Networking – Why is the LAN not gigabit

First and foremost it is absolutely critical that both ends match (at least with respect to wires that are actually used). If a given color originates on pin 3 on one end that same color must be tied to pin 3 on the other end. (This is contrary to phone cables where opposite ends are mirror images.) If you use a coupler to join two cables the coupler has a built-in "twist" so that this all works out.

Second (not quite as critical at lower speeds) the pairs must be kept together. That is, there is a "send" pair and a "receive" pair, and the wires of the pairs are twisted together. If you get one wire of the "send" pair twisted with one wire of the "receive" pair then you can get "crosstalk" which will introduce errors and slow things down (if not halt things entirely).

The pairs are not assigned to pins in an entirely logical sequence. The pairs are 1-2, 3-6, 4-5, 7-8. That is, three of pairs are adjacent pins, but one pair straddles the adjacent pins of the center pair.

Otherwise, electrons are color blind, so it doesn't matter which colors you tie to which pins, so long as the pairing is maintained. (Some claim that there is an ordering/positioning of the pairs in the cable and using the wrong pair of pairs can cause problems, but this argument is tenuous at best.)

However, it is a good idea to stick to a standard color assignment scheme, to make it easier to get cables terminated the same way on both ends. Unfortunately, there are 2-3 "standard" schemes.

This is most likely a problem with the cabling at some point. Here are the most common problems I have found to cause this in my experience (without certified test results on the installation, which I presume you do not have):

• Improper termination of the cable to the jack/plug.
• Physical problem with the cabling, often the result of the cable being pulled too hard during installation.
• Physical problem with the jacks or plug. This may be a manufacturing defect or the result of improper testing (I have often seen the pins in a jack bent when subject to the "probe" when toned out).
• One of the network cables you are using to connect the computer to the wall could have a problem (this is far more likely if the cable is hand made or older).
• Every once in a while you may come across a network cable that only has two pairs. They were sometimes manufactured this way because it was cheaper to do so.

While 10-BaseT and 100-BaseTX use only two pairs, 1000-BaseT uses all four of the pairs found in your standard Cat5e or higher cabling. If any one of the eight wires has a fault of some sort, this will prevent your connection from negotiating to 1000-BaseT and result in a 100-BaseTX connection.