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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
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.
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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Version 3.0 - Version Date: September 20, 2005
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