[asterisk-users] Wifi GSM handover

[Steve Kennedy]

On Sat, Oct 10, 2009 at 10:16:43AM +0200, Patrick wrote:

> Thank to Frank and Steve for your answers
> My understanding is that you need to place on operator premise an
> equipment that checks first the availability of the user on VoIP. If
> not registered, it's routing the call through the cellular network.
> Is it correct ?
> But during the handover (wifi to GSM), how does it works ? Is it the
> operator that initiates a call on the GSM network. If so, I guess the
> mobile device need to have some logic to seamlessly switch between the
> 2 channels, isn't it ?
> If it's the mobile device that initiates the call to the GSM network,
> it will also require some logic to do that.
> So my question is, is the handover something standard in every mobile
> device supporting GSM and VoIP or do you require an extra piece of
> software to do the trick ?
> Is this principal applies to every "transport technology", I mean VoIP
> through WIfi or VoIP over 3G ?

GSM calls are handled by an MSC (which is an SS7 switch) that talks to
BSCs (basestation controllers) which talk to BTS (basestations), of
course MSCs also talk to other MSCs.

The GSM operator will have a UMA gateway in the network.

A UMA phone will 'listen' for both GSM and WiFi and if it detects that
the WiFi is 'known' it connects to that and it will connect through to
the UMA gateway and the GSM network will switch the call to WiFi, if the
user wanders off the WiFi area it will switch back to normal GSM
operation.

So the phone has to be UMA aware and the operator has to support it.

On a normal GSM phone it is possible to write software that will switch
calls between VoIP and GSM but you then generally have to control the
endpoint of the call, so the GSM call usually goes through a VoIP access
system and the software will switch the call to VoIP if it can, but the
end-point is always the VoIP system that then calls the real number
dialed. i.e. when the user dials a number it doesn't really go to that
number directly, goes through the VoIP company who then can switch the
transport in-between them and the handset.

Steve

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