Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Get The TCP/IP Guide for your own computer.
The TCP/IP Guide

Custom Search







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

Previous Topic/Section
Key OSI Reference Model Concepts
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
"N" Notation and Other OSI Model Layer Terminology
Next Topic/Section

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 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 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.



Previous Topic/Section
Key OSI Reference Model Concepts
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
"N" Notation and Other OSI Model Layer Terminology
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.