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!

Enjoy The TCP/IP Guide? Get the complete PDF!
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
"N" Notation and Other OSI Model Layer Terminology
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Protocols: Horizontal (Corresponding Layer) Communication
Next Topic/Section

Interfaces: Vertical (Adjacent Layer) Communication
(Page 2 of 2)

Modularity and Inter-Layer Interactions

One of the primary goals of the OSI Reference Model is to allow the interconnection of different implementations of various layers. Thus, the intention is to have somewhat autonomous individual layers that you can “mix and match”—to a point. The only way to make this work is to have well-defined ways that the layers connect together, and that brings us back to the matter of interfaces. Each layer must present a consistent, well-documented interface to the layers above it so that any upper layer implementation can use the lower layer properly.

I'll provide an example from the world of TCP/IP to illustrate what I mean. The heart of the TCP/IP protocol suite is the Internet Protocol (IP). Whenever you use any application on the Internet—email, Web sites, FTP, chat rooms, and so on—you are indirectly using IP.

However, you never use IP directly—you generally use one of two transport layer (layer 4) protocols: the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). A standard interface exists between the network layer and the transport layer in the TCP/IP protocol stack, which defines how IP is to be used by upper layer protocols; this enables TCP and UDP to interface to it. Similarly, both TCP and UDP present a particular interface to the hundreds of higher-layer protocols and applications that use them at higher layers.

Many different types of communication actually take place between layers. Control information is passed to enable the higher layers to utilize the lower ones, and for the lower ones to pass status and results information back to the higher ones. Data is also passed in both directions across the interface. For transmission, it flows down to the lower layer, each time normally resulting in data encapsulation. Upon reception, the process is reversed, with data being sent back up across the interface from lower to higher layer.

Key Concept: In the OSI Reference Model, an interface defines the mechanism for vertical communication between adjacent layers. The existence of well-defined interfaces between layers is what permits a higher layer to use the services of any of a number of lower layers, without requiring knowledge of how those layers are implemented.



Previous Topic/Section
"N" Notation and Other OSI Model Layer Terminology
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Protocols: Horizontal (Corresponding 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.