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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  Key OSI Reference Model Concepts

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology

OSI Reference Model Layer Groupings

The OSI Reference Model does not formally assign any relationship between groups of adjacent layers. However, to help explain how the layers work, it is common to categorize them into two layer groupings:

• Lower Layers (Layers 1, 2, 3 and 4): The lower layers of the model—physical, data link, network and transport—are primarily concerned with the formatting, encoding and transmission of data over the network. They don't care that much about what the data is or what it is being used for, just about moving it around. They are implemented in both hardware and software, with the transition from hardware to software occurring as you proceed up from layer 1 to layer 4.
  
  
• Upper Layers (Layers 5, 6 and 7): The higher layers of the model—session, presentation and application—are the ones that are concerned primarily with interacting with the user, and implementing the applications that run over the network. The protocols that run at higher layers are less concerned with the low-level details of how data gets sent from one place to another; they rely on the lower layers to provide delivery of data. These layers are almost always implemented as software running on a computer or other hardware device.
  

  Figure 11: OSI Reference Model Layers

  The OSI Reference Model divides networking functions into a stack of seven layers, numbered 1 through 7 from the bottom up. To help illustrate the differing levels of abstraction between layers near the top and those on the bottom, they are sometimes divided into two layer groupings—the lower layers and the upper layers. Of course, not everyone agrees on exactly how the division should be accomplished. In particular, the transport layer is sometimes considered an upper layer and sometimes a lower layer.

  
  

There are some who would not necessarily agree with how I have chosen to divide the layers above. In particular, valid arguments can be made for including the transport layer in the upper layer group, since it is usually implemented as software and is fairly abstract. I believe it is better as part of the lower layer group since its primary job is still providing services to higher layers for moving data, however. Really, layer 4 is somewhat of a “transition zone” and is hard to categorize. Figure 11 shows how I divide the OSI Reference Model layers into groups and indicates the special position of layer 4 in the stack.

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology