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

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OSI Reference Model Networking Layers, Sublayers and Layer Groupings
(Page 3 of 3)

Relationships Between OSI Reference Model Layers

There are also certain OSI layers that have “natural” relationships to each other. The physical and data link layers, in particular, are closely related. For example, most people talk about Ethernet as being a “layer two technology”, but Ethernet specifications really deal with both layer 2 and layer 1. Similarly, layers three and four are often related; protocol suites are often designed so that layer three and four protocols work together; examples being TCP and IP in the TCP/IP protocol suite and IPX and SPX in the Novell suite.

In some areas, the layers are so closely related that the lines between them become blurry. This is particularly the case when looking at the higher layers; many technologies implement two or even all three of these layers, which is another reason why I feel they best belong in a group together. One important reason why the distinctions between layers five through seven are blurry is that the TCP/IP protocols are based on the TCP/IP model, which combines the functions of layers five through seven in a single, thick layer.

Key Concept: The four lower layers of the OSI model are most often discussed individually, because the boundaries between them are reasonably clear-cut. In contrast, the lines between the session, presentation and application layers are somewhat blurry. As a result, sometimes protocols span two or even all three of these layers; this is especially true of TCP/IP application protocols, since the TCP/IP model treats layers five through seven as a single layer.


Sublayers

Finally, note that some OSI Reference Model layers are further divided into sublayers to help define more precisely the internal details of protocols and technologies at those layers. This is commonly done at the lower layers, especially the physical layer and the data link layer.


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