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Why is the cable supporting my LAN experiencing abnormal attenuation?

A small company is experiencing difficulties on its LAN. After performing some tests, a technician has determined that the copper media supporting the LAN is experiencing abnormal attenuation. What are the possible causes of the problem? What is a cause of crosstalk in UTP cable? What is expected when crosstalk is present in networks with higher transmission frequencies?

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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.

This was first published in April 2006

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