Attenuation
Recent changes in the standards now use the term "insertion loss" and not attenuation. Given that test equipment manufacturers have used the term attenuation since 1993, attenuation will continue to be seen on test reports. Electrical signals transmitted by a link lose some of their energy as they travel along the link. Insertion loss measures the amount of energy that is lost as the signal arrives at the receiving end of the cabling link. The insertion loss measurement quantifies the effect of the resistance the cabling link offers to the transmission of the electrical signals.

Insertion loss characteristics of a link change with the frequency of the signal to be transmitted; e.g. higher frequency signals experience much more resistance. Stated a different way, the links show more insertion loss for higher frequency signals. Insertion loss is therefore to be measured over the applicable frequency range. If you test the insertion loss of a Category 5e channel, for instance, the insertion loss needs to be verified for signals ranging from 1 MHz to 100 MHz. For Cat 3 links the frequency range is 1 through 16 MHz. Insertion loss also increases fairly linearly with the length of the link. In other words, if link "A" is twice as long as link "B", and all other characteristics are the same, the insertion loss of link "A" will turn out twice as high as the insertion loss of link "B."

Insertion loss is expressed in decibels or dB. The decibel is a logarithmic expression of the ratio of output power (power of the signal received at the end of the link) divided by input power (the power launched into the cable by the transmitter). The table below demonstrates that the decibel scale is not a linear scale.

Results Interpretation
The attenuation in a cable is largely dependent upon the gauge of wire used in constructing the pairs. 24 gauge wires will have less attenuation than the same length 26 gauge (thinner) wires. Also, stranded cabling will have 20-50% more attenuation than solid copper conductors. Field test equipment will report the worst value of attenuation and margin, where the margin is the difference between the measured attenuation and the maximum attenuation permitted by the standard selected. Hence a margin of 4 dB is better than 1 dB.

Troubleshooting Recommendations
Excessive length is the most common reason for failing attenuation. Fixing links that have failed attenuation normally involves reducing the length of the cabling by removing any slack in the cable run.

Excessive attenuation can also be caused by poorly terminated connectors / plugs. A poor connection can add significant attenuation. Your clue to this cause is to compare the attenuation on the four pairs. If only one or two pairs have high attenuation, this suggests an installation issue. If all pairs have too much attenuation, check for excess length. However, impurities in the copper cable can also cause attenuation failures; again this typically happens on one pair only.

Temperature also affects attenuation in some cables. The dielectric materials, which form the conductor insulation and cable jacket, absorb some of the transmitted signal as it propagates along the wire. This is especially true of cables containing PVC. PVC material contains a chlorine atom which is electrically active and forms dipoles in the insulating materials. These dipoles oscillate in response to the electromagnetic fields surrounding the wires, and the more they vibrate, the more energy is lost from the signal. Temperature increases exacerbate the problem, making it easier for the dipoles to vibrate within the insulation. This results in increasing loss with temperature.

For this reason, standards bodies tend to specify attenuation requirements adjusted for 20 �C. Cables operating in temperature extremes can be subject to additional attenuation and where this is likely, the design of the cabling system should take this into consideration. You may not be able to run the maximum 90 meters (295 ft) defined in the standards. Many consultants try and keep runs below 80 meters (262 ft) to provide a safety margin. This of course is not always possible when space is a premium and the number of telecommunications rooms has to be kept to a minimum.