CCNA Crash Course: Day 4
Today we’re going to learn about the OSI 7-Layer Reference Model. Here’s a great link for reading about it (more links below too). This model exists for the purpose of simplifying the complex task of thinking about how data is transported over a network, and is used to help networking professionals troubleshoot problems. Here are the layers:
7. Application (Programs, such as a web browser, or chat client
6. Presentation (Data Formatting)
5. Session (Distinguishes individual sessions of communication; port numbers)
4. Transport (Splits outbound data into chunks called packets; responsible for end-to-end connectivity)
3. Network (Logical addressing, TCP/IP)
2. Data-Link (MAC addresses, CSMA/CD, LLC, Binary transcoding)
1. Physical (01101101)
Think of the very top layer (the application layer) as the actual programs you use on your computer, and the very bottom layer being the physical means (electricity, light, radiowaves, etc.). The image below helps to demonstrate the encapsulation process that takes place as data passes down through the 7 layers:
Let’s pretend I’m running a piece of chat software and talking to someone else through the Internet (I’m on the left of the above chart, and my chat buddy is System 2 on the right). I type something, press Enter, and what I type begins a journey down the series of layers. From the Application layer, my text passes through the Presentation layer which formats my text into ASCII characters (or whatever the application I’m using prefers) and passes it down to the Session layer. The session layer assigns a number to distinguish this individual (brief) data transmission from others. A small chunk of data is then appended to the front of the containing this session number tag (called a port number).
Layer 4 on down are considered the “lower network layers.” …and to be honest, I’m too lazy to write anymore about it. I’ll be focusing on the lower three layers more closely in the next blog. Use the links at the top for much more useful reading. (By the way, these CCNA posts are more or less for me later when I take the exam, so please excuse the poor quality of my notes). In the end, the data the is received at the physical layer of system two is sent back up, and layer by layer, each encapsulated chunk is pealed off until finally reaching the top and presenting the text I typed on my computer on System 2′s monitor.
Up next: A closer look at the Data-Link Layer, where we’ll talk about MAC addresses, the ARP protocol and Half/Full Duplex.
March 6th, 2009 at 10:10 pm
Hi Dave,
I am writing to ask permission to use this OSI encapsulation figure in an article on web application security for an internal publication by Noblis (noblis.org).
We would need your preferred attribution line.
Thanks,
Nancy
March 7th, 2009 at 12:56 am
I’m sorry Nancy, but the above image was not created by me. I found it the same way you found it: via a google search. Unfortunately, my website now seems to come up as hosting the most popular copy of this image (probably because of it’s width, as the original was much larger/sharper). I tried to dig deeper to find a the original source for you via google images search, but was unable to come up with another one. And I don’t remember where it came from originally now and there is no watermark on it that I can detect. I can’t speak for the original author of this image so I wouldn’t want to place you at the risk that I am taking of using the above image without permission.
And if the original author is out there… I’ll replace it with someone else’s drawing if you want me to.
March 27th, 2009 at 3:42 pm
Hmmm. Well, that’s a weird path to public domain, but I guess that’s how it happens. Perhaps with the myriad images of this concept, the original author no longer cares. I won’t cite this blog then….