Why is the cable supporting my LAN experiencing abnormal attenuation?

Abnormal attenuation basically means that as the signal is being transmitted across the wire, the signal is degrading. There are numerous causes for these problems, mostly around bad cabling or excessive cable length. Below is a copy of the specifications for particular lengths of cable. These are the maximum lengths for the various types of cabling standards. With this in mind, if you are under these length specifications and you're still experiencing degradation, inspect the cable itself. Any unnatural bends and/or improper termination will also cause attenuation to occur. Ethernet Cable Summary

Specification	Cable type	Maximum length
10BaseT	Unshielded twisted pair	100 meters
10Base2	Thin coaxial	185 meters
10Base5	Thick coaxial	500 meters
10BaseF	Fiber optic	2000 meters
100BaseT	Unshielded twisted pair	100 meters
100BaseTX	Unshielded twisted pair	220 meters

For untwisted pair cable (UTP), crosstalk can certainly cause severe issues as well as attenuation. Crosstalk occurs when the insulation and/or barriers between two signals degrades causing interference in the signals of both pairs of cables. Crosstalk tends to occur when the same cables are used for multiple connections, which are regarded as shortcuts by the standards bodies. In Category 5e and 6 cables, there are typically four pairs of wires for the termination. For some connections, I have seen that only two pairs are used for a single data connection and the other two pairs are used for a separate data connection, which does encounter problems. When signals are traveling across these wires, the crosstalk happens when the signal encounters another signal on a separate wire with additional noise. If a cable is properly installed, crosstalk and abnormal attenuation would not occur.

With proper cabling practices and attention to detail, higher transmission frequencies should not encounter a problem. Only poor cabling and installation would cause the network to behave improperly when higher transmit rates are required.

You can find more good information on this topic over at Carrie Higbie's expert page and in this guide from LANshack.com.