Wireless Access

I have some questions on the Coverage Index whose definition is copied here for reference.

"

Coverage Index: The AP uses this metric to measure RF coverage. The coverage index is calculated as

x/y, where "x" is the AP’s weighted calculation of the Signal-to-Noise Ratio (SNR) on all valid APs on a

specified 802.11 channel, and "y" is the weighted calculation of the Aruba APs SNR the neighboring APs

see on that channel."

1. Is x calculated as follows? An AP1 listens for beacons (or other traffic if there is any) from all other APs it can hear on the same channel, calculates the SNR (in dB) for each, and sums them. The final sum, x, includes AP1s own SNR (based on its current Tx power). This implies a channel has already been selected.

2. Is y calculated as follows? It is the sum of the SNR for all the other APs excluding itself (AP1).

3. Is this information then collected by the controller (from all APs) which then applies an algorithm to determine the new Tx power level for each AP? Is there any publicly available documentation on this algorithm?

4. The default coverage index is 10. If we are dealing in dB, does this mean that x >= y + 10?  Was this value chosen to minimize CCI?

Thanks.

---

@MortKaye wrote:

I have some questions on the Coverage Index whose definition is copied here for reference.

"

Coverage Index: The AP uses this metric to measure RF coverage. The coverage index is calculated as

x/y, where "x" is the AP’s weighted calculation of the Signal-to-Noise Ratio (SNR) on all valid APs on a

specified 802.11 channel, and "y" is the weighted calculation of the Aruba APs SNR the neighboring APs

see on that channel."

 

1. Is x calculated as follows? An AP1 listens for beacons (or other traffic if there is any) from all other APs it can hear on the same channel, calculates the SNR (in dB) for each, and sums them. The final sum, x, includes AP1s own SNR (based on its current Tx power). This implies a channel has already been selected.

 

2. Is y calculated as follows? It is the sum of the SNR for all the other APs excluding itself (AP1).

 

3. Is this information then collected by the controller (from all APs) which then applies an algorithm to determine the new Tx power level for each AP? Is there any publicly available documentation on this algorithm?

 

4. The default coverage index is 10. If we are dealing in dB, does this mean that x >= y + 10?  Was this value chosen to minimize CCI?

 

Thanks.

---

MortKaye,

The documentation gives a good general idea about how it works.  In truth, quite a few changes are made to account for the changing world that we live in.  We cannot explain further than the documentation.

Mort,

I second Colin that the documentation is a great place to start. All of Aruba's default RF configurations are ideal for their typical client base. So, you need to ask yourself if your network exemplifies their norm. The ideal coverage index value is somewhat of an ambiguous unit (not dB). The APs will modify their power levels so that their computed coverage index is as close to, if not matching, the ideal coverage index value.

Soooo, if you have a very dense deployment, the APs will compute a high coverage index that will likely exceed the ideal at first. This will cause the APs to lower their power until it gets near the ideal coverage index (or until it hits the minimum transmit power). The opposite behavior goes for sparse deployments.

Knowing this behavior allows for you to tweak your configuration to suit your needs. As an example, we have some buildings where APs had to be deployed in the hallways, not the rooms. The APs have line of sight to one another and compute a high coverage index. In order to prevent the APs from tuning power too low (and subsequently creating limited coverage in adjacent rooms), you need to either raise the minimum transmit power, increase the ideal coverage index (telling the APs to "look" or "expect" more neighboring APs), or both.

I hope this helps.

"Coverage Index: The AP uses this metric to measure RF coverage. The coverage index is 

calculated as x/y, where “x” is the AP’s weighted calculation of the Signal-to-Noise Ratio (SNR) on
all valid APs on a specified 802.11 channel, and “y” is the weighted calculation of the ArubaAPs SNR
the neighboring APs see on that channel."

Can someone explain, how x and y is calculated? 

---

@yogenpartha wrote:

"Coverage Index: The AP uses this metric to measure RF coverage. The coverage index is 

calculated as x/y, where “x” is the AP’s weighted calculation of the Signal-to-Noise Ratio (SNR) on
all valid APs on a specified 802.11 channel, and “y” is the weighted calculation of the ArubaAPs SNR
the neighboring APs see on that channel."

 

Can someone explain, how x and y is calculated? 

---

yogenpartha,

In general, it is just a way to assign a value to how many access points and their signal strength are seen on a specific channel by an access point.  It is just used to compare how much coverage exists on a channel.  How it is calculated is not specifically important; the fact that channels with more access points with a higher SNR is less desirable than a channel with less access points is what is important here.

Hi

Thanks for this interesting subject

Does the controller take into consideration information retreived from otap messages when calculating the current coverage index?

Brgds

Mehdi

What is an otap message?

Hi Colin

I meant over the air updates :

http://www.arubanetworks.com/techdocs/ArubaOS_63_Web_Help/Content/ArubaFrameStyles/ARM/ARM_Overview.htm

Brgds