Beating Signal Loss in WLANs

By Jim Geier

July 23, 2002

Don't let RF signal loss (attenuation) ruin your day. Learn the causes, impacts, and countermeasures of signal attenuation to better plan the deployment of wireless LANs.

Unfortunately when deploying wireless LANs, we have to abide by the laws of physics. Wireless signals propagating through the air lose strength while encountering natural and manmade obstacles. It would be nice if RF signals would propagate without an bounds, but that simply doesn't occur on Earth.

In order to deploy an effective wireless LAN solution, installers must have a good understanding of the causes of signal loss (attenuation) and how to implement applicable countermeasures. This knowledge becomes extremely important when performing an RF site survey, which technicians use to determine the optimum location of access points to provide necessary range. With familiarity of RF attenuation, you'll accomplish RF site surveys more efficiently and get higher performing wireless network installations as a result.

Attenuation basics

Attenuation is simply a reduction of signal strength during transmission. You represent attenuation in decibels (dB), which is ten times the logarithm of the signal power at a particular input divided by the signal power at an output of a specified medium. For example, an office wall (i.e., medium) that changes the propagation of an RF signal from a power level of 200 milliwatts (the input) to 100 milliwatts (the output) represents 3 dB of attenuation. Consequently, positive attenuation causes signals to become weaker when traveling through the medium.

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 bits 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 retransmit 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 retransmit 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.

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.

Discussion of the various algorithms to estimate indoor path loss is beyond the scope of this article. As a general rule of thumb, however, expect to encounter approximately 100dB of attenuation over distances of 200 feet when using 802.11b radios operating at 11Mbps. Keep in mind also that attenuation is not linear--it grows exponentially as range increases.

Typical office obstacles such as doors, windows and walls offer fairly known levels of attenuation. These values of attenuation are in addition to the path loss mentioned earlier. The following provides some examples of the attenuation values of common office construction:

Plasterboard wall

3dB

Glass wall with metal frame

6dB

Cinder block wall

4dB

Office window

3dB

Metal door

6dB

Metal door in brick wall

12.4dB

As a result, a typical small office could have several plasterboard walls equating to an additional 9 to 12dB of attenuation. Metal doors and glass walls could sometimes be in the way of the propagation of the signal, causing even larger amounts of attenuation. Of course this decreases the operating range of the access points.

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. Bear in mind also that higher levels of RF interference, such as that caused by 2.4GHz cordless phones or Bluetooth 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.

How much attenuation is acceptable? The mathematical method for determining this is to take into account EIRP (equivalent isotropically radiated power) and receiver sensitivity. Receiver sensitivity is different depending on whether you're using 802.11a or 802.11b and the data rate that users are operating. The higher the data rate, the lower the receiver sensitivity requirements. In other words, a receiver must be more sensitive to detect higher data rate signals.

For example, the EIRP of the source station could be 200 milliwatts (23dBm) and the receiver sensitivity would be -76dBm for 802.11b at 11Mbps. Thus, you can only afford to have 99dB of attenuation [23dBm -- (-76dBm)] before the signal drops below the receiver's ability to hear the signal. Thus at 200 feet from the access point, the user's 802.11b receiver will probably barely notice signals from the access point. If obstructions such as walls are present, then operating range will be less.

You can use these concepts to help with planning the location of access points. When setting up access points to operate near their maximum range, be aware that obstacles such as walls will offer additional amounts of attenuation that could cause loss of connectivity. For example if you're planning the range of a particular access point to be 200 feet, then having a few walls in between the access point and users will cause an additional 9dB or more of attenuation, which could likely be enough to push the signal power down below the receiver's sensitivity. As a result, place your access points closer together to ensure adequate coverage.

It's nearly impossible to accurately determine the range of wireless signals through indoor facilities without performing some live testing. As a result, be sure to accomplish an RF site survey to verify location estimates. The use of an 802.11 radio along with site survey software with successful test results proves that signal levels are above minimum requirements. Also consider using a wireless LAN analyzer, such as AirMagnet or AiroPeek to measure signal power at various points throughout the facility to ensure signal power levels are well above the receiver sensitivity.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.

Got a comment or question? Discuss it in the 802.11 Planet Forums


Comment and Contribute
(Maximum characters: 1200). You have
characters left.