Interference From Non-Overlapping Channels; Multipath And Antenna Diversity - Extreme Networks Summit WM Technical Reference Manual

DRM – Dynamic Radio Management
11 Mbps). This is why most 11b/g deployments use 3 channels (1, 6, and 11) or, where regulations allow (e.g.
ETSI), 4 channels 1, 5, 9, and 13.
The 802.11 protocol allows two Wireless APs with overlapping coverage to operate properly even when
the Wireless APs are using the same channel. This differs from other wireless technologies like cellular/
GSM and 802.16 WiMax where two base-stations with overlapping coverage must use distinct
non-overlapping channels or otherwise the co-channel interference will preclude any of them from
providing any service in the overlapping area. In a similar situation, the 802.11 Wireless APs will time-
share the channel so that both the Wireless APs will offer service in the overlapping coverage area.
The radio medium used by the WLAN is unlicensed, which means that the frequency bands used by
WLAN are not owned or reserved for the use by a service provider. Instead, it is public domain and any
device can use it provided that it meets certain requirements verified during regulatory-compliance
certification (i.e. FCC, ETSI, etc). This is another significant difference from other wireless technologies
like cellular/GSM and 802.16 WiMax that use licensed frequency bands in which there is no interference
from other systems. In a WLAN deployment, it is not only possible but very likely to encounter other
devices that use the same frequency and generate interference. These could be 802.11 devices — they
will share the bandwidth with the deployed Wireless APs — but they could be other devices like
Bluetooth, microwave ovens, cordless phones, video cameras, etc., which will cause a degradation of the
WLAN channel and result in reduced WLAN throughput or even loss of WLAN service.

Interference from non-overlapping channels

Two wireless devices that are very close to each other may interfere even if they operate on different
non-overlapping channels. This happens because when one of the devices is transmitting and the other
is receiving, the transmitted signal is so strong that it saturates the receiver of the other device. The
receivers are typically most sensitive to the adjacent channels, and the further away the frequency of the
transmitted signals the less sensitive the receiver is. Therefore, placing Wireless APs very close to each
other if they operate on adjacent channels is not recommended. In addition, placing Wireless APs near
strong sources of interference operating in adjacent bands is not recommended.
NOTE
Problems with adjacent bands have been observed with the lower and the higher channel in the 11b/g band because
the adjacent bands are used by licensed devices, which are allowed to transmit much higher power than 802.11
devices. A Wireless AP close to such a device must operate with a frequency that is as far away as possible from the
band edge.

Multipath and antenna diversity

The microwave radio signal propagates in a straight line as long as it does not encounter objects, but it
is reflected, refracted, or absorbed when it reaches any obstacle. Metal objects tend to reflect the signal,
while wood, concrete, ceramics, granite, or marble will tend to absorb or refract the signal. Due to these
propagation impairments, the wireless station (AP or client) will receive multiple copies of the same
signal with various delays and amplitudes. This phenomenon is called multipath and is a severe
impairment in traditional analog wireless communications. For example, multipath in analog TV causes
"ghosting" on the end image. If two signal copies arrive at the receiver with the same amplitude and
opposite phase, they will cancel each other. This effect is often noticeable in the short wave
communications and it is called "fading".
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