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

Interfaces: Vertical (Adjacent Layer) Communication 1 2 Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)

Each layer in the OSI Reference Model has a particular role (or roles)—a set of general tasks for which it is responsible. On each system on the network, hardware and software are running at many of the different levels in the model. The routines doing a particular job on machine “A” are designed to communicate with similar or complementary ones that are running on machine “B”. This horizontal communication is the very heart of what networking is about. It is what enables Web browsers and Web servers to talk, email applications to exchange messages, and so much more.

Of course, all types of communication function only if everyone agrees to the same methods of accomplishing it. Each set of rules describing one type of communication is called a protocol. You can think of a protocol as being like a language or a set of instructions. Each function or service of a network has its own language; like human languages, some are similar to each other while others are quite unique.

If you've done any reading at all about networks, you have probably seen the term “protocol” many, many times. Like the word “interface”, the word “protocol” can have many meanings. In fact, it is so fundamental to networking, and used in so many different ways, that I have a topic devoted to it in the chapter on Networking Fundamentals.

All that aside, we must remember that the OSI Reference Model is intended to be a formal way of describing networks. As such, the term “protocol” has a formal meaning in the context of the model. It refers specifically to a set of communication rules, instructions and/or procedures that describe communication between specific software or hardware elements running at the same layer on different machines within a network.

Interfaces: Vertical (Adjacent Layer) Communication 1 2 Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)