Why do output power levels vary across SSIDs?

[robin.deas]

Can anyone shed some light on why power output levels vary across SSIDs, on the same AP and frequency range? I'm specifically looking at an 1852i AP connected to an 8540 controller, if that is relevant.

What I see is that some SSIDs are always consistently lower than others - not talking about slight up and down variations over time. If anyone understands why this is, can you tell me if this is configurable? I can only see options to control output levels per radio in Prime.

Grateful for any advice.

[marce1000]

 - How are you measuring this, or which monitor tool do you use ? Meaning are you sure the same SSID from the 'local AP' is always observed and you have not accidentally roamed ?

 M.

[robin.deas]

Using a Netscout Aircheck tester, which I think is pretty accurate. Since I suspected this was happening I am focussing on one AP, definitely not roaming. I can see the same effect from a simple WiFi checker on my phone, tending to corroborate what I'm seeing on the Aircheck.

[Dan Rowe]

To add to the previous question from Marcee:

-- Is this behavior experienced across all wireless devices or specific to a certain model of wireless clients?

-- Is the wireless client experiencing this behavior completely upgraded in terms of their wireless NIC drivers?

[robin.deas]

I'm investigating complaints about connectivity to a specific SSID - I'm checking the power levels using a Netscout Aircheck tool, not looking at specific client NICs. I'm seeing the same behaviour on my own phone though.

[Scott Fella]

TX Power is defined per radio (2.4ghz and 5ghz), not per SSID. More like. Per BSSID since you can be comparing a particular SSID on a different radio bands.

[robin.deas]

I can see that power is controlled per radio  - I already checked that in Prime and that's why I'm asking the question. I'm not seeing what you describe - what I'm seeing is what I described in my post.

[Emilestegeman]

These values you see are simply probe response frames from the radio BSSID (SSID on a specific radio) combined with the received amplitude value (from the receiver, in this case the netscout). Theses frames are not received simultaneous thus amplitude can vary. (due to multipath, the spectrum and other environmental influences) Exactly why the RSSI of the 1 SSID is constantly lower than the the other, i don't know. By my knowledge multiple management frames on a single radio are always being sent with the same power output.

Are you actually seing performance differences on the 2 SSID'?

[robin.deas]

I'm getting complaints about dropouts on one SSID which, when I monitor it, appears to have consistently lower power than some others in one area. The difference in apparent signal strength appears persistent. I think I need to do some more testing to try and understand exactly what is happening, like maybe turning off any other APs within range and focussing on the behaviour of a single AP in isolation. I understand that signal levels may vary automatically per radio  - what I'm not understanding is why, in a single frequency range, in a single location, different SSIDs have different levels. I can probably fix it by looking at the AP positions again, but I'd like to understand what is going on a bit better. Might the apparent signal level be influenced by interference patterns - not seeing major interference but I can see how that would be a factor.

[Scott Fella]

I run a few differnt Netscout (NetAlly) products. What you need to do is use the filter and isolate the bssid. The signal will change even when you look at one SSID, that is normal, but should not fluctuate more than -5dbi.

[patoberli]

Typically the SSID your are attached to, can have a higher power (depending on the AP). Cisco calls that feature ClientLink. This feature should not influence the Beacon Frames if I remember correctly.
So if you measure the power, make sure the client is not attached to the SSID. I'm not sure how the Netscout Aircheck behaves when measuring.