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.
Sunday, March 30th, 2008