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OSI Reference Model Networking Layers, Sublayers and Layer Groupings
(Page 2 of 3)
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 modelphysical, data link, network
and transportare 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 modelsession, presentation
and applicationare 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 groupingsthe 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 Concept: The most fundamental concept in the OSI Reference Model is the division of networking functions into a set of layers, from layer one at the bottom to layer seven at the top. As you go up the layer stack, you move away from concrete, hardware-specific functions to ones that are increasingly abstract, until reaching the realm of user applications at layer seven. The seven layers are sometimes divided into groupings: the lower layers (one, two and three) and the upper layers (four through seven). There is some disagreement on whether layer four is a lower or upper layer.
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Version 3.0 - Version Date: September 20, 2005
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