Behavior over preference of APs and bands?

[Dan Williams]

On Mon, 2014-06-02 at 20:46 -0700, Rubin Abdi wrote:
> My environment:
> Thinkpad X220 - Intel Centrino Advanced-N 6205
> Debian Sid amd64 KDE
> firmware-iwlwifi	0.41
> wpasupplicant		1.1-1
> network-manager		0.9.8.10-3
> plasma-desktop		4:4.11.8-1
> 
> There are two behaviors in the way my machine connects to wifi that I'm
> trying to wrap my brain around. The first I think has a fairly straight
> forward answer.
> 
> If I wake up my machine from sleep, and there are multiple SSIDs with
> range that I've already connected to and remember through Network
> Manager, which is connected to first? Network Manager doesn't seem to
> have any sort of network priority function currently (unlike OSX or
> Android 4.x). My guess is that Network Manager will check to see if any
> networks it remembers are within range, then connect to the one with the
> least amount of noise.

At the moment NM's priority is "last used, if seen in a scan."  We're
going to add priorities in the near future, but that won't have any
effect on your setup if you're using a single SSID.  Signal strength
also doesn't factor into the decision at this time.

NetworkManager doesn't normally do anything with 5GHz vs. 2.4GHz bands,
that's left up to the supplicant and driver to figure out.  NM 0.9.8.10
and later *do* have the capability to lock a network to a specific
frequency band though.

> The more brain itching question I have is how does it deal with this
> connection preference when there are multiple APs on different bands
> broadcasting under the same SSID? I've thus far testing out my
> environment against Ubiquiti Unifi Pros and last generation Airport
> Extremes, both of which have dual 2.4GHz and 5GHz radios, and typically
> default to a config of using the same SSID for both radios. Neither to
> my knowledge employ any sort of band steering (though it sounds like
> that's an option Ubiquiti wants to add soon).
> 
> With both setups my laptop by default connects to the 2.4GHz network the
> majority of the time, even if I'm sitting with line of sight from the
> back of my laptop to the AP in the same room. Generally signal strength
> for 2.4GHz is higher than 5GHz and I totally get that in most cases when
> you're not in a part of the world saturated with technologically advance
> humans, 2.4GHz is generally better for in home use. Occasionally it'll
> roam over to one of the 5GHz networks, and when I go check out signal
> strength 2.4GHz still has somewhere around 10% higher signal as reported
> through NM.
> 
> Is any of this behavior documented everywhere? Is there any way to tweak
> this? Ideally I would set something up like go with the 5GHz network
> first unless 2.4 has a 20% lead in strength.

wpa_supplicant is the originator of this behavior, possibly in collusion
with the drivers.  But normally, the supplicant decides which specific
AP to connect to from the list of matching APs based on SSID and
security settings.  Based on wpa_scan_result_compar(), what appears to
happen is that normally, the highest signal strength AP within the SSID
is chosen.  If the signals are close to each other, the AP with the
fastest rates is chosen.  If the AP rates are similar, then 5GHz APs are
preferred.

Dan