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Hands on with the Cisco 3500 AP and CleanAir

The Cisco 3500 AP is an 802.11a/b/g/n dual band, 2 spatial stream, 2x3 access point with a high resolution spectrum analyzer built-in.

Cisco's release of the 3500 series AP has raised the bar for the competition when it comes to spectrum analysis, by integrating their stand-alone Spectrum Expert card abilities in the access point directly.

Cisco 3500i and 3500e

Cisco used a form factor for the 3500e (external antenna, support for high temperature install locations) and 3500i (internal antenna) that is similar to the 1142. Their new 1260 alsoThe 3500 is similar to the 1142 and 1252 in terms of servicing clients,  with 802.11n support on 2.4ghz and 5ghz support,  in 2x3:2 configuration (2 transmit antennas, 3 receive antennas, and 2 spatial streams), and supports a short guard interval of 400ns on both 40mhz and 20mhz channels. Why didn't Cisco go with 3x3:2? I'd have to guess because there isn't any real performance increase having a 3rd transmit antenna, when you can only handle 2 spatial streams.When Cisco announced the acquisition of Cognio, Inc. in September of 2007, I think most people could have guessed that Cisco's long term goal was not simply to sell PCMCIA cards at a list price of over $4,000 (Spectrum Expert). It could have been to simply lock up the 12 patents that Cognio had been granted, which reading the titles alone would make you wonder how anyone else on the planet is able to do spectrum analysis without licensing it from Cisco.  However it seemed to make logical sense to build the technology into the access points.I don't think anyone will argue that there is a portable Wi-Fi specific spectrum analyzer out there as good as Cisco's Spectrum Expert, which has dual band support and a frequency resolution of 10kHz. Most people would argue that you can get MetaGeek's Wi-Spy card for a tiny fraction of Cisco's price, or even AirMagnet's new USB analyzer, both of which are cheaper. Neither of those cards have the resolution of the Spectrum Expert card, but are often a perfectly fine choice depending on what level of digging you need to do. Wi-Spy is documented to have a frequency resolution of 328kHz, and Airmagnet's Spectrum XT is 156.3kHz, not to be confused with Airmagnet's "Spectrum Analyzer", which is the Cognio card re-branded.  Some may also argue that software wise, Wi-Spy and AirMagnet have nicer looking software. If you want to compare against a bench analyzer for Wi-Fi, take a look at the Tektronix RSA3408A (List $40,000), which is a monster of an analyzer and on my wish list. Frequency resolution is an important bench mark for spectrum analysis. If you're looking at 80mhz of spectrum, and can only see it in chunks of 20mhz..  That's 4 chunks.  When you consider broadband devices like Bluetooth that hop all over that 80mhz, or tiny devices like the non-Bluetooth 2.4ghz Logitech mice that use only 2-3mhz of bandwidth,  how can you uniquely identify these devices viewing in such large chunks? The smaller the chunks you can look at (resolution), the more precise you can be in identifying what's utilizing the spectrum, and have more specific signatures for devices seen. Cisco has integrated their high resolution Spectrum Expert chipset into the 3500 access point directly. Cisco's white paper goes into great detail here and I do recommend reading it. They've seemingly built a true spectrum analyzer into the access point. They have a current database that can identify by name 20 different non-wifi interference sources that they have built signatures for (I've seen it detect about 9 of those personally, but I haven't tried to test them all). Cisco's competition seems to be coming from Wi-Fi chipset vendors that are including some spectrum functionality in their wireless chipsets, such as what Atheros announced at CES in January of 2009. At least one chip specifically, their AR9002 chip with "iQUE", which is a marketing term for their new QoS techniques and Spectral analysis abilities. AP vendors are just now announcing support for this, and I personally have not yet seen it in action. I don't currently know of any vendor that is implementing a separate spectrum analyzer on their main board that isn't part of the offering from the 802.11 chipset vendor they utilize. There are certainly some questions I'd ask any vendor currently offering spectrum analysis, even if you're getting it for free as a software upgrade and no hardware required, be sure you're aware of the possibilities on the market. A few things I'd ask of any vendor, including Cisco:

uses this same form factor, leaving behind the removable radio, heavy, brick that was the 1252. Cisco has also left something else behind with the 1252, the need for 20 watts of power. The 3500s and 1260 all run off of normal 802.3af power.

  • What is the frequency resolution?
  • Can you identify the difference between 3 separate interference sources that are 1mhz apart, each 100khz wide,   2 with a pulse rate of 20pps, and 1 with a pulse rate of 80pps ?
  • Do you support 2.4/5ghz both?
  • Is there any performance hit to clients at all while all spectrum services are enabled?
  • Can you view a real time graph of FFT, Duty Cycle, and swept spectrogram?
  • Can you identify interference sources definitively and call out exactly what they are? (Xbox controller, Bluetooth, cordless phone, other Wi-Fi, microwave oven, etc)
  • Can you record the spectrum in detail when there is an issue?
  • Is the spectrum analysis being considered in your intelligent radio handling? (Automatic Channel selection, etc)
  • Does spectrum analysis run 24x7 on every AP while serving clients without human interaction?
  • Can it detect non-802.11 sources?
  • Can you locate the interference source utilizing the network of APs (while not impacting clients in any capacity) and show it on a map?
  • What tools exist to view the spectrum data offline and play it back?
  • Can you view the live FFT graphs and swept spectrogram on both bands at the same time with the provided software?
  • Can you view the live FFT graphs and other data while the AP is serving clients?

With Cisco's 3500 comes event driven radio resource management (EDRRM) and persistent device avoidance. Event driven RRM is exactly what it sounds like. A spectrum enabled AP will know more about its individual cell than a regular AP. It will know the type of interference, the true damage it's doing to 802.11, and be able to take faster action than a standard AP would.Persistent device avoidance is one thing that makes being able to specifically identify the type of interference important.  If a cordless phone is in use in your facility and the person walks hundreds of feet with it, do you want every single AP to change channel as it walks around? Probably not, and the AP knows that, because it knows the nature of cordless phones. However, consider a microwave oven.  Except for probably rare cases, most people's break rooms and microwave ovens stay stationary.  If your Microwave clobbers channel 11 every single day at noon for an hour,  which do you think is better for your environment: An AP that detects the interference, changes to channel 1 to avoid it.. and then an hour later, changes back to 11 when it clears up, to optimize the overall channel plan,  or an Ap that detects its a microwave oven, changes to channel 1, and gives some kind of weighting to channel 11 to avoid it it if possible, knowing this behavior will just keep happening every day?  Personally I'd rather my channels stay as static as possible than having it flop every day. At least if it changes to 1 permanently, the RRM algorithm can factor that into the nearby APs and smooth out the channel plan over time. If you aren't interested in EDRRM, or doesn't even use RRM today,  you can disable it, and still get the benefits of trapping on interference issue and all or some of the different classifiers. There still remains the question that must always be asked. What do you really need, and is it worth it? There are too many different situations to really say whether you need something like this or not. These APs are priced at $1,095 (internal antenna model) and $1,495 (external antenna model). You have to factor in antenna cost for the external model too, which could bump that AP upward of almost $2000 list, each. (Which oddly enough, is still HALF the price of the PCMCIA based Spectrum Expert card..  Lets hope Cisco goes ahead and lowers that stand-alone Spectrum Expert card to say.. $300 list?).   

You also have to consider if you'd deploy 100% of your facility with these APs, or only a percentage, and then consider the implications that some APs can detect things instantly and others can't.  One way to justify the 3500 was that there is a rugged version for manufacturing. warehousing, and other areas that are harsh and/or high tempterature but didn't require 20 watts like the 1252 does, so you could avoid buying EPOE switch if you didn't need them.  However Cisco released the 1260 as well, which lists at $995 without antennas, runs on 802.3af, and put the writing on the wall for the 1252, but also killing part of the cost benefit of the 3500 running on 802.3af and providing the benefits of 11n/MRC/legacy TxBF/MIMO.

3500 AP Pros:

  • Very high resolution spectrum analysis
  • 802.3af powered
  • Event Driven RRM on spectrum enabled APs will allow more informed decisions on RF handling over normal RRM
  • Persistent device avoidance, APs can avoid devices it knows don't move often (microwave ovens)
  • Can identify several different interference sources at a time across both 2.4ghz and 5ghz
  • Lighter than the 1252, easier to mount
  • Comes in 10 packs. This may seem silly, but time is money, and unboxing individual APs takes a considerably longer time than the 10 packs, especially when dealing with thousands of them. (Oh, there's the green aspect too.)
  • Won't really need that $4,000 Spectrum Expert card at sites that have 3500s installed
  • Integrates into WCS for locating and visualing interference sources and impact
  • Provides another metric (Air Quality Index) you can track long term to get a feel for your RF stability/quality
  • If fully deployed, you can pull up detailed spectrum view anywhere in your facility

3500 AP Cons:

  • 3500 series is first Cisco AP to drop support for an autonomous code load, meaning Unified installs only
  • Requires all new APs
  • 3500 is the most costly of Cisco's AP lineup, may be difficult to justify benefit vs cost depending on your business needs
  • Doesn't appear that older APs will have support for the Atheros style on-chip spectrum analysis abilities, even if limited compared to the 3500 features
  • If you want to connect into the access point with your fat client (Spectrum Expert) to see the nitty gritty detail beyond all of the integrated features discussed above, you have to switch the AP mode into Spectrum Analyzer, and it will stop serving clients. While this likely won't have an impact on clients in a normal layout that can sustain a perceived AP failure, it's annoying to have to wait the couple of minutes for it to reset.
  • When using the Spectrum Expert fat client, You can't view 2.4ghz and 5ghz at the same time. If you want to switch from one range to the other, you have to click on a different file in WCS to "Attach" to the other radio.

Regardless of whether this AP is right for your business, it's great to see that wireless vendors are making it easier to understand your RF environment and allowing you to more easily quantify issues that you may have, hopefully resulting in a more stable environment. Hopefully down the road when 3 and 4 spatial stream APs start coming out, we see things like in depth spectral analysis as a standard offering on all APs...  one can hope I guess.

(Note: Some of the items listed could change when the software is pushed to production)

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