Open Systems Interconnection (OSI)

The OSI reference model was created by the International ISO (Organization for Standardization) to help vendors create interoperable network device and software in the form of protocols so that different vendor networks could work with each other. It is the primary architectural model for networks. The OSI reference model breaks this approach into layers. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end.

Application Layer (Layer 7): Provide a user with interface (FTP, Telnet, SNMP)
The application layer acts as an interface between the actual application program and application layer protocols. For example, IE is a program that interfaces with the application layer protocols, it does not reside in the application layer. Some examples of programs that actually reside at the Application layer are FTP, TFTP, SNMP, Telnet, DNS, and NFS.

Presentation Layer (Layer 6): Present Data and Handles Processing such as encryptions
The Presentation Layer is responsible for data translation and code formatting. It’s essentially a translator and provides coding and conversion functions. Tasks like data compression, decompression, encryption, and decryption are associated with the Presentation layer.

Session Layer (Layer 5): Keeps different application data separate
The Session Layer is responsible for setting up, managing, and then tearing sown sessions between Presentation layer entities. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex. The Session layer basically keeps different applications data separate from other applications data.

Transport Layer (Layer 4): Provides reliable TCP or unreliable delivery UDP
The Transport layer is responsible for establishing and tearing down virtual circuits, example TCP. The term reliable networking can be used at the Transport layer. It means that acknowledgments, sequencing, and flow control will be used. Data integrity is ensured at the Transport layer by maintaining flow control and by allowing users to request reliable data transport between systems. Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host.

Network Layer (Layer 3): Provides logical addressing which routers use (IP Routing)
The Network Layer manages device addressing (examples are IP and IPX), tracks the location of devices on the network, and determines the best way to move data. Routers are specified at the Network layer and provide the routing services within an inter network.

Data Link Layer (Layer 2): Provides access to media using MAC address (Switch and Hub)
The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that translate messages from the Network layer into bits for the Physical layer to transmit. The Data Link layer formats the message into pieces; each called a data frame, and adds a customized header containing the hardware destination and source address (MAC addresses).