Previously,
we’ve defined RF interference and discussed general methods to minimize the
problem. Potential sources of interference include cordless phones, microwave
ovens, and Bluetooth-enabled
devices. The presence of these types of devices can degrade the performance
of an 802.11 wireless LAN.
Bluetooth provides relatively low data rates for supporting short-range, wireless
personal area network (PAN) applications. Bluetooth radios are starting to appear
more often now in laptops, headphones, cell phones, and PDAs, which results
in much less cabling for users to deal with.
The deployment of both Bluetooth and 802.11 networks in the same area is a
bit risky, though, because of the potential for interference.
What’s the problem?
Similar to 802.11b, Bluetooth devices operate within the 2.4 GHz band. The
difference is that that Bluetooth uses frequency hopping (at 1,600 hops per
second) to hop over the entire 2.4 GHz band. 802.11b, on the other hand, uses
direct sequence and only occupies approximately one third of the 2.4 GHz band.
As a result, Bluetooth hops all over 802.11b transmissions.
An 802.11 station (client or access point) is polite and first listens to the
medium before transmitting. If the 802.11 station doesn’t sense RF energy above
a certain threshold (meaning that the medium is idle), the 802.11 station can
transmit a frame. While the 802.11 station is sending the frame, other 802.11
stations will hold off their transmissions by following the same protocol. This
provides a fairly good method of sharing a common RF channel among devices complying
with the 802.11 standard.
A critical problem is that Bluetooth and 802.11b neither understand each other
nor follow the same rules. A Bluetooth radio may 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 when it realizes that
the receiving station is not going to send back an acknowledgement. This lack
of coordination is the basis for RF interference between Bluetooth and 802.11.
Interference impacts vary
Because of the potential for collisions, 802.11 (and Bluetooth) networks can
suffer 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 can only occur when both Bluetooth and 802.11b
devices transmit at the same time. Users may 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, only utilize the radio for a very short period of time.
In this case, the Bluetooth devices will generally not be active long enough
to noticeably degrade the performance of an 802.11 network. For example, a user
may synchronize their PDA to their desktop when arriving at work in the morning.
Other than that, their Bluetooth device may be inactive the rest of the day.
The biggest impacts are when a company implements a larger-scale Bluetooth
network, for example, 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 will likely suffer.
In addition to utilization, the proximity of the Bluetooth devices to 802.11
radio NICs and access points has a tremendous effect on the degree of interference.
The transmit power of Bluetooth devices is generally much lower than 802.11
wireless LANs. Thus, an 802.11 station must be relatively close (within ten
feet or so) of a transmitting Bluetooth device before significant interference
can occur.
A typical application fitting this scenario is a laptop user having Bluetooth
supporting connections to a PDA and printer and 802.11 for accessing the Internet
and corporate servers. The potential for interference in this situation is enormous,
especially when the user is operating within the fringe area of the 802.11 network
(i.e., at the range that just supports their required data rate). The low power
Bluetooth signal will likely drown out the weaker 802.11 signal due to the distance
of the access point.
Tips for minimizing interference from Bluetooth
What can you do to avoid interference from Bluetooth devices? The following
are some tips to consider:
- Manage the use of RF devices. One way to reduce the potential for
interference is to regulate the types of RF devices within your facility.
In other words, establish your own private regulatory body for managing unlicensed
RF devices. The extreme measure would be to completely ban the use of Bluetooth;
however, that is not practical or even possible in all cases. For example,
you can’t feasibly prohibit the use of Bluetooth in public areas. For private
applications, you could set company policies to limit the use of Bluetooth
to only specific applications, such as syncing PDAs to desktops. - Ensure adequate 802.11 coverage. Strong, healthy 802.11 signals throughout
the coverage areas helps reduce the impact of the Bluetooth signals. If wireless
LAN transmissions become too weak, then the interfering Bluetooth signals
will be more troublesome. As a result, perform a thorough RF
site survey, and determine the appropriate location for access points. - Move to the 5 GHz band. If none of the above steps solve the problem,
then consider using 5 GHz (i.e., 802.11a) NICs and
access points At least for the foreseeable future, you can completely avoid
RF interference in this band. You’ll also receive much higher throughput;
however, the limited range could require additional access points and higher
costs.
The future should bring harmony
For a couple years, the IEEE 802.15.2 task group, in conjunction with the Bluetooth
SIG, has been working on a "recommended practice" that describes
techniques to allow 802.11b and Bluetooth to coexist. Eventually the practices
will likely become part of the standard. They’re currently analyzing methods
that will provide the best solution; expect some firm practices hopefully by
the end of 2002.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
|
