On Tue, 27 Feb 2007 10:07:12 -0800
Jan Schaumann <jschauma@netmeister.org> wrote:

> "Steven M. Bellovin" <smb@cs.columbia.edu> wrote:
> 
> > Jan Schaumann <jschauma@netmeister.org> wrote:
>  
> > > I'm interested in getting a mobile broadband card for my Thinkpad
> > > X41. Does anybody have any experience with these things?  Which
> > > ones are known to work well under NetBSD?
> > > 
> > You don't say where you are...
> 
> Right now I'm in San Francisco, but I'd really like to have a card
> that works equally well in NY.

My question was more about country than city.
> 
> I hadn't mentioned this, b/c I was mainly concerned with finding a
> card that technically works with NetBSD, willing to postpone finding
> the proper service for later.
> 
I have a Verizon PC 5740, whic uses EVDO.  It shows up as ucom0, which
means that pppd just works -- in theory...

As Greg alluded in his post, there are problems with connections in
progress if a session drops.  (Hand-off if you use the card while
moving isn't always clean, though I regularly use it with reasonable
susccess from moving trains.)  As best we can tell, this is a property
of at least Verizon's service and probably EVDO in general.  The
problem seems to be that the card will drop the connection if if sees
packets with the "wrong" local IP address.  For quite a while, based on
Greg's observations, I've installed a packet filter in the ip-up
script, to block outgoing packets from "wrong" IP addresses.  The
problem was that if the EVDO connection ever dropped, the retransmits
from existing TCP sessions would get it annoyed.  This meant that
redialing would cause a connection drop again within about two minutes,
ad infinitum.  (Configuring pppd to ask for the old IP address again
made my card *very* unhappy.)

A couple of weeks ago, during one such episode, I discovered that
packets aimed at an old IP address were being delivered to the new IP
address.  It's as if some routing were taking place based on my
customer account, rather than just IP address...  I modified my filters
to send RSTs to moribund inbound connections, in the hope that that
will stop the problem; as yet, I have too little experience with the
new setup to know if it helps.

The best thing to do for a particular card is probably to Google for
"card Linux".

I know of three other possibilities in the US, Sprint, Cingular and
T-Mobile.  Sprint uses EVDO also; I suspect the distinction is based on
coverage areas and perhaps (to a small degree) cost.  By all accounts,
Cingular and T-Mobile's service is noticeably slower, though for
T-Mobile at least I think a subscription also lets you use their 802.11
hotspots.  Also, Cingular at least (not sure about T-Mobile) is
deploying a faster service.  I know nothing of NetBSD compatibility for
any of their cards; I do know one Linux user who successfully uses
T-Mobile.



		--Steve Bellovin, http://www.cs.columbia.edu/~smb