After installing a wireless LAN, you might find that it doesn’t support applications as expected. Users may complain of erratic connections and slow performance, which hampers the use and benefits of applications. When this happens, you’ll need to do some troubleshooting. Start by finding the root cause of the problems.
The table below gives you some pointers on what to look for, specifically the characteristics of signal level, noise level and retry rate that relate to the root causes of poor wireless LAN performance.
Bad Access Point
RF interference occupies the air medium, which delays users from sending/receiving data and causes collisions and resulting retransmissions. The combination of high noise levels and high retry rates generally indicates that RF interference is impacting your wireless LAN. You can use tools such as AirMagnet Analyzer or NetStumbler to measure noise. Also, AirMagnet has tools for testing retry rates, and most access points store retry statistics that you can view through the admin console.
If the noise level is above -85dBm in the band where users are operating, then RF interference has the potential to hurt performance. In this case, the retry rates of users will be above 10 percent, which is when users start feeling the effects. This can occur, for example, when wireless users are in the same room as an operating microwave oven.
Once you diagnose the problem as being RF interference, then figure out where it’s coming from and eliminate the cause. If the symptoms only occur when the microwave oven or cordless phone is operating, then try setting the access point to a different channel. That sometimes eliminates the interference.
Take a quick scan of other wireless LANs operating in your area. If you see that others are set to the same channel as yours, then change your network to non-conflicting channels. Keep in mind that there are only three channels (1, 6 and 11) in the 2.4GHz band that don’t conflict with each other. Most homes and small offices will have their access point set to channel 6 because that’s the most common factory default channel. So you may need to avoid using channel 6 with the access points near the perimeter of your enterprise.
If you can’t seem to reduce RF interference to acceptable levels, then try increasing RF signal strength in the affected areas. You can do this by increasing transmit power, replacing default antennas with units that have a higher gain, or placing the access points closer to each other. This increases the signal-to-noise ratio (SNR), which improves performance.
When there are large numbers of active wireless users, or the users are operating high-end applications such as Wi-Fi phones or downloading large files, the utilization of the network may be reaching the maximum capacity of the access point. With this condition, the retry rates will be relatively high (greater than 10 percent), even if signal levels are high and noise levels are low (i.e., high SNR). The result is lower throughput per user due to the additional overhead necessary to retransmit data frames.
You can increase the capacity and resolve this problem by placing access points closer together with lower transmit power to create smaller radio cells. This “micro-cell” approach reduces the number of users per access point, which enables more capacity per user.
Another method for handling high utilization is to move some of the applications to a different frequency band. For example, you might consider having Wi-Fi phones interfacing with a 5GHz 802.11a network and data applications running over 2.4GHz 802.11b/g.
After installing a wireless LAN, changes may take place inside the facility that alter RF signal propagation. For example, a company may construct a wall, which offers significant attenuation that wasn’t there before. Worse, perhaps an RF site survey was not done prior to installing the network. These situations often result in areas of the facility having limited or no RF signal coverage, which decreases the performance and disrupts the operation of wireless applications.
Indications of a coverage hole include low signal level (less than -75dBm) and high retry rates (greater than 10 percent), regardless of noise levels. The signal in this situation is so low that the receiver in the radio card has difficulties recovering the data, which triggers retransmissions, excessive overhead, and low throughput. For instance, a user will likely experience a 75 percent drop in throughput when operating from an area having low signal levels.
To counter coverage holes, you need to improve the signal strength in the affected areas. Try increasing transmit power, replacing the antennas with ones having higher gain, or moving access points around to better cover the area. Keep coverage holes from popping up unexpectedly in the future by performing a periodic RF site survey, possibly every few months.
Bad Access Point
In some cases, the root cause of poor performance may be an access point that has failed. Check applicable access points for broken antennas, status lights indicating fault conditions and insufficient electrical power. Try rebooting the access points, which often resolves firmware lockups. Make sure that the firmware is up to date, however, to minimize lockups in the future!
Jim Geier is the principal of Wireless-Nets, Ltd. (www.wireless-nets.com), a consulting firm focusing on the implementation of wireless mobile solutions and training. He is the author of the books, Deploying Voice over WLANs (Cisco Press), Wireless LANs (Sams) and Wireless Networks – First Step (Cisco Press).