Seamless switching from gprs to wifi calling

Luit van Drongelen openmoko at
Fri Jun 8 10:48:13 CEST 2007

To get back to what Mathew asked: I don't think a T-Mobile
HotSpot at home is switching calls back and forth as you get in reach of
a hotspot, and walk away from it. Secondly: this only works with
T-Mobile! (for now) T-Mobile has probably set up a HotSpot at home call
server near their GSM traffic backbone, on which your phone logs in,
and through which your GSM traffic goes (with that UMA protocol) while
you're logged in (in reach of a public (T-mobile) hotspot). Calls that
already take place can't be re-routed I guess...

Anyway, what I'm trying to say is that firstly: you need T-Mobile as
your operator. Secondly: you need that T-Mobile HotSpot @home plan.
Thirdly: you need a phone that's capable of routing your GSM traffic
through UMA, to the T-Mobile UMA server/backbone/whatever they call
it. As for the Neo1973 and OpenMoko: The phone can most likely do it,
because the software just needs to know how to do it. BUT, I don't
think T-Mobile will tell you how to log in. T-Mobile makes the phone
software themselves for a reason. If they show you how to make a phone
log in, you can make a program that logs your computer in too. So a
FOSS solution for this probably won't come easily.


PS: sorry for the double post Johnson, it bounced because I mailed
from the wrong account

On 6/8/07, Al Johnson <openmoko at> wrote:
> Check the archives for a full discussion of this. In short GPRS is unsuitable
> for VoIP because of the high latencies, often in seconds. The GSM data mode
> is more suitable even though it's only 9600. It should be possible to have
> Asterisk route calls to the right VoIP endpoint, or to a GSM voice call if it
> can place calls to the PSTN. The trick comes in knowing when to hand over,
> and having a unified client that'll get Asterisk to do it.
> On Friday 08 June 2007 06:21, kenneth marken wrote:
> > mathew davis wrote:
> > > Dear community,
> > >
> > > I am not sure if this is a widely known thing or not, but I just found
> > > out about it and had some questions about this working on the neo.
> > > T-mobile has hotspots all around my area, but have been experimenting
> > > with a new service called T-mobile HotSpot @Home.  It uses a UMA
> > > (unlicensed mobile access) technology to allow phones to switch from
> > > cellular connection to Wi-Fi connection.  And also makes it possible for
> > > VoIP calls.  So this is something that is very interesting to me only I
> > > would like it to be a little different, I don't want to use T-Mobile's
> > > service I would like to use my Wi-Fi connection to my VoIP of choice.  I
> > > know this has been talked about before with some options including an
> > > Astrex box forwarding the call to your cellphone until your in range
> > > then switching to Wi-Fi but that was not a very seamless transistion
> > > from my understanding.  So I guess my question is could we impliment a
> > > UMA type of technology for the neo that is customizable to use our VoIP
> > > provider?  Or since that particular part is locked we wouldn't have
> > > access to that part?  Just curious. When I get the phone I will be
> > > playing with trying to find a solution to this problem.  I have very
> > > limited knowlege about this kind of thing.  I am not an experianced
> > > programmer yet.  I only have about 3 yers of indestry experiance, but
> > > none of that is mobile development and almost none of it is linux
> > > related, so I have a bit of a learnign curve so that is why I am asking
> > > the question here.
> >
> > while not fully up to speed on how it all works, here is my quick take
> > on it:
> >
> > as long as its a voip connection, and said voip service allows two ip's
> > to share a account and call, there should be little to no problem having
> > both a wifi and gprs connection open at the same time as one moves about
> > (in my experience a gprs connection can be held open but not used).
> > hell, one may even use bluetooth if it can handle the data transfer.
> >
> > the problem here is that ip thing. UMA has a normal mobile phone
> > connections as one option so therefor dont have to think about multiple
> > ip's. it just need to have a internet connected cell so to speak, and
> > only hand the call over when the ip based connection is fully in place.
> >
> > however im guessing there are some issues with going between two wifi
> > zones/networks or something similar...
> >
> >
> > so mostly you need a voip service that allows you to log in from another
> > ip without booting the old connection off or hanging up any calls. after
> > that its mostly a case of the client software figuring out what of the
> > two connections to send on. or maybe just send on both, expecting the
> > service to throw away the data thats a duplicate. something that i think
> > is a basic feature in mobile phone systems.
> >
> > one funny thing is that if your using voip, and have a flat rate data
> > plan for your mobile phone, there is no need to go wifi anyways as the
> > mobile data connection will probably be more reliable given that its
> > already built to do what one is trying to make the wifi system do
> > (handover, multiple connections and overlapping zones).
> >
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