Is it adequate for an enterprise to scan only the channels that it is using? Why or why not?





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There are 2 main reasons for monitoring the entire frequency bands for spectrum management, instead of just narrowly focusing on the channels that are being used by the enterprise.

1) Paint the complete picture of the entire frequency band to empower IT staff members to make sound channel planning decisions. In a situation where the AP needs to switch its operation from one channel to another for any reason, maintaining that detailed information on the “cleanliness” of the new channel is very critical to be able to continue to operate the network at its top performance.

2) Many of the non Wi-Fi interference sources operate on more than a single channel or hop around different frequencies in the entire spectrum. Even though the center frequency of the interference source may not be the same as that of the corporate or authorized channels, the RF energy from these sources may bleed over or be a part of the hopping sequence, leading to disturbances on the corporate channels. By monitoring the entire frequency range, the enterprise IT staff can expand their “point of vision” for the frequency band and determine the root cause for lowered performance in their WLAN, which maybe due to interference sources that may not be operating on the corporate channels.

Dilip - can you think of any pros/cons to getting this 24x7 monitoring capability from a third party such as AirMagnet/Fluke rather directly from the WLAN systems vendor - either from a functional standpoint, cost perspective, or something else?


1. Independent 3rd party monitoring: With overlay spectrum monitoring or management solutions like the ones offered by AirMagnet/Fluke, users get an independent and unbiased view of the RF environment as seen by the AirMagnet sensors. The important thing to remember is that it can also be used as an overlay solution for any WLAN Infrastructure vendor, so this is a solution for every WLAN deployment even if the deployed AP vendor does not offer any spectrum monitoring capability.

2. Zero-downtime for upgrades: When spectrum capability is built inside the AP that is providing client data access, any new spectrum features, including any new signatures needed to classify new RF interference sources will call for upgrading the AP’s firmware/software or, in certain cases, even the hardware. This system-wide upgrade of an active WLAN deployment may not be feasible or acceptable for most users. On the other hand, AirMagnet is a dedicated monitoring overlay system, that delivers best-of-breed air monitoring, with easy updating to the latest features and signatures, and most importantly with zero downtime or upgrades to the existing active WLAN infrastructure.

3. Dedicated active spectrum development and innovation: This has been proven in the industry with the AirMagnet spectrum solution being the first and the only 24 X 7 dedicated spectrum monitoring solution in the last 4+ years. AirMagnet has continued innovating in the RF spectrum analysis field, with the best example being the introduction of classification for new interferers, Wi-Fi impact analysis (visualization of impact of RF interference on Wi-Fi performance) and many more. Also being an overlay solution does not limit any innovations or updates to the solution to solve new interference issues, as compared to infrastructure solutions that may have to time these updates with general AP updates (which don’t happen very frequently).

4. Complete troubleshooting solution combining spectrum and Wi-Fi intelligence: With dedicated and specialized spectrum monitoring solutions, like the those offered by AirMagnet, users can get the best-of-both worlds, layer 1 and layer 2 information and how they correlate to each other. Not only can users detect RF interference sources, but at the same time they can correlate that information with detailed Wi-Fi statistics for corporate devices. For example, users can see if an RF jammer wipes out signals for the corporate AP or leads to excessive errors or retries on the AP or its operating channel. This makes troubleshooting RF issues very quick and efficient, allowing users to prioritize their troubleshooting activities in the detection and location of interference sources.

5. Total cost: The total cost of a spectrum solution from a WLAN Infrastructure vendor may include the purchase of new AP hardware with spectrum capability and/or analysis engine servers, making the overall purchase very expensive. On the other hand, software based spectrum solutions may appear to be cheaper, but lack any true RF spectrum capabilities to solve any real RF problems. With overlay spectrum solutions, users are powered not only with a feature and benefit-rich application that can actually troubleshoot RF performance and security problems in their network, but also a very affordably priced product.

6. Companion troubleshooting tools: With the AirMagnet solution, users can also get the companion field spectrum analyzer tool, AirMagnet Spectrum XT, that can travel to the location of the problem for on-site location specific troubleshooting. The mobile tool, as mentioned in the previous topic also works great for pin-pointing the location of interference sources.


The only con, in my mind, would be the need for a specialized sensor, as it is an overlay solution. But with the AirMagnet solution, the sensors serve a dual-purpose, and not only monitor the RF environment for interference issues, but also perform layer 2 security and performance analysis.

This bullet-proof security, performance, compliance and RF monitoring solution is definitely worth the investment for an overlay solution.

OK, looks like you're agreed that all frequencies should be scanned. Neil, is the 24x7 scanning capability available from Cisco and, if so, do customers have to pay extra for it?

No. From a security standpoint, it is important to scan all frequencies for rogue RF devices. For example, a proprietary bridge operating on an off frequency may represent a security breach to your network.

It is also beneficial to scan additional channels beyond where your wireless network is operating so that you can monitor performance and prepare for future interference. For example, some devices such as cordless phones and wireless video cameras have the intelligence to try and operate on channels that will not interfere with your Wi-Fi network; however if someone later manually changes the operating channel of the device, it may suddenly start causing interference. Since new devices may enter your Wi-Fi network at any time, and current devices can randomly switch channels and interfere with your network, scanning outside your Wi-Fi networks channels plays an important role in proactively detecting and avoiding interference.

OK, looks like you're agreed that all frequencies should be scanned. Neil, is the 24x7 scanning capability available from Cisco and, if so, do customers have to pay extra for it?

Yes, Cisco CleanAir is based on a 24x7 scanning capability. If the AP is operating in Local Mode (serving traffic), then the AP performs 24x7 scanning on the active channel. If the AP is operating in Monitor Mode, then the AP performs 24x7 scanning on all channels. In either case, this 24x7 spectrum information is reported up to the controller, and then to the WCS management server where it is stored in a database for reporting.

There is no charge for the CleanAir capability, other than the requirement that you use the 3500 series AP.

In the event that you are operating in "Local Mode," is there an option for changing the active channel in the event that severe interference is detected?

If so, is this advisable? I can see some advantages to the automation, but I'm also sufficiently "old-school" to be a little bit nervous about too much automation.

Yes, the Cisco system has the capability to change the active channel in the event that severe interference is detected. We refer to this feature as "Event Driven RRM".

We feel very confident in this feature, because it only reacts when the interference: 1) has been classified, and 2) is seen to be causing a major impact.

In the past, systems without spectrum analysis capability were reluctant to implement a feature like this for fear that they would react accidentally to heavy WiFi traffic on a nearby AP, and thereby cause the system to be unstable.

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