Features Help Visualize Interference Issues

By Andrew Garcia  |  Posted 2010-08-12

Cisco's CleanAir Spectrum Analysis Offers Outstanding RF Visibility

Cisco CleanAir-based on the networking giant's newest Aironet3500 series access points and the 7.0 version of the Unified Wireless Network software-provides outstanding RF visibility combined with reporting, tracking and assessment tools that help wireless administrators build stable wireless networks ready to host mission-critical systems and applications.

For providing a premium and distinct solution (even at a premium price) in an increasingly commoditized marketplace trending toward lower prices and similar feature sets, Cisco and CleanAir earn eWEEK's Analyst's Choice.  

With many wireless LAN vendors now touting spectrum analysis detection features, Cisco stands alone as the only vendor providing that capability via integrated, purpose-built hardware. Leveraging its 2007 acquisition of Cognio, Cisco added a customized chip set to the new Aironet 3500 series AP (access points) that works in concert with the standard WiFi implementation to provide reporting, assessment and actions based on visibility into raw spectrum energy and WiFi-related sources.

I tested CleanAir using six Aironet 3502i dual-band 802.11n access points deployed throughout eWEEK's San Francisco offices, connected to a 5508 WLC (Wireless Controller) running version 7.0.98 of its Unified Wireless Network software. To the testbed, I added a Cisco WCS (Wireless Control System) installed on a Windows XP-based laptop and a 3300 series MSE (Mobility Services Engine).

Prices for the 3500 series Aironet APs start at $1095 each for the 2.4 GHz-only 3501i with internal antennae, and go up to $1495 for the dual-band 3502e with external antenna connectors. Our 3502i is right in the middle at $1295 apiece.  According to Cisco representatives, a network that comprises 250 Aironet APs, a single 5508 WLC, WCS software and wireless-management licenses, and a 3300 MSE with Context Aware licenses would cost $414,225 when using Aironet 3502i APs.

Aironet 3500 APs and a 2100, 4400 or 5500 series WLC with 7.0 software are the only required elements for CleanAir. The MSE and WCS are optional, but they do add mapping, systemwide air-quality data aggregation and storage, and an analysis of historical data that are not otherwise available.

CleanAir APs generate a couple of distinct pieces of information that are transmitted to the WLC to provide the RF perspective. The first, the IDR (Interference Device Report), generates a unique device identifier for detected interferers, while reporting the interferer classification along with affected radio band and channel. The IDR also reports an interferer's severity index, based on its relative transmit power and its duty cycle.

Air Quality Qualifies How Usable APs RF Environment Is


The second element, Air Quality, essentially aggregates the severity index of all identified interferers to provide a score to help qualify how usable the AP's RF environment is.

The WLC aggregates these information elements from all attached CleanAir APs, presenting per-band views on the average and minimum Air Quality assessments for the channels scanned by AP. Air Quality scores update every 15 minutes, unless an admin is looking at a specific AP and radio, which bumps the update to every 30 seconds, providing relatively real-time assessments.

The channels scanned by an access point depend on its mode.  APs in local mode (servicing wireless clients) only monitor the RF for the channel that the AP transmits on, while monitor-only mode APs constantly cycle through all channels, passively listening to both WiFi and non-WiFi traffic.

In my tests, CleanAir identified an assortment of the usual suspects of interferers, providing impact assessments for each: Bluetooth links and discovery actions, dozens of DECT phones in the 2.4 and 5 GHz bands, and microwave ovens. CleanAir also classified some X-box controllers, analog wireless cameras and generic time-division duplex transmitters.

Because interferers could be detected by several CleanAir APs listening on the same channel, the WLC attempts to merge together interferer records to avoid multiple listings for the same interferer. For local-mode APs, neighbor lists are used to determine if detecting APs are near each other. The WLC tries to cluster records it thinks are for the same interferer into one object, with the AP most affected by the interferer as the cluster center.  Merging is also done at the MSE for larger networks supporting multiple controllers, and the MSE is also required to merge records provided by a monitor-mode-only AP.    

I found merging to be somewhat hit and miss, not unexpected given the newness of the capability.  Although frequently able to correctly merge my interference sources, CleanAir left numerous like items orphaned. It also merged a few interferers incorrectly, such as the time an Apple Magic Pad that had never before entered the building was merged with a two-week-old otherwise unknown Bluetooth source.

CleanAir also helps automate interference avoidance, as CleanAir is integrated into Cisco's RRM (Radio Resource Management) feature set, thereby allowing the network to change an AP's channel if Air Quality drops too low for too long due to either WiFi and non-WiFi causes.

CleanAir provides two types of interference avoidance for RF sources. An event-driven RRM is utilized for intermittent interference, as it detects interference overpowering AP and moves the AP to a different channel, providing a cleaner Air Quality for at least three hours--even if the new channel is the same as that of a neighboring AP.

Persistent Device Avoidance, meanwhile, tracks interferers that tend to be always on and rather immobile-such as wireless cameras or Canopy outdoor networking equipment. If CleanAir classifies the interferer as persistent, the channel change will last until seven days have passed since the last time the interferer was detected.

Changing channels is tricky business in a large network, and Cisco put the time thresholds in place to avoid APs constantly thrashing between channels, which has implications for the performance of other parts of the network. And CleanAir RRM makes its changes with some smarts, avoiding channel changes due to interference from frequency-hopping devices that affect many channels by bouncing their transmissions around the spectrum. 

In tests, I found CleanAir RRM worked quite well, quickly identifying and avoiding interference provided by a pair of incredibly noisy wireless cameras-one using the 2.4 GHz band, the other 5 GHz. In practice, I have some qualms about using event-driven RRM in the 2.4-GHz band, however; there simply aren't many alternate channels to choose from. For instance, I set the 2.4-GHz camera on a channel that affects WiFi channels 1-9, so four of my six APs immediately flopped over to channel 11, where they stayed for the next three hours.

Since the other channels were swamped with a high-power, high-duty cycle interferer, the alternative was not ideal either, but four adjacent APs on the same channel in a multicell architecture is a practice that's typically frowned upon. At the very least, I'd recommend tuning the non-WiFi RRM thresholds very low, say, in the 2.4 GHz band.

Thankfully, CleanAir trap and action thresholds are highly configurable, which allowed me to tailor alarm and RRM actions differently for each band or for each controller. Typically, many interferers-particularly Bluetooth devices-are relatively low risk for network performance, so I could also configure the system to avoid reporting IDRs for those devices, while still accounting for their presence in the Air Quality score.

Features Help Visualize Interference Issues


Adding a WCS server and an MSE into the mix adds a number of features to help visualize and respond to interference issues in larger networks.  The WCS's CleanAir dashboard offers clean graphical depictions of both the average and minimum AirQuality scores in both bands across the network over the last seven days and raw interferer counts with detailed lists of the worst interferers and their effects, as well as potential security risks. 

The mapping capabilities of the WCS are also extended to CleanAir, with the system attempting to place detected interferers on a floor plan, provided at least three APs detected the source. With an MSE in place, I could also track movements of an interferer over time, provided sources were merged properly.  I typically found location estimates for non-WiFi sources weren't as accurate as those presented for WiFi devices, but, in most cases, the plots on the my map fell within 5-10 meters of the actual source location with a few egregiously wrong outliers. However, since local-mode APs only listen on the transmit channels, I'm not surprised about the inaccuracies, given the lack of points of triangulation in my small test network.  

Perhaps my favorite CleanAir feature is the integration between CleanAir and the new Cisco Spectrum Expert 4.0 software for Windows PCs (price with PC card is $4260, or the software is a free download to use only with CleanAir sensors), called Spectrum Expert Connect. While previous iterations of Spectrum Expert required the network administrator to capture RF information by wandering around with a laptop armed with the software and a spectrum analysis PC Card sensor, version 4.0 lets administrators sit at a desk and use as the sensor a 3500-series AP deployed anywhere in the network. 

Connected remotely to the AP in this way, an administrator gains access to the raw spectrum analysis data that CleanAir works so hard to simplify and qualify. Through Spectrum Expert Connect, the administrator gains access to the real time Fast Fourier Transforms, Duty Cycle graphs and Swept Spectrograms, as well as visualizations of channel utilization, interference power, and channel utilization versus time.  Spectrum Expert also provides good analysis of the RF characteristics of detected interferers.

Unfortunately, the massive tradeoff with this feature is that the AP must be taken offline and rebooted to run new code by placing it in Spectrum Expert Connect mode from the WLC or the WCS. The AP cannot service clients nor perform regular WiFi scanning while in this mode, potentially leaving a hole in one type of coverage or the other. The disappointment is doubled, then, because I could only view a single bands' RF detail at one time, leaving the other radio in the SE-Connect-mode AP only able to report CleanAir IDRs and AirQuality back to the controller. 

Down the road, I suspect Cisco will find a way to let administrators use SE Connect to attach to local mode access points as well, to at least be able to analyze the RF details for the channel in each band on which the AP is transmitting.

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