When wireless LANs first became available in the
early 1990s, primary applications were wireless bar code solutions for needs
like inventory control and retail price marking. Data transfers for these types
of applications don’t demand very high performance. In fact, 1Mbps data rates
are generally sufficient to handle the transfer of relatively small bar codes
for a limited number of users.
Today, enterprises are deploying wireless LANs
for larger numbers of users with needs for corporate applications that involve
e-mail, Web browsing, and access to various server-based databases. The need
for higher data rates and techniques to improve performance of wireless LANs
is becoming crucial to support these types of applications. To get that extra
performance, you have a lot to consider.
- Choose the Right 802.11 Physical Layer.
An important element that impacts the performance of a wireless LAN is the
selection of the appropriate Physical (PHY) Layer (i.e., 802.11a,
802.11b, or 802.11g).
802.11a offers the highest capacity at 54Mbps for each of twelve (maximum)
non-overlapping channels and freedom from most potential RF interference.
802.11b provides 11Mbps data rates, with only three non-overlapping channels.
802.11g will eventually extend 802.11b networks to have 54Mbps operation,
but the three non-overlapping channels limitation will still exist. Of course
requirements dictate needs for performance, which will point you toward a
particular PHY. If you need maximum performance, then 802.11a is the way to
go, but you may need more access points because of the weaker range it has
compared to 802.11b.
- Properly Set Access Point Channels. The
802.11b standard defines 14 channels (11 in the U.S.) that overlap considerably,
leaving only three channels that don’t overlap with each other. For access
points that are within range of each other, set them to different channels
(e.g., 1, 6, and 11) in order to avoid inter-access point interference. You
can also take advantage of the automatic channel selection features that some
access points offer. I often see companies setting their access points all
to the same channel. The problem with this is that sometimes roaming will
not work as users move about the facility, and the transmission of a single
access point blocks all others that are within range. As a result, performance
degrades significantly. With 802.11a, this is not an issue because the 802.11a
standard defines separate, non-overlapping channels.
- Provide adequate RF coverage. If access
points are too far apart, then some users will be associating with the wireless
LAN at something less than the maximum data rate. For example, users close
to an 802.11b access point may be operating at 11Mbps; whereas, a user at
a greater distance may only have 2Mbps capability. In order to maximize performance,
ensure that RF coverage is as spread out as possible in all user areas, especially
the locations where the bulk of users reside. The completion of an effective
RF site survey will aid tremendously with this exercise. The proper setting
of transmit power and selection of antennas will also aid in positioning access
points for optimum performance.
- Avoid RF interference. Cordless phones
and other nearby wireless LANs can offer significant interfering signals that
degrade the operation of an 802.11b wireless LAN. These external sources of
RF energy in the 2.4GHz band periodically block users and access points from
accessing the shared air medium. As a result, the performance of your wireless
LAN will suffer when RF interference is present. So obviously you should strive
to minimize sources of RF interference and possibly set the access point channels
to avoid the interfering signals. Again, an RF site survey will help you discover
interference problems before designing and installing the wireless LAN. If
it’s not possible to reduce potential interference to an acceptable level,
then consider deploying 5GHz, 802.11a networks.
- Consider RTS / CTS. The optional request
to send / clear to send (RTS / CTS)
protocol of the 802.11 standard requires a particular station to refrain from
sending a data frame until the station completes a RTS / CTS handshake with
another station, such as an access point. RTS / CTS reduces collisions associated
with hidden nodes and may improve performance. Collisions can occur when hidden
nodes blindly transmit when another station (blocked by some obstruction or
significant range) is already transmitting. This causes a collision and results
in each station needing to retransmit their frames, doomed again by a possible
collision due to the hidden node scenario. The outcome is lower throughput.
If you suspect hidden nodes are causing collisions / retransmissions, then
try setting the RTS / CTS threshold lower through a trial and error process
while checking the impacts on throughput.
- Fragmentation. An 802.11 station can
use the optional fragmentation
protocol to divide 802.11 data frames into smaller pieces (fragments) that
are sent separately to the destination. Each fragment consists of a MAC
Layer header, FCS (frame check sequence), and a fragment number indicating
its ordered position within the frame. With thresholds properly set, fragmentation
can reduce the amount of data that needs retransmission. RF interference often
causes only a small number of bit errors to occur. Instead of resending the
entire data frame, the station implementing fragmentation only needs to retransmit
the fragment containing the bit errors. The key to making fragmentation improve
throughput is to set the thresholds properly. A threshold too low will result
in smaller fragments (making retransmissions efficient), but the greater number
of fragments requires substantial overhead because of the additional headers
and checksums. As with RTS / CTS, use a trial and error process to set the
threshold while keeping an eye on consequential throughput. If there is no
appreciable RF interference, then it’s best to deactivate fragmentation.
Jim Geier provides independent consulting services to companies developing
and deploying wireless network solutions. He is the author of the book, Wireless
LANs and offers computer-based
training (CBT) courses on wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
