2.6.6 Attenuation and Interference Issues PDF

2.6.6 Attenuation and Interference Issues If a cable link is too long, decibel (dB) loss (or insertion loss) may mean that the link experiences signal degradation problems with high error rates and retransmissions (frame or packet loss) resulting in...

2.6.6 Attenuation and Interference Issues If a cable link is too long, decibel (dB) loss (or insertion loss) may mean that the link experiences signal degradation problems with high error rates and retransmissions (frame or packet loss) resulting in reduced speeds and possibly loss of connectivity. Insertion loss is measured in decibels (dB) and represents the ratio of the received voltage to the original voltage. A dB expresses the ratio between two values using a logarithmic scale. A logarithm is a mathematical tool for performing complex multiplication and division exponential operations as simpler additions and subtractions. The essential point is that a logarithmic scale is nonlinear, so a small change in dB value represents a large change in the performance measured. The following reference points are useful to remember: +3 dB means doubling, while -3 dB means halving. +6 dB means quadrupling, while -6 dB relates to a quarter. +10 dB means 10 times the ratio, while -10 dB is a tenth. For a longer primer on dB math, view the presentation at internetsociety.org/wp- content/uploads/2017/10/dB-Math.pdf. The maximum value allowed for insertion loss depends on the link category. For example, Cat 5e at 100 MHz allows up to 24 dB, while Cat 6 allows up to 21.7 dB at 250 MHz. When you are measuring insertion loss itself, smaller values are better (20 dB insertion loss is better than 22 dB, for instance). A cable certifier is likely to report the margin, which is the difference between the actual loss and the maximum value allowed for the cable standard. Consequently, higher margin values are better. For example, if the insertion loss measured over a Cat 5e cable is 22 dB, the margin is 2 dB; if another cable measures 23 dB, the margin is only 1 dB, and you are that much closer to not meeting acceptable link standards. Higher grade or shielded cable may alleviate the problem; otherwise, you will need to find a shorter cable run or install a repeater or additional switch. Careful cable placement is necessary during installation to ensure that the wiring is not subject to interference from sources such as electrical power cables, fluorescent lights, motors, electrical fans, radio transmitters, and so on. Electromagnetic interference (EMI) is something that should be detected when the cable is installed, so you should suspect either some new source that has been installed recently or some source that was not taken into account during testing (machinery or power circuits that weren't activated when the installation testing took place, for instance). Interference from nearby data cables is also referred to as alien crosstalk. Radio frequency interference (RFI) is EMI that occurs in the frequencies used for radio transmissions. Copyright © The Computing Technology Industry Association, Inc. All rights reserved.