users and application processes to access network services.
Application layer supports application and end-user processes.
Application layer examples include WWW, SNMP, Telnet, HTTP, FTP.

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