Re: Changing Auto-Connect feature

[Dan Williams <dcbw redhat com>]

On Tue, 2010-05-25 at 13:45 -0300, Franco Miceli wrote:
> Hi,
> 
> I'm part of a project in Uruguay (Plan Ceibal), that aims to give
> educational connectivity to primary school students
> 
> We've been working with the XO platform (Sugar, Fedora 9 -soon 11-),
> and have had the following issue regarding NM auto-connect feature.
> 
> We provide connectivity in the school yard via large outdoor wireless
> equipment and indoors with smaller AP. It has come to our attention
> that NM auto-connects to the last AP the XO connected to. Since the
> outdoor equipment is still visible indoors the XO connects to it even
> if it has less quality than the indoor equipment. This gives us
> problems in capacity planning and coverage.
> 
> We consider the auto-connect feature to be most useful at the school,
> but we want the choice of connection to be more than just "the last
> one it connected to". Therefore we want to to make some changes to the
> NM code in order to take into consideration other factors such as:
> SNR, loss %, etc.

This is tricky because actually not all kernel drivers really support
good reporting of SNR and loss %.  But in your case since you're
targeting specific hardware where you can assume that the driver reports
the correct values, you can locally patch NetworkManager in a way that
we cannot do upstream.

You want real_get_best_auto_connection() in nm-device-wifi.c.  You're
passed a list of connections that get sorted through.  The list is
already sorted by timestamp.  What you want to do is to run through the
*entire* list (and not stop at the first compatible AP that's found by
nm_ap_check_compatible() near the bottom) and instead keep a pointer to
the "best" connection as you define it.

This is a bit complicated because remember that connections are simply
packages of settings necessary to connect to a specific network, they do
not describe actual wifi APs.  So the basic procedure would be something
like:

NMConnection *best_yet = NULL;
guint32 best_ap_strength = 0;

for each connection in 'connections':
    <do normal device checking up to AP list iteration>

    for each ap in the device's AP list:
        if (nm_ap_check_compatible (ap, connection)) {
            if (nm_ap_get_strength (ap) > best_ap_strength) {
                best_yet = connection;
                best_ap_strength = nm_ap_get_strength (ap);
            }
        }

    return best_yet;

Dan

> We already have the code of the NM OLPC uses on the XO. What we wanted
> to know is where inside all the *.c files to look for this
> functionality and also if anyone has had any experience with this in
> order to give us some advice on how to approach this subject.
> 
> Thanks in advance,
> 
>  
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