The Physical Layer: Understanding the OSI Model

Thanks for your comments on my first EtherGeek post on understanding the OSI Model. I especially enjoyed learning new ways to remember the OSI Model; for example: Please Do Not Throw Stale Pizza Away. Now, that’s a philosophy I can agree with both in networking and at breakfast time!

Just in case you’ve never heard it before, I’ll say it now....The first three rules in network troubleshooting are:

  1. check the cable
  2. check the cable
  3. check the cable

The cable (the media that the network is running across), and the electrical specs for the device connecting to it are the most basic building blocks of the network connection. The Physical Layer includes cable specs, pin-out patterns, voltages, hubs, repeaters, network cards, protocol standards (RJ45, V series modem standards) and even host bus adapters (HBA).

A lot of the network administrators I’ve met have never had to think much about Physical Layer networking. Many weren’t classically educated in electrical engineering and have learned about network administration through on-the-job training. Additionally, patch cables are easily available online in whatever length, color, and pin-out you want.

How many of us have had to make an Ethernet cable lately? I was once asked to make 100, four-foot-long, CAT5 rollover cables. Not to be confused with patch cables or crossover cables -- but rollover cables. I made all 100 of them perfectly, exactly the same – and all 100 percent wrong as I misunderstood the pin-out.

So, I had to re-terminate one end of each of them and I ended up with 100 cables that were almost four-feet-long. Doh! Pardner, it's darn hard to forget lessons that you learnt through blood, sweat, and tears.

If the devices you are trying to get to communicate aren’t physically connected, then you don't have a snowball's chance in Hades of making that connection work (assume we’re talking about physical media rather than wireless connections like 802.11 and Bluetooth). Always start here – at the physical layer – and work your way up. If you’re new at working with this Layer I suggest:

  1. Get yourself a crimper, some RJ-45 connectors, and some CAT5, CAT5e, or CAT6 cable and make some patch cables, crossover cables, and rollover cables. Test each and every one of them to ensure that you’ve made 'em right and keep those crimpers and connectors handy ‘cause you’ll never know when you’ll need them.
  2. Get yourself a book on network fundamentals and yes, actually read it! Three good ones are Network Warrior, Networking Bible, and Telecommunications: A Beginner’s Guide.
  3. When in doubt, swap out the cable.

With each different type of cable, connector, and unique communication standard there comes a distinct set of problems. Back when I was in the U.S. Air Force we had an FDDI backbone that ran around the base. I spent countless nights chasing “wraps” around the base on my government issued Kawasaki Mule and polishing the ends of those fiber runs hoping for a clear connection. Luckily, nowadays the vast majority of the networks are leveraging Ethernet running over twisted pair copper.

One more thing to remember, the Layers build on each other. For instance, if the cable looks good and you’re able to see the machine in question in the ARP cache of the neighboring route, then you can check the Physical Layer off your list and continue investigating on up the stack.

Flame on…


Josh Stephens is Head Geek and VP of Technology at SolarWinds, an IT management software company based in Austin, Texas. He shares network management best practices on SolarWinds’ GeekSpeak and thwack. Follow Josh on Twitter @sw_headgeek and SolarWinds @solarwinds_inc.

Copyright © 2011 IDG Communications, Inc.

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