Q103881: The ISO Model: Theory and Function of Layered Design

Article: Q103881
Product(s): Windows for Workgroups and Windows NT Networking Issues
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Last Modified: 30-JUL-2001

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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