07-19-2013 05:09 AM
When I deploy wireless networks in high density enviromnents I allways use Mode Aware ARM on the 2,4ghz radio. I'm surprised that this isn't a default setting, what are your recommendations regarding Mode Aware and environments where AP density is high for example schools who use 1 AP per classroom?
And in Instant it seems like Mode Aware ARM is gone. How should I account for the density if deploying instant? Just put minimum power to 3dbm? I want to limit co-channel interferense as much as possible.
07-19-2013 05:40 AM
ARM Mode Aware is designed to deal with interference when you have too much coverage, not capacity. If you have too much 2.4 ghz coverage on a dual-radio AP, it will only turn the 2.4ghz side of the AP into an air monitor. The 5ghz side will continue to serve clients. To wit, if you are fully covered on the 2.4ghz spectrum, you would not add more capacity by adding more access points...just more interference. In the 5ghz spectrum it is much less likely to happen, so all your APs would normally remain on.
07-19-2013 05:48 AM
The ARM “Mode Aware” option is a useful feature for single radio, dual-band WLAN networks with high density AP deployments. If there is too much AP coverage, those APs can cause interference and negatively impact your network. Mode aware ARM can turn APs into Air Monitors if necessary, and then turn those Air Monitors back into APs when they detect gaps in coverage.
Mobile clients need to be switched from access point to access point in an efficient and expeditious manner when roaming. Aruba offers instantaneous hand-offs to allow undisturbed roaming without hand-off delays or dropouts, and automatic load balancing to ensure that roaming clients don't slow down access point performance.
If one AP in the above picture was to fail ARM would increase the power and adjust the channels of the neighboring AP’s until the area which was covered by the failed AP is covered by the neighboring AP’s.
07-19-2013 05:55 AM
Yes, I'm deploying AP-105s for medium to high density environments and where we put APs very close to serve alot of clients on the 5ghz band I want the 2,4 ghz band to go into AM if there's enough coverage on 2,4ghz just like you say.
My questions remain though:
Are you recommending Mode Aware arm on a typical AP-105 high density deployment for the 2,4ghz radio? Are there any drawbacks on enabling it?
What about instant? Why doesn't it have Mode Aware functionality and how do I compensate?
07-19-2013 06:16 AM
This is one of the most common myths about Wi-Fi that I've ran into so far and you are far from the first one to think that CCI is the most important thing to avoid.
So, there are two things that need to be taken into account when building a HD environment, channel utilization and actual interference. The thing with Wi-Fi based interference is that the standard is built to handle it and a client will not transmit if another device is currently using the air.
So with that in mind, what we actually need to do is save air time and not lower interference. This means getting the client off of the network and done transmitting as soon as possible.
Most client devices broadcast between 15 mw and 30 mw, which is around an EIRP of 12 or 15 dB for us (you can use TPC to limit that if the client suppots 802.11h, but thats another story). So we always try to have symmetry power-wise with the client devices and because of that, I try never to go lower than en EIRP of 9 dB.
So when I build HD networks, I will leave Mode-Aware ARM off, set min-tx to 9 and max-tx to 15 dB and instead cut data rates on all the SSIDs.
This will save air time and allow clients to get off of the network faster. This will also reduce the usable cell-size of a client and also increase the speed of the beacon packets (which Wi-Fi sends quite a bit of).
Finally, if you are in a very high density environment that you really feel needs Mode-Aware ARM, I would go ahead and set min-tx to 9 and max-tx to 12 in that ARM profile to help Mode-Aware ARM make a better decision.
ACMX, ACDX, CISSP, B. Eng.
07-19-2013 06:20 AM
At Texas A&M our approach was to leave mode aware off in HD areas
For a given area we have surveyed, we come up with a budget of how many users we want per radio to provide on overall acceptable user experience
So in a case like that where you have deployed radio coverage to support user density vs actual area/square footage, by leaving mode aware on you run the risk of loosing radio density that you have deployed for and in the end affect the performance/client experience when the area becomes saturated with use
mode aware turns ap's into air monitors and only changes them back to ap's if another ap goes down
this has been my experience anyway
Network Engineering|Texas A&M University
07-19-2013 06:24 AM
In 2.4 Ghz the chances of having Co-channel Interference (CCI) are very high and as we know it occurs when 2 or more access point BSSIDs in range of each other also transmit/receive on the same channel. So the first problem in mode aware ARM is to detect edge APs, i.e. APs that are in the edge of the building. This is important to ensure that edge APs are not turned off by ARM. This will automatically cause non-edge APs to be turned off if coverage is very high. The AP edge status is determined by the controller based on information received during ARM update messages from APs. This is done based on number of valid APs seen by each AP. The AP with the least number of valid APs seen is classified as an edge AP
Converting APs to APM
- If no active clients AND
- not an edge AP AND
- tx-power closer to arm-min than arm-max AND
- coverage is greater than acceptable on all channels on the band without itself
Transition to APM
- Converting APMs to APs
- If empty channel available (no interfering APs) OR
- zero coverage channel available
- tx-power closer to arm-min than arm-max AND
- Min covered channel has coverage less than acceptable OR
- Classified as Edge AP then transition to AP
About IAP: In IAP we don't have a concept of AM.
07-19-2013 06:56 AM
Thanx for all the feedback!
Regarding cutting data rates, in a typical school where we deploy 1 AP per classroom (very dense according to me). What rates would you put as basic and transmit on the 2,4ghz radio?
Also, if we in the above scenario put min-tx to 9 and max-tx to 15, won't there be situations where clients for example sit int he hallway connected to a classroom 2,4ghz radio and become a hidden node on that channel for other APs/clients?
Thank you for your very detailed response!
What about you, would you recommend enabling Mode Aware on 2,4ghz as standard in a HD installation?
07-19-2013 07:26 AM
In such a scenario, I would feel comfortable with turning off all data rates up to 12 mbps.. Meaning that I would set basic rates at 12 and 24 and all transmit rates but anything under 12 mbps. The ones that really need to be removed are 1-2mbps, after that the gains drop off quite quickly.
As for the hidden node scenario, I do not think that will become a huge issue, and when it does happen, it will be much less with the setup as it is right now with smaller cells. This is much more likely to happen with large cells such as you usually have with Mode-Aware ARM.
ACMX, ACDX, CISSP, B. Eng.