The OSI Reference Model

One of the greatest functions of the OSI specifications is to assist in data transfer between disparate hosts—meaning, for example, that they enable us to transfer data between a Unix host
and a PC or a Mac.

The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network. It also provides aframework for creating and implementing networking standards, devices, and internetworking schemes.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end.

The following network devices operate at all seven layers of the OSI model:

• Network management stations (NMSs)
• Web and application servers
• Gateways (not default gateways)
• Network hosts
  

Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as Ms. Post wrote the book setting the standards—or protocols—for human social interaction, the ISO developed the OSI reference model as the precedent and guide for an open network protocol set. Defining the etiquette of communication models, it remains today the most popular means of comparison for protocol suites.

The OSI reference model has seven layers:

• Application layer (layer 7)
• Presentation layer (layer 6)
• Session layer (layer 5)
• Transport layer (layer 4)
• Network layer (layer 3)
• Data Link layer (layer 2)
• Physical layer (layer 1)