• Choose a category
• All
• Classic-Fiction

Once you have your mind wrapped around the problem, it’s time to fix it. Unless the problem is either simple (network cable unplugged) or something you’ve seen before and know exactly how to fix, you’ll almost certainly need to research it. The Internet makes this easy. I use one of my favorite tricks is when I get some bizarre error text: I type the error message into my search engine—that would be Google, of course—and most times find a quick fix!

Make the Fix and Test

Once you have a good idea as to the problem and how to fix it, it’s time to do the fix. Always make backups—or at least warn the user of the risk to the system. If possible, try to remember how the system was configured before the fix so you can go back to square one if the fix fails to work. After you perform the fix, do whatever you need to do to make sure the system is again working properly. Make sure the user sees that the system is working properly and can sign off on your work.

OSI Seven-Layer Model

A lot of people think about networks and troubleshoot networking issues by using the OSI seven-layer model. Using this model (or my four-layer model, described in the next section of this chapter) helps you isolate problems and then implement solutions. Here are the seven layers of the OSI model:

Layer 1 Physical

Layer 2 Data Link

Layer 3 Network

Layer 4 Transport

Layer 5 Session

Layer 6 Presentation

Layer 7 Application

The Physical layer defines the physical form taken by data when it travels across a cable. Devices that work at the physical layer include NICs and hubs. Figure 23-46 shows a sending NIC turning a string of ones and zeros into an electrical signal, and a receiving NIC turning it back into the same ones and zeros.

Figure 23-46 The Physical layer turns binary code into a physical signal and back into ones and zeros.

* * *

NOTE Basic switches reside at Layer 2 (Data Link) of the OSI model. They provide filtering based on MAC. More advanced switches that can perform InterVLAN and protocol support operate at Layer 3 (Layer 3 switch). Routers are often called Layer 3 switches.

The Data Link layer defines the rules for accessing and using the Physical layer. MAC addresses and Ethernet’s CSMA/CD operate at the Data Link layer.

The Network layer defines the rules for adding information to the data packet that controls how routers move it from its source on one network to its destination on a different network. The IP protocol that handles IP addressing works on Layer 3.

The Transport layer, Layer 4, breaks up data it receives from the upper layers (that is, Layers 5–7) into smaller pieces for transport within the data packets created at the lower layers. In TCP/IP networks, the protocols that typically handle this transition between upper and lower layers are TCP and UDP.

The Session layer manages the connections between machines on the network. Protocols such as NetBIOS and sockets enable a computer to connect to a server, for example, and send and receive e-mail or download a file. Each different task you can perform on a server would require a different kind of session.

The Presentation layer presents data from the sending system in a form that a receiving system can understand. Most Layer 6 functions are handled by the same software that handles Layer 7 functions.

The Application layer is where you (or a user) get to interact with the computers. These are programs that make networking happen, such as Web browsers and e-mail applications. Chapter 25, “The Internet,” covers these applications in a lot more detail.

The key to using the OSI seven-layer model is to ask the traditional troubleshooting question: What can the problem be? If Jill can’t browse a Web site, for example, could this be a Layer 7 issue? Sure: If her browser software was messed up, this could stop her from browsing. It could also be a lower-level problem, though, and you need to run through the questions. Can she do anything over the network? If her NIC doesn’t show flashing link lights, that could point all the way down