Aruba’s Adaptive Radio Management, ARM is responsible of transmit power management and planning of radio channels to access points.
It maintains two measures for every channel : a coverage index and an interference index  .
Transmit power control over a given channel is based mainly on the coverage index.
Both indexes, coverage and interferance, are processed by the controller, in a controller-based architecture, and matched to access points per channel.
Where to get coverage information
This processing is based on the information reported by access points , every 10 seconds and for 110 millisecond scan time duration by default .
In addition to this channel scanning, access points exchange proprietary messages, ARM Over The Air Updates  , on every supported channel.
These messages when received by other access points are relayed back to the controller for further processing. These messages are IEEE 802.11 management messages and carry this information:
- Channel currently used by reporting access point for wifi clients
- Current transmit power EIRP over this channel
- A list of detected neighboring access points over this channel
Access points report also this information   that evaluate the quality of transmission in both control and data plane :
- % of retransmissions,
- % of low rate frames,
- % of non-unicast frames,
- % of fragmented frames,
- Channel rate in kbps,
- % of PHY errors,
- % of MAC errors,
- Level of floor noise.
Processing of coverage information
All this information is consolidated at controller level in a table  that matches for every access point and per channel the percentage of retransmissions, physical errors, mac errors, noise level and processes an evaluation of these indexes:
- util (Qual): that is an indication of the channel quality based on effective usage of it.
- cov-idx : coverage index.
- intf_idx : interference index.
The decision of increasing or decreasing the transmit power level on a given channel is based on the processed coverage index as compared to the “ideal” coverage index, noted cov-idxideal, and “acceptable” overage index, cov-idxacceptable, both configured at controller level .
As a general rule the current coverage index should be greater than cov-idxacceptable and equivalent to cov-idxideal, .
Coverage index, cov-idx, that acts directly on transmission power level on a given channel is a function of detected access points SNR on the same channel as it is measured relatively by each other.
It corresponds to the sum of two variables « x » and « y ». « x » being the weighted average of all other access points SNR as being measured by the current access points. « y » is the weighted average of processed « x » variables by other access points from the same vendor and on the same channel .
Example of power processing
As an example let’s consider a set of 3 access points : AP1, AP2, and AP3, that reports respectively these SNR measures : (SNR12, SNR13), (SNR21, SNR23), and (SNR31, SNR32). Then the coverage indexes are written as follows for every access point:
- cov-idx(AP1) = x1/y1
- cov-idx(AP2) = x2/y2
- cov-idx(AP3) = x3/y3
- x1 is a function of SNR21, SNR23, SNR31, SNR32
- x2 is a function of SNR12, SNR13, SNR31, SNR32
- x3 is a function of SNR12, SNR13, SNR21, SNR23
- y1 a function of x2 and x3
- y2 a function of x1 and x3
- and y3 a function of x1 and x2
SNRs and EIRPs
SNRs when they are processed at the same time as EIRPs (sent previousely in ARM Over the Air Updates) can help accurate processing of xn and cov-idx(APn). It is to notice that SNR measures are based on received signal and are processed locally whereas EIRP are reported measures at transmission that are sent over ARM management messages to neighboring access points and back to controller for consolidation.
 http://fr.slideshare.net/Venudhanraj/3-aruba-arm-and-cm, 2014.
 Aruba Network, techdocs, ARM Coverage and Interference Metrics.
 Aruba Network, techdocs, ARM Yourself to Increase Enterprise WLAN Data Capacity.
 Aruba Network, techdocs,rf arm-profile.
 Aruba Network, techdocs,show ap arm rf-summary.
 Aruba Network, techdocs,Understanding ARM.
 Aruba Network, techdocs,Adaptative Radio Management (ARM)