By Jim Geier
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.