The OSI Model

The Open Systems Interconnect model is based upon a standard promulgated by the ISO (Organization Internationale de Standards). In practice the standard has never been fully implemented. However it has been influential in the development of networks and distributed systems. For this reason it is important to understand the model.

The Model is based upon seven layers. These are (from bottom to top)

1. The physical layer which defines the standards required for physical interconnections (the wire).
2. The data link layer which defines the protocols for exchanging data frames over a "wire". This includes the important Medium Access Layer defining protocols for access to the shared wire of a LAN.
3. The Network Layer. This is where routing take place.
4. The transport layer. This layer provides for end to end connection between machines. Conceptually it is outside the network.
5. The Session layer. This layer provides for dialog control between processes.
6. The Presentation Layer provides for translation of data structures between differing architectures.
7. The Application Layer Provides application level access to the network, file transfer, remote terminals etc.

Other Models

Two other important protocol stacks for distributed systems are:

1. The TCP/IP suite This is in fact the most widely used networking protocol. IP datagrams are used to interface to the network and TCP defines the transport layer. The applications are built on top of TCP.
2. ATM (Asynchronous Transfer Mode). This is a fast packet switched network. TCP/IP can interface to it. It is quite likely that ATM could become the network of choice.

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Last Changed: 11 August 1995