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The ISO Model: Theory and Function of Layered Design

In 1978, the International Standards Organization (ISO) introducedthe ISO model for Open Systems Interconnect (OSI) as a first steptoward international standardization of the various protocolsrequired 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 thesame layer without affecting the operation of layers above or below.

In addition to explicitly defining protocols and interfaces atselected layers, the OSI model also serves as a concept, providing areference for how data communication should take place. It provides acommon basis for the coordination of standards development for thepurpose of systems interconnection, while allowing existing standardsand architectures to be placed in perspective within the overallreference 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.

Article ID: 103881 - Last Review: 06/13/2014 06:09:00 - Revision: 2.1

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