The Physical Layer


The Physical Layer

by Brad Fair

I’ve decided to deviate from my blog-posting standard in two different areas today. First, I’m going to actually post this, which goes against the norm for me. Second, I’m going to talk about some underlying network theory instead of a break/fix review.

The OSI Model

When network techs troubleshoot a networking issue, often times it’s beneficial for them to troubleshoot beginning at the bottom of the OSI model. The OSI model is a seven layer model that divides communications systems into more manageable, less complex pieces. The model is often displayed like this:

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Since each layer of the model builds on all previous layers (as if building foundations for a house), the layers are numbered from the bottom up, 1-7. The layer I’m writing about now is the Physical layer, otherwise known as Layer 1.

Layer 1 – The Physical Layer

The physical layer’s primary purpose is to create end-to-end connectivity for communication. When you access, your communication has to travel a physical path from your computer all the way to the InterWorks servers. This physical path can include network cards, cables (and cable length), switches, routers, media access converters, a sattelite or two, and any number of other things on which your communication can travel.

The physical layer defines things such as:

  • Cabling Standards
  • Flow Control
  • Carrier Sense/Collision Detection
  • Bit Synchronization
  • Duplex/Speed
  • Network Topology
  • Simplex/Half Duplex/Duplex, and
  • Autonegotiation

In troubleshooting network issues, it can make good sense to start with this layer, since any issue on this layer (such as a broken cable or mismatched speed/duplex settings) will affect all other layers. 

While this post describes the Physical layer in a very simple manner, in reality it is much more complex. If you ask yourself “Does this communication have a valid path for end-to-end transmission” you might realize that you will need to dive deeper into any one of the things mentioned in the list above. Remember, Google is your friend!

More About the Author

Brad Fair

Solutions Architect
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