Network Troubleshooting Flowchart
Warning! You must unplug your ATX power supply from the wall before working inside the case.
Copyright 2010 by Morris Rosenthal
All Rights Reserved
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Network Cable and Hub Troubleshooting
Note that these steps correspond with decision points on the flowchart and are reached directly by clicking on the diamond symbols. The text below cannot be read sequentially.
Does the PC (Workstation) see any network resources, servers, other workstations? Note that some versions of networking software display "remembered" resources, even when the PC can't access them, so you'll need to actually click on a given resource to see if it's really available.
Have you recently added a network hub? Is the workstation the first workstation on a new hub, being stacked or chained to existing hub(s)? When connecting hubs or switches with twisted pair (RJ-45 connector) cabling, whether 10BaseT or 100BaseT, make sure that you either connect to an "X" port (uplink port), or use a special crossover cable. A crossover cable, unlike a straight through cable, connects pins 1 and 2 on one end to 3 and 6 on the other end, and vice versa. You must use a twisted pair for each for noise protection. If you do have an X or uplink port, you normally see that it is connected to an adjacent port by a line or other symbol. You can only use one or the other, since they are using the same physical circuitry, with the X port making the pair reversal. Some people seem to think that hubs are bullet proof, but I've seen as many bad hubs and bad ports on hubs as bad network adapters. Hubs also have a power transformer that needs to be plugged into a live outlet.
Most network adapters have one or more onboard LEDs to show the status of the link and network activity (traffic). If your documentation tells you that you have such a link light, is it lit? No link light indicates there's a actual break in your physical layer. Check the physical connectors at all points on your network in the failed path, and make sure that you are within all of the limits for your physical layer in terms of number of workstations and distances. On a 10BaseT or 100BaseT network, swap the workstation cable to another port on the hub and see if it works. While it's possible the adapter or the next port or device to which it's connected is bad or powered off, the problem is usually caused by the cable. Wireless and IR adapters may fail simply due to the physical location (blind spot) or distance from the transceiver. Old coax networks can have the wrong or missing termination (the most common, Thin Ethernet, requires 50 Ohm terminators at segment ends).
Have you cloned the software configuration from another workstation on the network (everything but the unique portion of the IP address, assuming you're set up for TCP/IP)? It's too easy to make a mistake with which protocol should be the default or with the spelling of a Workgroup, etc. At an active workstation, go through every option in the network setup and print screen every page and sub page that comes up. Keep it around for future reference when you run into networking problems with a similar workstation. If this is the first workstation on the network, or the second on a peer-to-peer, go with the defaults and make use of the operating system's built in troubleshooter, at least in Windows versions. Your problem is most likely software configuration, which is far too in depth to address in the chart. When in doubt, reboot.
Does the Device Manager see the network adapter and report no conflicts? Try reinstalling the driver and rebooting. In Windows, start by deleting the existing network device in Device Manager. If Windows still won't recognize the network adapter, it could be a conflict with another hardware adapter or it could be faulty. If the adapter is built-in, either on the motherboard or in a notebook, try restoring the defaults in CMOS Setup. Proceed to the Conflict Resolution flowchart.
Have you tried a known good cable? Even if the link light is lit, it doesn't mean your cable is capable of carrying network traffic. An incredible number of techs make these cables wrong out of sheer laziness or ignorance. Don't say, "But it's a new cable!" Four conductors are actually used for normal implementations 10BaseT and 100BaseT, and the wiring is straight through, 1-1, 2-2, 3-3, 6-6. Pins 1 and 2 and pins 3 and 6 must each use a twisted pair, or the longer runs will fail and shorter runs will act unpredictably. Visually inspect connectors to make sure they are solid and wired properly (i.e. two shared pairs for 1-2, 3-6). Squint into the transparent connector and try to take note of the color coding for pins 1, 2, 3 and 6. Then go look at the other end of the cable and make sure that the color coding is the same, AND that a pair (i.e, blue, blue stripe) is used for the pair 1 and 2 and the pair 3 and 6.
Take the PC (just the system box which some people call the CPU) to another workstation location and swap it out with that PC. If you get right on the network, that tells you that the physical link to the location where it failed is bad. That could be the patch cable, the in-wall wiring, or the port on the hub it connects to. If it doesn't work at the new location, that tells you it's either the network adapter or the software configuration. If it's an add-in adapter and you have a spare, by all means try swapping it out, but the software settings are more often the culprit. Make sure the driver is up-to-date and the correct version for the OS, make sure that you have cloned all the settings (except the machine name or final IP address) from a working machine, and try going through the OS troubleshooting steps.
Are your network access problems of a random or intermittent nature? Check for loose connectors. It's very easy to install a RJ-45 connector improperly or fail to crimp it tightly enough to hold to the cable such that it loosens up with just a minor physical movement. The problem might also be interference somewhere in the cable run. Make sure it's not draped over the back of a CRT or running directly over florescent lights or other noisy RF emitters. You could be experiencing software conflicts with other processes on the PC. You can try eliminating all tasks except the minimal network configuration and do some large file movements to see if the hardware layer is solid. More likely it's simply the loading of the network, a traffic jam, or you're exceeding the number of simultaneous users supported by the hardware (including wireless) or the software.
Are you using Shielded Twisted Pair (STP) or any other cable type with a non-signaling shield? Note that this is not the usual case for twisted pair cabling. Make sure that the shield is grounded at one end only, or you could end up with a ground loop and a constant leakage current. If it's not grounded at either end, it may act as an antenna to pick up and disperse interference. Also, make sure that your cables, even when grounded, are intelligently routed. Stay away from transformers, high current junctions, heavy equipment that can induce lots of electrical noise, though it's primarily the higher frequencies you need to worry about.
Are you within the physical layer limits for your network? This applies to both wired and wireless networks. Don't go by the number in the IEEE standard, use the limit in the hub, switch or base station documentation. Be aware that the distance limitations are based on a normal operating environment with the proper cabling or antennas installed. If your cables are made wrong, routed poorly, or are low quality, the limits will be reduced. Rerouting cables, adding repeaters (amplifiers) or eliminating sources of interference can increase the reach of your network.
Have you tried a different port on the hub? There's no rule that says hubs have to fail all at once, and even though a performance degradation of a single port is a rarity, it's worth trying. It could also be that the cable end plugged into the hub wasn't crimped on as tightly as it could have been, causing the performance of the link to be dependent on the exact position of the cable, an unacceptable situation.
Does the problem, be it lost connections, slow performance or anything else, occur during periods when network traffic is high or a large number of users are logged on? There are many reasons a network can bog down or have trouble in high traffic or high user count situations, including the natural limitations of the technologies being used. In general, if you are using a passive hub, you can greatly increase your network performance during high traffic periods by swapping the hub for an active switch. Also, if you are running a hybrid LAN, with a mix of 10BaseT and 100BaseT adapters, you should upgrade them all to 100BaseT, providing the cable plant is all Cat 5, which it better be!
Is the PC flaky when it's not on the network? If so, don't waste any more time on network diagnostics, proceed to Motherboard, CPU and RAM failure and look for the symptoms the PC is displaying. This isn't a good test of software problems, since you run different applications and have different resource usage when you're connected to the network.
You should always have a proven long bypass cable for testing, that you can run directly from the workstation to the hub without going through walls, ceilings, etc. Make sure you are within the distance limits for twisted pair, wireless and IR, and within the total number of active stations limit for wireless and IR. Check for physical cable damage. The sheathing on the Cat 5 cables is thin and the inner conductors can be easily broken if the cable is stretched or crimped.
Does a new network adapter fix the problem? New PCI network adapters cost less than $10, so there's no reason not to try one. If you're running a wireless network with notebooks and add on wireless adapters, borrow one from a good unit. If the new network adapter hasn't fixed the problem and you've gone through all the physical layer diagnostics to get here, it's a software issue.
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