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In 1978, the International Standards Organization (ISO) introduced
the ISO model for Open Systems Interconnect (OSI) as a first step
toward international standardization of the various protocols
required for network communication.
The OSI ISO model:
- Was designed to establish data communications standards that
would promote multi-vendor interoperability.
- Consists of seven layers, with a specific set of network
functions allocated to each layer, and guidelines for
implementation of the interfaces between layers.
- Details a specific set of protocols and interfaces to implement
at each layer. So far, only the lowest four layers have been
explicitly defined. The upper layers, and their interfaces to
the lower ones, have not yet been completed. The overall model
has become the basis for the government's required standard
environment, GOSIP, beginning in August 1990.
Each layer of the OSI model can be viewed as an independent module.
You may (theoretically) substitute one protocol for another at the
same layer without affecting the operation of layers above or below.
In addition to explicitly defining protocols and interfaces at
selected layers, the OSI model also serves as a concept, providing a
reference for how data communication should take place. It provides a
common basis for the coordination of standards development for the
purpose of systems interconnection, while allowing existing standards
and architectures to be placed in perspective within the overall
reference model.
The principles that led to the creation of seven layers are:
- A layer should be created only where a different level of
abstraction is required.
- Each layer should perform a well defined function.
- The function of each layer should be chosen with an eye
toward defining internationally standardized protocols.
- The layer boundaries should be chosen to minimize the
information flow across the interfaces.
- The number of layers should be large enough that distinct
functions need not be thrown together in the same layer out
of necessity, and small enough that the architecture does
not become unwieldy.
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