Troubleshooting Basic Networking

By Ronald Pacchiano | Posted January 06, 2004

The other day I was setting up a PC for a new associate at our company and had the most difficult time getting the PC onto our network. The first thing I had checked was to see if the Ethernet cable was in correctly and that I had a green Link light on the network adapter (which I did). I then proceeded to check the computer's transmission control protocol/Internet protocol (TCP/IP) configuration settings, updated the network drivers, and even went so far as to swap out the network adapter, yet I still couldn't get it to login.

Finally, after over an hour of testing I decided to replace the Ethernet cable. To my surprise, the system booted right to a login prompt. After so much frustration, to have the problem corrected simply by replacing the Ethernet cable is infuriating. Particularly since the first thing I had checked for was to see that I had a solid LED on the network adapter. This brings me to my question: Why would the link LEDs be on solid if there was a faulty cable connecting the two Ethernet devices? I'd appreciate any insight you might have on this matter.

Ah yes, welcome to the wonderful world of network troubleshooting — sometimes interesting, always frustrating. We have all been in a similar situation at one time or another, and I have nothing but empathy for the aggravation you experienced. Many network mysteries go unexplained; however, this one I think I might actually be able to shed a bit of light on.

As you are no doubt aware, a Solid Link Light typically indicates a good connection between two network devices. This is true regardless of whether you are connecting a PC to a hub/switch or linking a PC directly to another PC. But as you discovered, this is not always the case.

You see, 10BASET and 100BASE-TX Ethernet interfaces have two transmit pins (both + and -) and two receive pins (also + and -). The rest of the pins are unused. Positive (+) transmit pins must be connected to + receive pins and Negative (-) transmit pins must be connected to - receive pins. This is why this cable type is refereed to as a straight-through cable.

Hence when you see a solid link LED on two Ethernet devices (e.g. a network adapter and a switch) that are connected together, this indicates that the two transmit pins are connected to the correct two receive pins. It does NOT, however, guarantee that the cable has been constructed properly or that it will reliably transmit data. For example, the Ethernet standard specifies that the transmit pins be connected to corresponding receive pins with wires from the same twisted pair. It is certainly possible to connect a set of pins using one wire from one pair and another wire from a different pair.

The reason it is possible to get solid LINK LEDs and unreliable data transfers is that link determination is made with a link integrity test pulse that is transmitted at a much slower rate than the actual Ethernet signals that transfer data. It's also interesting to note that broken, disconnected, improperly terminated (coax), or "mis-wired" cables are responsible for a large percentage of most LAN problems.

In the future you should also remember that in addition to the cable connecting the PC to the switch going bad, the port on the hub/switch that you're connecting to could also be bad. While it is rare for a port to go bad, I have seen it happen. So if you're experiencing a conductivity problem, you could try connecting it to a different port on the hub/switch or even a different hub/switch entirely, even if it's only to rule the possibility of a bad hub out.

One last thing, in most office environments there are actually two Ethernet cables connecting your PC to the switch. The first is going from the PC to the wall jack. The second is going from the patch panel located in the server closet to the switch. So don't overlook either of the two cables during your troubleshooting.

I have three Windows 98 machines that are currently networked together in a peer-to-peer (P2P) configuration. I have another computer I want to network, but this one must run DOS and not Windows. The reason is that this machine is going to control some automatic test equipment that is only available in DOS.

Unfortunately, I don't have a lot of experience using DOS, so I'm not really sure how to go about setting up the drivers and configuring the protocols in order to do this. The only thing I can really tell you is that the DOS machine is using a 3Com 3C509 ISA network card. Any help you can provide would be helpful.

As the Windows 9x operating systems took hold in the late 1990s, installing DOS-based network drivers became something of a lost science (or is it a lost art?). In actuality, installing and configuring DOS-based network drivers isn't very hard; it just requires a bit of trial and error.

The toughest part about the process is trying to load as many of the DOS network driver files as possible into the high memory area (HMA). High memory resides in the area between 640 KB and 1 MB. This is necessary because if the multitude of driver files takes up too much of the space below 640 KB, there often won't be enough memory remaining to run an application. In order to load drivers into the HMA, you'll need DOS 5.0 or later.

How to configure devices in the HMA is a bit beyond the scope of this column, but you should be able to find resources on how to do it in the DOS help files or in your network adapter's user guide. Searching the web for information on EMM386.EXE may also help.

As far as the actual installation is concerned, it can vary a bit depending on which vendor's NIC you use. Some vendors will automate most of the configuration for you, while other vendors will force you to manually modify all of the DOS configuration files (CONFIG.SYS and AUTOEXEC.BAT) yourself.

As a result, it helps to have a basic understanding of DOS before you begin. You didn't say what network protocol you were using, but it's probably either internetwork packet exchange/sequenced packet exchange (IPX/SPX) or TCP/IP, and I believe both are included with the DOS driver package.

You're lucky to have an older 3Com card. Many newer and inexpensive network adapters don't bother including DOS drivers anymore. If you happen to have the original driver diskettes that came with your network card, you will probably find DOS drivers included on them. If not, they are still available on 3COM's Website.

In addition to the DOS drivers, copies of the original documentation are also available for download and should contain enough information to help you successfully get your DOS drivers installed and functioning. There are a couple of different variants of the 3C509 adapter, so double-check which model you have before downloading them. They can be found online. I hope this helps.

Adapted from PracticallyNetworked.com, part of the EarthWeb.com Network.

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