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The OSI Model > The OSI Model - Pg. 156

156 CHAPTER 10 Network Troubleshooting Some questions you'll probably want to ask include the following: 1. What is the exact nature of the problem? Try to be as specific as possible, and ask follow up questions to gather as many details as possible. 2. How many computers are affected by this problem? If the issue is isolated to a single computer, it is likely that the cause of the problem will be related to the computer itself. If it is affecting all computers on a particular subnet or those connected to a particular hub or switch, you can use this information to help you in the troubleshooting process. 3. When did the problem begin to occur? More specifically, you should find out what changed on the network when the issues first began. Analyzing and Responding to a Problem To analyze all of the data to determine the cause of the problem, you should examine each of the following layers in the open system interconnection (OSI) model: Layer 1 ­ Check your physical connectivity, like cables, patch panels, wall jacks, and hubs. Layer 2 ­ Verify that any switches or switch ports are configured and appear to be operating properly. Layer 3 ­ Verify that your routers are configured and functioning properly. Layer 4 and above ­ Check the actual application itself. To assess the situation at each layer, you need to determine the proper trou- bleshooting tools to use at each layer. For layer 1, it is often useful to start with simple physical inspections looking for issues with the naked eye. However, fluke meters and other tools can be used to check if wiring is correct or if it has degraded. For layers 2 and above, start with basic connectivity tools like ping, moving on to other tools once you've determined that basic connectivity is in place. Net- work discovery will help you to document the devices on your network and how they are configured. It is strongly recommended to invest in a network discovery tool; the amount of time it will save you in troubleshooting network connectivity issues will often pay for the cost of the tool. THE OSI MODEL You can use your understanding of the OSI model to improve your troubleshoot- ing techniques. When it comes to network troubleshooting, the most important layers of the model are the physical, data link, network, and transport layers. The physical layer It is the lowest layer of the OSI model, and it involves the actual electrical signals that are going from the network cables into the network interface card (NIC) of a computer, switch, router, or hub. A failure at the hardware level will usually involve the physical components of a computer or device. The physical layer is responsible for a number of functions, which are as follows: