Re: Some Cleanair Questions.

dhurshowy · ‎07-14-2011

I've read a lot and watched the recent webinar, but I still have a few questions:

1) During and Event Driven RRM "The channel change is not random. It is picked based on device contention, thus it is an intelligent alternate choice.". Could you please explain how it is based on device contention? Does the AP listen before it retunes (CleanAir can only listen to the channel of operation normally so it has no idea about the status of other channels)?

1b) If it just listens to WiFi channels before retuning, could a Non-Clean air AP not make the same "intelligent alternate chose" in the case of a necessary retune?

2) The CleanAir network design guide uses a WiFi Jammer example to explain ED-RRM. How would a Non-CleanAir AP (such as the 1260) react to the same jammer? Would it not change channels as well?

http://www.cisco.com/en/US/products/ps10315/products_tech_note09186a0080b4bdc1.shtml

3) If you have a large deployment in a shopping mall with CleanAir on all APs, how does the system mitigate a channel change on one AP which would then essentially effect every other AP in the whole mall in a chain reaction?

3b) How would things be different if all APs were Non-CleanAir APs?

3c) How would things be different if there were a mix between CA and Non-CA APs?

Thanks again.

Leo Laohoo · ‎07-14-2011

1b) If it just listens to WiFi channels before retuning, could a Non-Clean air AP not make the same "intelligent alternate chose" in the case of a necessary retune?

No it won't. CleanAir AP will only talk to CleanAir AP because some APs, like the 1140, 1250, 1260 for example, don't have the Spectrum Expert chip built-in.

2) The CleanAir network design guide uses a WiFi Jammer example to explain ED-RRM. How would a Non-CleanAir AP (such as the 1260) react to the same jammer? Would it not change channels as well?

Non-CleanAir will not change channels. It "may" be able to report a jammer based on wIPS but otherwise it'll just take it as it goes.

3) If you have a large deployment in a shopping mall with CleanAir on all APs, how does the system mitigate a channel change on one AP which would then essentially effect every other AP in the whole mall in a chain reaction?

3b) How would things be different if all APs were Non-CleanAir APs?

3c) How would things be different if there were a mix between CA and Non-CA APs?

CleanAir is good if all of your APs support CleanAir. This is because the APs will "talk" or coordinate with each other and change channels necessarily. For example, if an AP detects a shopper with a bluetooth hands-free walking at one end of the mall. This AP will then tell talk to the rest and say, "Who sees this clown with a bluetooth? Y'all hommies who see this move to different channels." So if I was another CleanAIr AP but I don't see this bluetooth interferrer, (like I'm located at the opposite end of this grand mall), there's nothing for me to do but sniff the air.

Nicolas Darchis · ‎07-14-2011

I'll add some precisions.

2)

Non cleanair APs can also change channels, this is often a confusion from reading all companies marketing blablah.

The difference is that non-cleanair APs don't have a clear picture, they can only measure the amount of packet drops and the amount of non-802.11 noise.

So if your environment is total crap, both cleanair and non cleanair APs will change channel. The subtle difference in the example is that the wifi jammer is said to have "very low duty cycle". I.e. it transmits very few signal (it's not "always on") but it's enough to kill all wifi packets anyway. CleanAir APs will notice it better than non-cleanair APs which will maybe report a interference level of 5% (while those 5% are enough to kill the channels but in some other cases a 5% would be acceptable).

3)

The channel change problem happens the same with or without cleanair. Since 6.0 especially, the algorithm evaluates the impact of a channel change BEFORE actually doing it. Meaning that for every AP that would change channel, the WLC evaluates the impact to surrounding APs. If a neighbor AP will sit on the same channel it can be ok if that neighbor is not very used but would not be ok if that neighbor is in a meeting room full of people. In this last case, the WLC would have to change this neighbor AP channel too.

A cascade calculation is then made to evaluate if changing channel is a good idea and what will be the new overall channel plan.

This calculation avoids the very old mechanism where one AP would change channel. Then it would impact a neighbor AP. some minutes later, that neighbor AP changes channel too and then it causes another problem ...

thinking before is better :-)

dhurshowy · ‎07-15-2011

Thanks to both for your replies. The more you learn, the more questions you end up having so I may post some more next week.