Re: Which AP is chosen when several are found for the same SSID?

[Dan Williams <dcbw redhat com>]

On Tue, 2008-10-07 at 10:06 -0400, Jose Aliste wrote:
> Hi, 
> Finally I tried at office with nm0.7 packages from launchpad ppa and
> it works! Just added the desired AP address to the BSSID and nm
> connected without problems. Some last questions: 
> - Is wpa_supplicant used even if the wireless network is totally
> open? 

Yes.  It makes no sense to have two different codepaths for connecting
to APs, thus wpa_supplicant is always used.  Fewer codepaths == fewer
opportunities for bugs.

> - If nm0.7 can lock the AP why not trying to do something like chosing
> the AP that would be beter in terms of rate and then connecting. I
> understand the driver is free to roam to another AP but then one just
> trusts the driver because if not we would have some nasty bugs like in
> Mac (having the AP changed every two seconds!)

wpa_supplicant doesn't this, but the reason NM doesn't do this is
partially because drivers historically (and some still do) suck a lot
and didn't return good signal strength or rate information.  The
supplicant currently sorts APs based on strength so you'll get that at
least.

> just one last piece of info. In the iwlist scanning the fastest ap( in
> terms of signal quality / type of router) in my network appear the
> first. Nevertheless, nm tries to connect to the slow one(which is the
> best in terms of signal quality) if I don't lock the AP by hand in the
> connection editor.  

Technically it's wpa_supplicant that tries to connect to the "slower"
one but since it's only sorting by signal strength this somewhat
expected.  Probably best to see if we can optimize the supplicant's
choices by hinting what we'd like (or at least being able to require
specific bands like a/b/g) and having the supplicant filter it's scan
results with that requirement.

Dan
 
> Greetings, 
> 
> Keep up the good work!
> 
> José 
> 
> 
> 
> On Fri, Oct 3, 2008 at 4:08 PM, Dan Williams <dcbw redhat com> wrote:
>         
>         On Fri, 2008-10-03 at 11:42 -0400, Jose Aliste wrote:
>         > Thanks for the info ... I just have one more question, see
>         inline.
>         >
>         > On Thu, Oct 2, 2008 at 3:18 PM, Dan Williams
>         <dcbw redhat com> wrote:
>         >         On Wed, 2008-10-01 at 19:14 -0400, Jose Aliste
>         wrote:
>         >
>         >         > Hi all,
>         >         >
>         >         > I want to ask the question in the subject:
>         >         > Which AP is chosen when several are found for the
>         same SSID?
>         >         > I ask this because I see three AP from mi wireless
>         network
>         >         at my
>         >         > office. Two of them are "802.11G" AP are the other
>         one is
>         >         only
>         >         > "802.11B" the signal for the slow one is better(it
>         is
>         >         nearer) and NM
>         >         > always connects to it. However, I would like to NM
>         to
>         >         connect to one
>         >         > of the others because even with less signal the
>         connection
>         >         is
>         >         > better( 33mb/s against the 11mb/s of  a b
>         connection!).
>         >
>         >
>         >         At this moment, this is left up to the driver and
>         the
>         >         supplicant.  The
>         >         supplicant will apparently choose the AP with the
>         best signal
>         >         strength
>         >         (since the scan results for the same SSID are sorted
>         by signal
>         >         strength), but the driver can also choose to roam
>         from AP to
>         >         AP based on
>         >         other criteria.
>         >
>         >
>         >
>         >         > So, how can I tell NM to use the AP I need ?
>         >
>         >
>         >         You can try to set the BSSID in the connection
>         editor to lock
>         >         it onto
>         >         one of the APs, but the driver has the discretion to
>         ignore
>         >         that and
>         >         roam to an AP it thinks is better.  We don't yet
>         have the
>         >         facility to
>         >         lock to 802.11g at _any_ level of the stack, not
>         just
>         >         NetworkManager.
>         >
>         > I am using NM 0.6 in Hardy and If I do as you or aaron
>         said(keeping in
>         > th bssid only the address of the ap I want), but it does not
>         work.
>         > However, If I go with manual config and I set the  AP by
>         using the
>         > wireless-ap option in the /etc/network/interfaces  then
>         everything
>         > works... Would I be more lucky in NM 0.7, I mean does NM 0.7
>         try to
>         > force the AP? Btw wpa_supplicant is used even if I am not
>         using WPA?
>         
>         
>         NM 0.6 doesn't have the capability to lock to a particular
>         AP.  NM 0.7
>         has the capability, but is still subject to driver behavior.
>          Give NM
>         0.7 a shot and let us know if locking to your AP's BSSID
>         works.
>         
>         Dan
>         
>         
>         > Thanks
>         >
>         > José
>         >
>         >
>         >
>         >         You could try setting the data rate, but that
>         doesn't always
>         >         do what you
>         >         want since you can only set it to one value, and the
>         card
>         >         would be
>         >         unable to rate-scale based on signal quality of the
>         locked AP.
>         >
>         >         In short, can't really be done, but this is only
>         partly NM's
>         >         fault.
>         >         There's a lot more work in the stack from drivers,
>         to WEXT, to
>         >         wpa_supplicant to make sure this works the way you
>         want it.
>         >
>         >         Dan
>         >
>         >
>         >
>         >
>         
>         
> 
>