User guide

AirMagnet Spectrum XT User Guide
Chapter 10: Device Detection 145
As the number of these devices increases, the 2.4-GHz IMS band is becoming more and more
congested. As a result, network performance degradation has become a major issue facing
network administrators managing 802.11b/g WLANs. It has long been recognized that the
main culprit for WLAN performance degradation is RF interference caused by these
competing devices in the 2.4-GHz band. RF interference occurs when two or more RF devices
are transmitting at the same frequency at t he same time. RF interference causes over-the-air
collision which can lead to data corruption and loss.
802.11b APs can operate on one of 13 (11 in the US) channels in the 2.4-GHz IMS band, each
being 22 MHz wide and 5 MHz apart. Because each of these channels takes up roughly a
quarter of the 2.4-GHz spectrum and adjacent channels tend to interfere with each other,
802.11b WLANs are typically installed using one of three non-overlapping channels, namely
Channels 1, 6, and 11.
The table below shows the operating channels for 802.11b WLANs in North America, with
Channels 1, 6 and 11 highlighted in grey as non-overlapping channels.
RF Spectrum Pattern
802.11b APs use Direct Sequence Spread Spectrum (DSSS) signal modulation method which is
very susceptible to signal multipath. Signal multipath occurs when radio signals are reflected
on their way between the transmitter and the receiver. This could happen when radio signals
from an AP are blocked by metal furniture, dry walls, and other structural elements common
in office buildings. Signal multipath has a huge impact on data quality and WLAN
throughput because it causes transmission errors and requires retransmission.
802.11b WLAN APs typically use up to +20 dBm (100mW) transmit power and -80 dBm ~ -
90dBm of receive sensitivity. Their bit transfer rate is 11 Mbps (maximum).
Figure 10-20 shows the typical RF pattern of an 802.11b AP.
Figure 10-20: RF spectrum pattern of an 802.11b AP
Impact on WiFi Networks
Since 802.11b APs use a fixed bandwidth of 22 MHz in the 2.4-GHz spectrum, the probability
of collisions or interference between 802.11b APs or with other 2.4-GHz devices largely
depends on the channel they operate. If they are on the same channel or overlapping channels,
the probability is high. Otherwise, the chances of collision are low.