Open system interconnection. A seven layered model of communications networks defined by ISO. The seven layers are:

Layer 7 Application: provides the interface for application to access the OSI environment.

Layer 6 Presentation: provides for data conversion to preserve the meaning of the data.

Layer 5 Session: provides user to user connections.

Layer 4 Transport: provides end to end reliability.

Layer 3 Network: provides routing of data through the network.

Layer 2 Data Link: provides link access control and reliability.

Layer 1 Physical: provides an interface to the physical medium.

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