Re: WiFi roaming & Modem handover

[Dan Williams <dcbw redhat com>]

On Mon, 2015-06-29 at 16:59 +0200, Pieter Cardoen wrote:
> > Subject: Re: WiFi roaming & Modem handover
> > From: thaller redhat com
> > To: pieter cardoen hotmail com; networkmanager-list gnome org
> > Date: Mon, 29 Jun 2015 15:42:55 +0200
> >
> > On Mon, 2015-06-29 at 12:56 +0200, Pieter Cardoen wrote:
> > > I would like to have some information on how NetworkManager takes 
> > > care of handover between Access Points and between Networks:
> > > How does NetworkManager handle WiFi handover between different APs of 
> > > one network?
> >
> > Do you mean roaming? That is entirely done by wpa_supplicant.Yes I do but I also want to know how the 
> > roaming is managed by the wpa_supplicant. What metric do they use to decide to change to another Access 
> > Point?> 
> > > How does NetworkManager handle handover between different networks: 
> > > i.e. a mobile network and a WiFi network.
> >
> > What do you mean with "handover" here? You can have different networks
> > ("connections" in NetworkManager speak) active at the same time.
> > In that case, priorities are determined based on routing and route
> > metrics. The same is true for the default-route.This is indeed what I want to know but I wonder when it 
> > is decided that a connection is down? It could be that the wifi can just be detected but almost no data 
> > goes over the network while a good 3G connection is available. Which metric is used to detrmine that the 
> > connection is down?
> >
> > Not sure if that answers your questions :)
> >
> > Thomas

Attempting to reply to Pieter's HTML-only message...

> Yes I do but I also want to know how the roaming is managed by the
wpa_supplicant. What metric do they use to decide to change to another
Access Point?

wpa_supplicant roams to another AP of the same SSID+security when the
current AP's signal strength drops below a threshold, and the other AP's
signal strength is better.  The supplicant attempts to balance stability
with throughput, to ensure that it doesn't jump between APs too often
(since doing so causes latency and increases the chance for
disconnections) but still uses the best possible AP for max throughput.

> This is indeed what I want to know but I wonder when it is decided
that a connection is down? It could be that the wifi can just be
detected but almost no data goes over the network while a good 3G
connection is available. Which metric is used to detrmine that the
connection is down?

NetworkManager decides that the WiFi connection is down when either the
driver or the AP disconnect the connection.  Throughput or bitrate is
not currently taken into accound.

Dan