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!

The whole site in one document for easy reference!
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
OSI Reference Model Networking Layers, Sublayers and Layer Groupings
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Interfaces: Vertical (Adjacent Layer) Communication
Next Topic/Section

"N" Notation and Other OSI Model Layer Terminology
(Page 1 of 2)

As a theoretical model, the OSI Reference Model comes complete with a set of terminology that is used to describe it and its constituent parts. This is sort of both good news and bad. The good news is that if you understand this terminology, it can help you comprehend much better most OSI model discussions, and also how technologies relate to the model. The bad news is that the terminology can also increase confusion—especially since it isn't always used consistently.

Common OSI Model Terminology Concepts

Let’s discuss a few of the terminology concepts you will often see used to refer to the OSI Reference Model. Many of these are illustrated in Figure 12.


Figure 12: OSI Reference Model Layer Relationships and Terminology

This diagram shows the relationships between OSI model layers and the terminology used to refer to adjacent layers in the context of any particular layer. Each layer (except layer 7) provides services the layer above it; each layer (other than layer 1) uses services provided by the layer below. Another way of saying this is that each layer “N” provides services to layer “N+1” and uses the services of layer “N-1”. Taking the example of layer three, the network layer, we see that it provides services to layer four, and uses services of layer two. From the standpoint of the network layer, the transport layer is layer “N+1” and the data link layer “N-1”.

 


Layer Names and Numbers

The various layers of the OSI Reference Model are referred to in a variety of ways. They may have their names spelled out in full, or they may be abbreviated. They are also often simply referenced by their layer number. So for example, all of these refer to the same thing: “data link layer”, “Data Link Layer”, “DLL”, “L2”, “layer two”, “layer 2”.

Similarly, you will often see layer names being used as adjectives to describe protocols and technologies. A “layer 3 technology” is one that operates primarily at the network layer.

“N” Notation

The letter “N” is often used to generically refer to a number within the computer world. With respect to the OSI Reference Model, it's common to see this letter used in discussions that relate generically to individual layers without mentioning a specific layer. You will hear terms like “N-functions” and “N-services”, which just refer to the functions and services provided within a particular layer.

As another example, you might see someone say that a particular technology “provides a useful service to the N+1 layer”. This just means it provides a function to the layer above the one at which it operates. Conceptually, every layer but the first and seventh have an “N-1” layer and an “N+1” layer, and so on. If you are looking at the network layer (layer 3) then the “N+2 layer” is the session layer (layer 5). See the discussion of model layer interfaces for more information.


Previous Topic/Section
OSI Reference Model Networking Layers, Sublayers and Layer Groupings
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Interfaces: Vertical (Adjacent Layer) Communication
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.