The OSI Reference Model
In 1984, the International Organization for Standardization (ISO) adopted the Open Systems Interconnection (OSI) Reference Model (or simply, the OSI model) to facilitate interoperability between network devices independent of the manufacturer. The OSI model defines standard protocols for communication and interoperability by using a layered approach. This approach divides complex networking issues into simpler functional components that help the understanding, design, and development of networking solutions and provides the following specific advantages:
Clarifies the general functions of a communications process, instead of focusing on specific issues
Reduces complex networking processes into simpler sub-layers and components
Promotes interoperability by defining standard interfaces
Aids development by allowing vendors to change individual features at a single layer, instead of rebuilding the entire protocol stack
Facilitates easier (and more logical) troubleshooting ...