Minimize WLAN Interference

Radio frequency (RF) interference can lead to disastrous problems on
wireless LAN deployments. Many companies have gotten by without any
troubles, but some have installations that don’t operate nearly as well as
planned. The perils of interfering signals from external RF sources are
often the culprit. As a result, it’s important that you’re fully aware of
RF interference impacts and avoidance techniques.

Impacts of RF interference

As a basis for understanding the problems associated with RF interference
in wireless LANs, let’s quickly review how 802.11 stations (client radios
and access points) access the wireless (air) medium. Each 802.11 station
only transmits packets when there is no other station transmitting. If
another station happens to be sending a packet, the other stations will
wait until the medium is free. The actual 802.11 medium access protocol is
somewhat more complex, but this gives you enough of a starting basis.

RF interference involves the presence of unwanted, interfering RF signals
that disrupt normal wireless operations. Because of the 802.11 medium
access protocol, an interfering RF signal of sufficient amplitude and
frequency can appear as a bogus 802.11 station transmitting a packet. This
causes legitimate 802.11 stations to wait for indefinite periods of time
before attempting to access the medium until the interfering signal goes
away.

To make matters worse, RF interference doesn’t abide by the 802.11
protocols, so the interfering signal may start abruptly while a legitimate
802.11 station is in the process of transmitting a packet. If this occurs,
the destination station will receive the packet with errors and not reply
to the source station with an acknowledgement. In return, the source
station will attempt retransmitting the packet, adding overhead on the
network.

Of course this all leads to network latency and unhappy users. In some
causes, 802.11 protocols will attempt to continue operation in the presence
of RF interference by automatically switching to a lower data rate, which
also slows the use of wireless applications. The worst case, which is
fairly uncommon, is that the 802.11 stations will hold off until the
interfering signal goes completely away, which could be minutes, hours, or
days.

Sources of RF interference

With 2.4 GHz wireless LANs, there are several sources of interfering
signals, including microwave ovens, cordless phones, Bluetooth-enabled
devices, FHSS wireless LANs, and neighboring wireless LANs. The most
damaging of these are 2.4 GHz cordless phones that people use extensively
in homes and businesses. If one of these phones is in use within the same
room as a 2.4GHz (802.11b or 802.11g) wireless LAN, then expect poor
wireless LAN performance when the phones are in operation. (Refer to a
previous tutorial for results of testing interference from a cordless
phone.)

A microwave operating within ten feet or so of an access point may also
cause 802.11b/g performance to drop. Of course the oven must be operating
for the interference to occur, which may not happen very often depending on
the usage of the oven. Bluetooth-enabled devices, such as laptops and PDAs,
will cause performance degradations if operating in close proximately to
802.11 stations, especially if the 802.11 station is relatively far (i.e.,
low signal levels) from the station that it’s communicating with. The
presence of FHSS wireless LANs is rare, but when they’re present, expect
serious interference to occur. Other wireless LANs, such as one that your
neighbor may be operating, can cause interference unless you coordinate the
selection of 802.11b/g channels.

Use tools to “see” RF interference

Update002-RF_Interference.jpg

Figure 1.

Unless you’re Superman, you can’t directly see RF interference with only
your eyes. Sure, you might notice problems in using the network that
coincide with use of a device that may be causing the interference, such as
turning on a microwave oven and noticing browsing the Internet slow
dramatically, but having tools to confirm the source of the RF interference
and possibly investigate potential sources of RF interference is
crucial. For example, MetaGeek’s Wi-Spy is a relatively inexpensive
USB-based Wi-Fi spectrum analyzer that indicates the amplitude of signals
across the 2.4GHz frequency band. Figure 1 is a screenshot of the
Wi-Spy display with a microwave oven operating ten feet away.

This clearly shows relatively high-level signals emanating from the
microwave oven in the upper portion of the 2.4GHz frequency band, which
indicates that you should tune any access points near this microwave oven
to lower channels. To simplify matters, MetaGeek has an interference
identification guide that you can use with Wi-Spy to help pinpoint
interfering sources. The benefit of using a spectrum analyzer in this
manner is that you can identify the interference faster and avoid guessing
if a particular device is (or may) cause interference.

Take action to avoid RF interference

The following are tips you should consider for reducing RF interference
issues:

  1. Analyze the potential for RF interference. Do this before
    installing the wireless LAN by performing an RF site survey. Also, talk to
    people within the facility and learn about other RF devices that might be
    in use. This arms you with information that will help when deciding what
    course of action to take in order to reduce the interference.
  2. Prevent the interfering sources from operating. Once you know the potential
    sources of RF interference, you may be able to eliminate them by simply
    turning them off. This is the best way to counter RF interference; however,
    it’s not always practical. For example, you can’t usually tell the company
    in the office space next to you to stop using their cordless phones;
    however, you might be able to disallow the use of Bluetooth-enabled devices
    or microwave ovens where your 802.11 users reside.
  3. Provide adequate wireless LAN coverage. A good practice for reducing
    impacts of RF interference is to ensure the wireless LAN has strong signals
    throughout the areas where users will reside. If signals get to weak, then
    interfering signals will be more troublesome, similar to when you’re
    talking to someone and a loud plane flies over your heads. Of course this
    means doing a thorough RF site survey to determine the most effective
    number and placement of access point.
  4. Set configuration parameters properly. If you’re deploying 802.11g
    networks, tune access points to channels that avoid the frequencies of
    potential interfering signals. This might not always work, but it’s worth a
    try. For example, as pointed out earlier in this tutorial, microwave ovens
    generally offer interference in the upper portion of the 2.4GHz band. As a
    result, you might be able to avoid microwave oven interference by tuning
    the access points near the microwave oven to channel 1 or 6 instead of 11.
  5. Deploy 5GHz wireless LANs. Most potential for RF interference today is in
    the 2.4 GHz band (i.e., 802.11b/g). If you find that other interference
    avoidance techniques don’t work well enough, then consider deploying
    802.11a or 802.11n networks. In addition to avoiding RF interference,
    you’ll also receive much higher throughput.

The problem with RF interference is that it will likely change over
time. For example, a neighbor may purchase a cordless phone and start using
it frequently, or the use of wireless LANs in your area may increase. This
means that the resulting impacts of RF interference may grow over time, or
they may come and go. As a result, in addition to suspecting RF
interference as the underlying problem for poor performance, investigate
the potential for RF interference in a proactive manner.

Don’t let RF interference ruin your day?keep a continual close watch on the
use of wireless devices that might cause a hit on the performance of your
wireless LAN.

Author Biography: Jim Geier provides independent consulting services and
training to companies developing and deploying wireless networks for
enterprises and municipalities. He is the author of a dozen books on
wireless topics, with recent releases including Deploying Voice over
Wireless LANs (Cisco Press) and Implementing 802.1x Security Solutions
(Wiley).

Article courtesy of Wi-Fi
Planet

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