Page 75 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 4: Wireless Networks Types and PAN Technologies

operating in the 2.4 GHz band, such as 802.11 wireless LANs. A critical problem is that Bluetooth Notes
and 802.11b neither understand each other nor follow the same rules. A Bluetooth radio might
haphazardly begin transmitting data while an 802.11 station is sending a frame. This results in a
collision, which forces the 802.11 station to retransmit the frame. This lack of coordination is the
basis for radio frequency (RF) interference between Bluetooth and 802.11.
Because of the potential for collisions, 802.11 and Bluetooth networks suffer from lower performance.
An 802.11 station automatically lowers its data rate and retransmits a frame when collisions occur.
Consequently, the 802.11 protocol introduces delays in the presence of Bluetooth interference.
The full impact of RF interference depends on the utilization and proximity of Bluetooth devices.
Interference occurs only when both Bluetooth and 802.11b devices transmit at the same time.
Users might have Bluetooth devices in their PDAs or laptops, but no interference will exist if their
applications are not using the Bluetooth radio to send data.
Some Bluetooth applications, such as printing from a laptop or synchronizing a PDA to a desktop,
utilize the radio for a short period of time. In this case, the Bluetooth devices are not active long
enough to noticeably degrade the performance of an 802.11 network. For example, a user might
synchronize her PDA to her desktop when arriving at work in the morning. Other than that, their
Bluetooth radio might be inactive and not cause interference the rest of the day.
The biggest impact is when a company implements a large-scale Bluetooth network, such as one
that enables mobility for doctors and nurses using PDAs throughout a hospital. If the Bluetooth
network is widespread and under moderate-to-high levels of utilization, the Bluetooth system
will probably offer a substantial number of collisions with an 802.11 network residing in the
same area. In this case, Bluetooth and 802.11 would have difficulties coexisting, and performance
would likely suffer.
In addition to utilization, the proximity of the Bluetooth devices to 802.11 radio NICs and
access points has a tremendous affect on the degree of interference. The transmit power of
Bluetooth devices is generally lower than 802.11 wireless LANs. Therefore, an 802.11 station
must be relatively close (within 10 feet or so) of a transmitting Bluetooth device before significant
interference can occur.
A typical application fitting this scenario is a laptop user utilizing Bluetooth to support
connections to a PDA and printer and 802.11 to access the Internet and corporate servers. The
potential for interference in this situation is enormous, especially when the user is operating
within outer limits of the coverage area of the 802.11 network. Figure 4.4 illustrates this situation.
The signal from the Bluetooth device will likely drown out the weaker 802.11 signal because of
the distance of the access point.

Figure 4.4: RF Interference Can Occur Between Bluetooth and 802.11 Wireless LAN Devices

Source: http://etutorials.org/Networking/wn/Chapter+4.+Wireless+PANs+Networks+for+Small+Places/Wireless+PAN
+Technologies/

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