An important challenge with WLANs is radio signal interference. In
metropolitan area building-to-building designs, it is possible to have
third-party interference from other companies using wireless technology
. In this
situation, network managers must ensure that different channels are utilized.
Interference cannot always be detected until the link is actually implemented.
Because the 802.11 standards use unlicensed spectrum, changing channels is the
best way to avoid interference.
Many other devices such as portable
phones, microwave ovens, wireless speakers, and security devices, also use
these frequencies
. The
amount of mutual interference that will be experienced by these and planned
networking devices is unclear. Interference between wireless speakers and other
devices is common today. As this unlicensed band becomes more crowded, it is
likely that other types of interference will appear. Physical objects and
building structures also create various levels of interference.
Operation in unlicensed bands carries with it an inherently higher risk of
interference, because the controls and protections of licensing are not
available. In the United States, for example, the Federal Communications
Commission (FCC) does not have any rule that specifically prohibits a new user
from installing a new unlicensed-band radio link in an area and on a frequency
that is already occupied. This may result in interference. There are two
warnings to be aware of.
The first warning is, if someone installs a link
that interferes with a wireless link, the interference is probably mutual. With
point-to-point links that employ directional antennas, any signal source of a
comparable power level that might cause interference would have to be
physically aligned along the transmission path axis. In unlicensed bands, the
potential for interference from another unlicensed user is becoming greater
than for licensed bands. The difference is one of control. The license holders
essentially own a licensed channel. They have a legal right to defend
themselves against performance-robbing interference. Due to the popularity of
WLANs, the use of unlicensed bands is increasing. Network administrators should
be aware that there are other, licensed users who sometimes also operate in the
unlicensed bands. The unlicensed bands are allocated on a shared basis. While
there may be no requirement to obtain a license to operate a low-power data
communications application using approved equipment, licensed users may be
allowed to operate with significantly higher power.
The second warning
is, it is possible for electromagnetic interference (EMI) to be generated by
non-radio equipment operating in close proximity to the Cisco Aironet WLAN
equipment. While it is theoretically possible for this interference to directly
affect the reception and transmission of signals, it is more likely to affect
the components of the transmitter. To minimize the possible effects of EMI, the
best course of action is to isolate the radio equipment from potential sources
of EMI. The equipment should be located away from such sources if possible, and
a source of conditioned power to the WLAN equipment should be provided to help
lessen the effects of EMI.