802.11 Signal Attenuation Causes and Corrective Measures

Article ID: 91566153

Question

What is causing 802.11 signal attenuation?

Facts

Attenuation is simply a reduction of signal strength during transmission. When signal power decreases to relatively low values, the receiving 802.11 radio will likely encounter bit errors when decoding the signal. This problem worsens when significant RF interference is present. The occurrence of bit errors causes the receiving 802.11 station to refrain from sending an acknowledgement to the source station. After a short period of time, the sending station will re-transmit the frame, possibly at a lower data rate with hopes of extending the range of the transmission. Excessive attenuation causes the network's throughput to decrease because of operation at a lower data rate and the additional overhead necessary to re-transmit the frames. Generally, this means that the user is operating within the outer bounds of an access point's range. There's enough attenuation present to decrease signal power below acceptable values. At worst case, signal power loss due to attenuation becomes so low that affected users will lose connectivity to the network.

Answer

1. Causes of attenuation:Both signal frequency and range between the end points of the medium affect the amount of attenuation.  As either frequency or range increases, attenuation increases.  Unlike open outdoor applications based on straightforward free space loss formulas, attenuation for indoor systems is very complex to calculate.  The main reason for this difficulty is that the indoor signals bounce off obstacles and penetrate a variety of materials that offer varying effects on attenuation.  Keep in mind also that attenuation is not linear--it grows exponentially as range increases.
  • Counteracting attenuation:The main goal of combating attenuation is to avoid having signal power within the area where users operate to fall below the sensitivity of the 802.11 radio receivers. You need to ensure that the receiver is always able to hear the transmissions. Keep in mind that higher levels of RF interference, such as that caused by 2.4GHz cordless phones or Bluetooth (Read More>>) devices, will negatively impact the ability for the receiver to decode the signal. As RF interference signal levels become higher than 802.11 signals, an 802.11 receiver will encounter considerable bit errors when trying to demodulate the 802.11 signals.

2. It's nearly impossible to accurately determine the range of wireless signals through indoor facilities without performing some live testing such as an RF site survey to verify location estimates. The use of an 802.11 radio along with intuitive site survey software will successfully prove that signal levels are above minimum requirements.