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The Open System Interconnection (OSI) reference model is a principle of internetworking that you must understand to appreciate the way Cisco devices operate. The OSI reference model is a seven-layer architectural model developed by the International Organization for Standardization (ISO) and the International Telecommunications Union-Telecommunications (ITU-T). It is used universally to help individuals understand network functionality. The OSI reference model adds structure to the many complexities involved in the development of communications software. The development of communications software involves many tasks, including dealing with multiple types of applications, transmission strategies, and physical network properties. Without structure, communications software might be difficult to write, change, and support.
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ISO is an international organization founded to promote cooperation in technological developments, particularly in the field of communications. ITU-T, on the other hand, is a global organization that drafts standards for all areas of international analog and digital communications. ITU-T deals with telecommunications standards.