OSI Model: The OSI Model is a conceptual model widely used in computer communications design. It defines a seven layer structure of hardware and software that is implemented on each computer with the corresponding layers on each computer communicating between each other using the lower layers.

The strength of the OSI model is that it results in some commonality of approach between different hardware and software producers. For example, Seventh Layer Applications software designers design their programs to talk to the 6th Layer Presentation Layer, not directly to the (First Layer) Physical Layer. The weakness is that there is some difficulty accommodating exceptions. For example, diagnostic applications need to access physical layer information intentionally hidden from them by the intervening layers.

In the conventional picture, the layers are as follows:

7. (Top) Application Layer: Provides the network interface seen by applications; flow control; error recovery.

6. Presentation Layer: Network Services. Redirection of information to the network; protocol conversion; data encryption/decryption; data compression/decompression.

5. Session Layer: Establishing a connection between two computers; controls when the connection is established; who talks when; and how the session is terminated.

4. Transport Layer: Packetizes and depacketises data. handles transmission errors at the packet level.

3. Network Layer: Handles addressing, receiving, routing transmission of data. Handles traffic control and reassembly of transmitted data into packets for the transport layer

2. Data Link Layer: Controls the interface to the physical layer. Moves data to from Transmission devices. Manages Service Access Points (SAPs)

1. Physical Layer: Transmits and receives data of the physical network medium, handles cables, cards, transmitters, receivers, etc.

Return To Index Copyright 1994-2002 by Donald Kenney.