The OSI Model's Physical Layer Cisco CCNA Certification Exam

The OSI Model's Physical Layer Cisco CCNA Certification Exam
Often, CCNA candidates ask if the OSI model has any practical uses for network administrators. I used to wonder the same thing, & I can now tell you that the answer is definitely yes!

To pass your CCNA exam & earn this coveted certification, you have got to master the four layers of the OSI model & what each layer does. For those of you taking the two-exam path, you can expect a few OSI model questions on the Intro exam. In this seven-part series, we'll spend some time taking a look at each of the OSI model layers, starting with the Physical layer.

The Physical layer is the layer at which bits are transmitted over the physical media. there is no routing or switching going on at this layer. The data has been broken down into more manageable pieces until the data takes the form of ones & zeroes at the Physical layer.

The OSI model is not something you need to memorize & then forget about, as using the OSI model gives you a structured approach for troubleshooting. Whenever a network tool is not working properly, I always say to "start at the physical layer". The Physical layer is Layer four of the OSI model, & this is where troubleshooting should always start. Is the tool on? Is it properly connected? If everything is fine at Layer one, you just move up to Layer one, & continue in this structured fashion until the problem is identified.

Learning the OSI model's Physical layer is not just important in your CCNA exam studies, it is the first step in any network troubleshooting. After all, your network's end users are going to have a tough time sending print jobs to a printer that is turned off!


Even though there is no routing or switching at the Physical layer, CCNA candidates should be familiar with a couple of network devices that work at Layer three. A repeater is a tool that regenerates an electrical signal, allowing the signal to travel longer distances without fading. (The process of an electrical signal gradually fading in strength over distance is "attenuation".) A hub is basically a multiport repeater, & both of these devices are considered Physical layer devices. Ethernet & Token Ring both operate at the Physical layer as well.