Mapping your wireless local area network: How to make your WLAN shine
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In the first two parts of this series on WLAN performance optimization, wireless expert Lisa Phifer explains the
use of Wireless Intrusion Prevention Systems (WIPS) and the need to proactively troubleshoot WLANs. In this final portion of the series, we look at tools that take applications into account when measuring WLAN performance.
Many WLAN managers focus on fixing problems and avoiding security breaches, but WLAN performance optimization falls somewhere further down on the to-do list. Not only is this practice shortsighted, overlooking optimization is a recipe for disaster. Sub-par performance may be tolerable when each AP supports a dozen users surfing the Web, but as APs near capacity and applications grow more demanding, bandwidth hogs and bottlenecks become hard to ignore.
To address WLAN performance optimization, network managers and admins should use new WLAN measurement and testing tools that examine network performance in relation to the type of applications running on the network. Seeking those tools could force network admins to look beyond their current vendors and testing methods to more innovative offerings. While these tools may be costly, they are likely to pay for themselves many times over.
Why the need for better WLAN performance testing and network design?
Early WLANs were often designed by rough rules of thumb, using circles of coverage per AP, where radius represents minimum signal strength. Site surveys were then used to plot actual signal strength, measured by passive (scan) or active (associated) methods. Tools used to conduct surveys were manual and labor-intensive; for example, one method required a tester to walk a site twice to gather passive and active measurements.
Clearly, these methods don't scale well to larger WLANs. More importantly, they do not reflect the realities posed by 802.11n and more diverse applications that depend on high bandwidth and low latency. With 802.11n, data rates can differ in each direction, and strong signal is not necessarily an indication of acceptable application performance.
WLAN performance measurement tools gauge application need
This is where new performance measurement tools designed specifically for 802.11n and important business mobility applications can be a big help. For example, AirMagnet Survey not only uses iPerf to measure uplink and downlink performance, but it analyzes measurements to offer 802.11n recommendations and a voice-readiness assessment.
Veriwave WaveDeploy is an agent-based site assessment tool that generates test streams to real client devices to measure not just TCP throughput but "quality of experience" for Web, voice and video. By taking one pass through a facility with WaveDeploy, testers can generate coverage maps that depict which areas meet application requirements. This is measured with metrics for each application (e.g., MOS and jitter for voice).
WLAN performance optimization: Ongoing client testing
Optimizing WLAN performance can be a tricky business. These new survey tools should continue to be used after WLAN deployment to verify that performance targets continue to be met and to assess the impact of planned tweaks and updates. Over time, you may find that different clients achieve vastly different data rates and throughputs, even when sending the same kind of traffic from approximately the same location.
For example, clients that experience unusually low data rates may be refusing to roam to a better AP when they could. These "sticky" clients not only achieve low throughput, but they require more airtime to send the same data. When airtime competition is high, every other client gets penalized by that one sticky client. This is where a mobile tool like AirMagnet WiFi Analyzer can analyze client roaming behavior, identifying sticky clients and their actual impact on application performance (e.g., MOS score).
Using airtime fairness algorithms for RF measurement
Periodic test results can be useful for fine-tuning processes like disabling low data rates found to cause problems or enabling beneficial 802.11n options in clients. However, some performance issues are transient and sensitive to client mix and location. If a visitor carries a troublesome device into an office, or just tries to use the WLAN from too far away, one slow client could drag everyone else down. The only effective way to deal with this kind of issue is ongoing measurement backed by real-time response.
Some WLAN vendors now offer proprietary "airtime fairness" algorithms that take real-time RF measurements and use them to automatically adjust client transmit opportunities. For example, when Aerohive's Dynamic Airtime Scheduling is enabled, transmit opportunities are allocated based on the airtime each client actually consumes. As a client's data rate starts to fall, faster clients can be given more transmit opportunities, optimizing total throughput. Aerohive's Performance Sentinel then ties those real-time measurements to per-user demand and goodput targets. If SLAs are not met for heavier-traffic clients, compliance alerts can be generated to alert NOC operators.
Combining automated and drill-down WLAN testing tools
In the end, most enterprises will combine automated/centralized and mobile/drill-down test tools and methods. For some tasks, it makes sense to leverage WLAN and WIPS infrastructure to test continuously and adjust in near real time. This approach can be highly efficient in large distributed networks where periodic testing just doesn't cut it.
But for other tasks, third-party or handheld tools can still be extremely valuable. Third-party test tools can provide an "outside-looking in" perspective. Mobile tools can provide greater flexibility than AP or sensor-based tests. While these tools may require additional purchases or training, they can be invaluable to finding holes, debugging problems or isolating performance problems that would otherwise prove elusive.
Perhaps the most important thing is to avoid being myopic, simply using whatever your vendor offers or tools that you have always used to test your WLAN. This rapidly changing market is expected to expand along with enterprise WLAN adoption.
About the author: Lisa A. Phifer is vice president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment and the use of emerging technologies and best practices.