More often than not, it is when you have too much co-channel interference. This is mostly on the 2.4 Ghz radio and can be because of a number of factors like:
- density of APs
- interfering/rogue APs in the area
- non 802.11 interfering devices like cordless phones and microwaves
- vertical placement of the APs floor to floor
If you look at the APs out there now and a certain area (or all of them) are set to the min Tx value, that is telling you that there is enough reason to use the lowest possible power setting on that radio. This may or may not be 100% clean in your network. A good report/chart you can reference in Airwave is RF capacity and RF health report. This will tell you the channel utilization that is being seen. The controller will also show this in the dashboards but it is mostly "real-time" and doesn't keep a long history of this data. During normal client activity, it is acceptable to see spikes in channel utilization, however, if you are seeing a consistent high percentage even overnight or without users, than you proabably are suffering with co-channel interference.
By lowering the Tx value from 12 or 9 to 6 as an example, will tell the APs that IF you need to lower your power from the current value to a lower number, we will allow it to happen. Keep in mind that this setting is done PER AP GROUP. If you have a mixed environment and you are seeing high channel utilization in only a few environments, then my advise would be to either break off the APs into a new group or just modify the group seeing the issue with this new ARM profile.
Other than reducing the min Tx value, there are other settings that you can modify to alleviate this should this be the issue.
1. You can drop all broadcasts/multicasts in the air. This will reduce the amount of packets that many consider to be unnecessary in a shared wifi network
2. You can enable "deny broadcast probe". This prevents a client from sending a broadcast SSID discover packet on the network and EACH AP that can hear it, must respond.
3. You can enable band steering to reduce utliization on the 2.4 and move capable clients to the 5 radio
4. You can disable the low transmit and basic rates of the 2.4 bands.
5. You can set a local probel request threshold so that associating clients must have a minimum SNR value.
A discussion with your SE, partner, or TAC should help put you on the right path as to what to do with the RF. ARM is a great technology but it isn't a 100% set it and forget it. However, once those initial settings and tweaks are done, you should be in for optimized RF and happy users!