Wireless Access

We focus on a school building scenario in which multiple AP's were deployed at the same building level. We were in the building and put two Samsung smartphones collocated to each other (both devices are associated with APs --- may or may not be the same AP). Then we checked their snr values that were pulled out of the controller.

 

Considering both devices were collocated, I am assuming that almost the same set of AP's can see both simultaneously and then report their snr values. However, we found that, for one device within a focused time window, only one AP was reporting its snr values to the controller, while for the other device at the same time window there were quite a few AP's that were reporting its snr values.  

 

Can anybody give me some hints why this could occur? Thank you.

Hi ,

 

Can you clarify couple of things here,

1. What is the model of those APs

2. What is the distance between those APs.

 

Please feel free for any further help on this.

Thank you. 

 

For the first, I have no idea. I am a student researcher and I am blind to any controller/ap model, policy and performance. But I guess my school should be able to have up-to-date HW/SW, albeit probably not purchasing full-fledged features. 

 

For the second, I believe there are more 5 AP's around two examined devices. The nearest one(s) is located in the same room as devices and is only 3 meters (line-of-sight distance) away from both. There are a few that are located in neighbor rooms - the distances (line-of-sight) are around 8-15 meters.  

 

Two devices are Galaxy S5 whose snr values were greatly recorded, and Note 3 whose snr values were recorded by only one AP within the discussed time window. Before the window, however, there had been quite a few AP's that were reporting N3's snr values to the controller; and after the window, for some reasons AP's started to report N3's snr again. It seems to me that the transition from reporting to not reporting was when N3 got stationary and was stably connected, while the transition from not reporting to reporting was unclear to me. 

 

I did one more check - N3 scans AP's at every 4~6 seconds. The above can be reproduced. Interestingly enough, the "disappear" AP's albeit not reporting N3's snr replied to scans.

 

Is there any doc or whitepaper that may explain all such and relevant policies? Thank you. 

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@nzhang27 wrote:

We focus on a school building scenario in which multiple AP's were deployed at the same building level. We were in the building and put two Samsung smartphones collocated to each other (both devices are associated with APs --- may or may not be the same AP). Then we checked their snr values that were pulled out of the controller.

 

Considering both devices were collocated, I am assuming that almost the same set of AP's can see both simultaneously and then report their snr values. However, we found that, for one device within a focused time window, only one AP was reporting its snr values to the controller, while for the other device at the same time window there were quite a few AP's that were reporting its snr values.  

 

Can anybody give me some hints why this could occur? Thank you.

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There are a number of reasons why this would occur.  If you do not have access to the controller, these reasons cannot be explained definitively.

Thank you.

Could you please give me several possible causes so that I can do more tests to guess, or any doc to explain such policies? Thank you.

Start with this:  https://en.wikipedia.org/wiki/Signal-to-noise_ratio

 

Should I? Thank you, but by starting from here how long do you think I need to take to know what I really need to know? You do not have to reply if you do not know.

I do not have enough information to answer your question.  I need more information to narrow down the possible answers.

 

What command are you using to report the client SNR values?

 

Hi, You mentioned there are some possible reasons. Could you name a few? I am a networking researcher - I am 200% fine with that AP's/controller is black box to me. Actually I am happy with this. If you can give me a few possible reasons/expert-level insights I can design experiments and make a guess what the root cause is.

My situation was stated in earlier posts. Let me know if it is still unclear.

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@nzhang27 wrote:
Hi, You mentioned there are some possible reasons. Could you name a few? I am a networking researcher - I am 200% fine with that AP's/controller is black box to me. Actually I am happy with this. If you can give me a few possible reasons/expert-level insights I can design experiments and make a guess what the root cause is.

My situation was stated in earlier posts. Let me know if it is still unclear.

However, we found that, for one device within a focused time window, only one AP was reporting its snr values to the controller, while for the other device at the same time window there were quite a few AP's that were reporting its snr values.

Access points scan other channels periodically (every 10 seconds) to see other clients.  If an access point has not seen a client, that could mean that it has not yet scanned its channel.  That is probably the reason.

 

My time window was one hour. All AP's were not able to see N3 for one hour? This answer would not be correct.

There are reasons why access points pause off-channel scanning like High channel utilization, or clients on a voice or video call. The client maybe was not transmitting or off channel when an access point scanned it's channel. Interference could have also prevented the access point from seeing any frames of the client.

Great. Those are what I should care about. Thank you for your help.

"The client maybe was not transmitting or off channel when an access point scanned it's channel."

 

Are you assuming the client is connected? Thank you

The assumption is that the client is connected to something at the time.

I know there are some reasons that makes an ap increase scan interval. Among them are load amount and voip, just like you mentioned. However, is it possible that an ap goes sleep and increases scan interval *for saving itself energy such as over night hours (so guessing none would come and connect)*?

- The off-channel scanning that is described above is Aruba platform specific.  All manufacturers do some sort of off-channel scanning, but they all do it differently.

- Off-channel Scanning is for IDS/IPS purposes and is completely optional.  It is not necessary for the proper functioning of the network.

- The clients typically do not know the difference, because clients also go off-channel and scan at the same time while they are sending and receiving data;  that is how they can populate what wireless networks are available in their supplicant.

- The access point never goes to sleep to save energy or overnight.  Only clients do that.

Keep using the setting example: ap1(on channel 1) and ap2(on channel 2), a client is connected to ap1. So, there are two situations in which ap2 can *meet* the client:

 

1: ap2 scans channel 1, and the client is transmitting at the time. If the client is quiet/recieving or off channel, then they canNot meet each other.

 

2: ap2 (passive) scans another channel 3, and the client (active) scans channel 3 at the time. 

 

Am I correct? Is there any other situation in which ap2 can meet the client? Thanks for your patience. 

Let's understand the goal here:

 

If the goal is to report a client's SNR, AP1 will always report the client's SNR because the client is connected to that AP.  What is the role of AP2 in this scenario?

 

Sorry, should be clear in the first place ---

 

I am USING *all APs* (including the connected one and a set of off-channel ones) snr reportings to localize clients. So off-channel snr reportings may not be needed for common tasks such as association and roaming, but do be critical in my system.

 

Hope now everything gets clear.

nzhang27,

 

If an access point is configured as an "Air Monitor" it will not serve any clients, but it will scan  channels every 500 miliseconds, and give you better location quality.

Yes, it will. But I am trying to understand when an off-channel ap can *meet* (i.e. report snr) the client. Could you please help me make sure the following two situations as well as bringing other situations I may not know? Thank you.

 

Situation 1 for meeting: ap2 scans channel 1, and the client is normal transmitting at the time. If the client is quiet/recieving or off channel, then they canNot meet each other.

 

Situation 2 for meeting: ap2 (passive) scans another channel 3, and the client (active) scans channel 3 at the time. 

#1 and #2 are the two situations when this can happen.