Network Basics: The Seven Layers of the OSI Reference Model
OSI sounds like the name of a top-secret government agency you hear about only in Tom Clancy novels. What it really stands for in the networking world is Open Systems Interconnection, as in the Open Systems Interconnection Reference Model, affectionately known as the OSI model.
The OSI model breaks the various aspects of a computer network into seven distinct layers. These layers are kind of like the layers of an onion: Each successive layer envelops the layer beneath it, hiding its details from the levels above. The OSI model is also like an onion in that if you start to peel it apart to have a look inside, you’re bound to shed a few tears.
The OSI model is not a networking standard in the same sense that Ethernet and TCP/IP are networking standards. Rather, the OSI model is a framework into which the various networking standards can fit. The OSI model specifies what aspects of a network’s operation can be addressed by various network standards. So, in a sense, the OSI model is sort of a standard of standards.
|1||Physical||Governs the layout of cables and devices such as repeaters and hubs.|
|2||Data Link||Provides MAC addresses to uniquely identify network nodes and a means for data to be sent over the Physical layer in the form of packets. Bridges and switches are layer 2 devices.|
|3||Network||Handles routing of data across network segments.|
|4||Transport||Provides for reliable delivery of packets.|
|5||Session||Establishes sessions between network applications.|
|6||Presentation||Converts data so that systems that use different data formats can exchange information.|
|7||Application||Allows applications to request network services.|
The first three layers are sometimes called the lower layers. They deal with the mechanics of how information is sent from one computer to another over a network. Layers 4 through 7 are sometimes called the upper layers. They deal with how application software can relate to the network through application programming interfaces.
WHY IS UNDERSTANDING THE OSI REFERENCE IMPORTANT?
A lot of networking books and other resources gloss over the OSI Reference Model, including only passing mention of it, or relegating it to an appendix. The usual stated reason for this is that the OSI model is “too theoretical” and “doesn’t apply to modern networking protocols like TCP/IP”.
I believe that this is a misguided notion. While it is certainly true the OSI model is primarily theoretical, and that networking protocols aren’t always designed to fit strictly within the confines of its layers, it’s not true that the OSI model has little applicability to the “real world”. In fact, it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.
As just a few examples: the OSI Reference Model provides the basis for understanding how technologies like Ethernet and HomePNA have some important similarities; it explains how a PC can communicate using any of several different sets of protocols, even simultaneously; it is an important part of understanding the differences between interconnection devices such as repeaters, hubs, bridges, switches and routers; and it also explains how many WAN technologies interoperate.
Far from being obsolete, the OSI model layers are now showing up more than ever in discussions of technology. In fact, some protocols are even named specifically in terms of their place in the OSI Reference Model! For an example, consider the Layer Two Tunneling Protocol. Also, switches are now commonly categorized as being layer 2, layer 3 or even higher-layer switches.
In theoretical discussions, the OSI Reference Model helps you understand how networks and network protocols function. In the “real world”, it also helps you figure out which protocols and devices can interact with each other. So, I encourage you to read on. It’s time well spent.