Troubleshooting With OSI

Computers, devices and networks are just great.  We have come a long way in the past twenty years.  In fact, I think the Internet was first launched back in 1991.  During the 90s, it was complicated just to get a computer with Internet capability, go on line and actually do something.  There were no search engines back in 1991.  As far as what you can do with the Internet today did not even exist back then.

Nowadays, a fifth grader can setup a wireless network for the house.  Most of it is plug n’ play.  You don’t need to know about the OSI model to setup a network.  Most everything, even IP address assigning, is all done automatically.  But, it is only a really neat thing when it works.  When it doesn’t, it is a nuisance.

However, if you have a fundamental understanding about the OSI model, you can use that to your advantage to troubleshoot your network.  Since the OSI model is used to implement network in layers, you can do troubleshooting in layers as well.

If you really like this post after reading it, you might want to print it out.  The problem with troubleshooters is that when you need them, you can’t access them.  Your network has to be working in order to access my website .

Before we go into the OSI model as a troubleshooter, I recommend doing the following first: figure out what the scope of your problem is before fixing it.  Sure, following the steps of the OSI model will do this for you.  I just feel that 90% of all problems can be solved much faster by following a few steps first.  This guide and steps are not meant to solve every single problem out there.  With this, I am assuming that your Internet was working great, then all of a sudden, it just stop working.  I am also assuming that you have a network at your house (the OSI model is pointless otherwise.)

Step 1: Figure out the scope of your problem

OK, so you cannot get on the Internet with your computer.  Is this a problem with every computer in your house?  Figuring out if it is your computer not connecting or if every computer is not connecting will save you a lot of time.

Step 2: Reboot the scope

So many problems and aggravation can be easily solved by just rebooting a computer.  Who knows?  If all of your settings were working before, and nobody made any changes, they should still be working now.  If your computer is the only one not connecting to the Internet, just restart the computer.  If everyone cannot connect to the network, just restart your router and modem or your gateway (home gateway devices are usually routers with built-in cable/DSL modems in them.)

Step 3: limit the scope further

If step 2 doesn’t help,  chances are you indeed have a problem.  But, don’t panic.  There is still more that can be done.  In this step, it is a good idea to check on your task bar, look at your network connection, and see if you are connected.  That right there will give you a clue as to where the problem is at.  If you are connect, try opening up another browser (if you have one installed) and see if that browser works.  If it does, you know that it is software related and not necessarily your network.

OK, at this point, we’ll assume that you still have a problem and that you have no connection.  If you do have connection on your task bar, you may skip to level 3 of the OSI model.  If not, you have to start at level 1.  As you might guess, when using the OSI model to troubleshoot your network, you will start at level 1 and work your way up to level 7.

Level 1: Physical

You cannot use your network if you are not physically connected to it.  That would be a big problem.  If you are connected to a switch/router via the Ethernet port, check the lights next to the port of your computer and switch/router.  Usually, there will be a green light that indicates there is a connection and a flashing amber light indicating that data is being transferred.  If these lights are not lighting up,  gently push on both connections to make sure that they are not loose.  Then, try plugging the cable into another port on the switch.  Ports on switches seem to go bad, particularly after power failures.  Try using another laptop/computer (if available) and see if it works.  If it does, try the cable that your computer is using.  If that cable works, you are probably looking at a bad NIC (Network Interface Card,) and it needs to be replace.  You can get one internally using PCI, or if a laptop, they make them for your PC card slot.

If you do have lights and it looks like it is working, check your modem/router/gateway and make sure that all lights are normal.  Who knows.  Maybe your Internet is just down and you need to call your ISP.  Most Internet devices have a green light that is labeled ‘Internet’ that will be on if you do have Internet connection.

Layer 2: Data Link

Assuming that everything looks good on the physical layer, it is now time to check the data link.  There is a one in a million possibility (but still possible) that more than one computer has the same MAC address.  If you have just added a new computer to your network, try turning it off and reset your modem/router/gateway/switch.  If this is not an issue, the only other thing you can do at this layer is to check the drivers of your NIC.  You can do this in Windows by right-click ‘my computer’ or ‘computer’, go to properties, click on the hardware tab, and click on the ‘device manager’ button.  Look under ‘Network devices’ and see if you have any warnings (there will be a question mark or an exclamation.)  If there is, try reinstalling your drivers via the CD that came with the NIC card.  If you do not have one, you will have to go to the manufacturer’s website of the NIC card (or your computer manufacturer’s website if you have a pre-built like a Dell, HP, etc.)

If reinstalling the driver does not fix the problem, you might have a bad NIC card.  Or, it could be a registry problem.  Who knows?  If you have a warranty or you just don’t want to fix it yourself, check your computer in for service.

If you are using wireless, check to see if you can connect to your router.  If you can’t, your router might need to be reconfigured.  Try to connect to your router via the Ethernet port and go to the administrator page.  If your password is not working, you will have to restore your router to it’s default factory settings (usually by holding down a button with a pin) and reconfigure it.  Check the instruction manual or manufacturer’s website (see how useful it would be to have this printed out already) for more details.

Layer 3: Network

Assuming that there is nothing wrong with your network card and that you have physical connection and are able to connect to your wireless router, we should now see if we have a logical network connection.  In Windows, go to start->run and type in ‘cmd.’  This will bring up a command window.  Type in ‘ipconfig’ (or ‘ifconfig’ on non-Windows machines.)  This will tell you if you are getting an IP address or not.  If do not have one and are using DHCP, type in ipconfig /release, followed by an ipconfig /renew.  This should reassign you an IP address.  If you do not receive one, either your computer is not configured for DHCP or something is wrong with your DHCP server (aka your router.)

If you do receive one, ping the gateway address by typing in ‘ping x.x.x.x’ (where x.x.x.x is the IP address of the gateway that you can get by typing ‘ipconfig.’)

If you receive four replies, then you have a logical connection between you and your router.  If not, type in ‘ping 127.0.0.1′.  This is the loopback address.  If you can’t ping yourself, then there is something wrong with the TCP/IP protocol stack on your computer.  Fixing this problem is beyond the scope of this post.  You might want to Google this.

If you can ping your router, we should now see if you have connection outside your network.  Type in ‘ipconfig /all.’  This will give you more details, including a DNS server.  Try to ping the DNS server by typing ‘ping x.x.x.x’ (where x.x.x.x is the IP address of the DNS server.)  If you get replies, you are able to get out to the Internet.  If not, then something is preventing you from getting out.  You can verify this by typing ‘tracert x.x.x.x,’ (where x.x.x.x is the IP address of the DNS server.)  Tracert will ping every device between your computer and the DNS server.  If it stops at the first or second IP address, chances are good that it is your router/gateway/modem.  Fixing this problem is beyond the scope of this post, but at least it will get you started.

Layer 4: Transport

This next step is really not on layer 4, but I like to do it here anyways.  If you can ping your DNS server, we now want to see if DNS is indeed working.  Try pinging a known website (like ping www.awebsiteofyourchoosing.com).  I don’t want to say a website because I don’t want everyone reading this post pinging it at once .  Even if you get ‘destination not reachable,’ you should still get an IP address that your computer will try.  Most websites will have ‘ICMP’ turned off in order to prevent an attack (hence destination not reachable.)  If you get a message that says ‘can’t find the host,’  then there is something wrong with DNS.  You can do a Google search to find a tutorial that will flush your DNS cache.  However, the problem could be with your ISP’s DNS server.  Give them a call.

Layer 4 is also about ports.  Maybe your firewall is blocking you.  If you have one installed, try to disable it.  If that does not work, we need to go to the next layer.

Layer 5: Session

There is not a whole lot you can do at this layer.  However, you can see if you can do file sharing on your network.  This is especially important if you have a computer that you are using to share the Internet to the other computers (old school.)  If you cannot share files with this computer, then you cannot share the Internet with it as well.

Layer 6: Presentation

Again, there is not a lot you can do at this layer.  However, you might want to try to delete your temporary Internet cache and cookies.  Maybe there is an SSL certificate that has expired and is preventing you from going out to the Internet.

Layer 7: Application

You are still down?  Even after you have determined you have a physical connection, no driver issues, you can ping, and your DNS is working?  Wow!  The good news is that there is probably nothing wrong with your network.  The bad news is that it is software related, and probably the hardest problem to fix!  If you only have one web browser installed (such as Internet Explorer,) try downloading another one (like Firefox or Safari.)  If they work and Internet Exporer does not, then there is something wrong with that particular browser.  BTW, you should be able to download another browser off another computer, burn it on CD or put on a USB drive, then copy it on your computer (hard to get them if you can’t get on the Internet.)

If your Internet is still not working, then it can be anything from a virus to a registry error.  If it were my computer, I would just re-image it (get any data you want off of it first!!!!)

If it is a specific Internet service, you might want to try their help desk.  I was having problems with watching movies from Netflix, and their help desk had me delete a file, and then it worked perfectly.

Like I said, this guide is not intended to fix every problem out there.  I would need to write a 50,000 page document in order to do so.  Hope this helps!