UMA works very well. Take the data that would be sent over the cell radio and send it over a wifi IPSEC connection instead. That is pretty much it. Heck it will even do realtime handoff between cell and wifi (in either direction).
The reason why it isn't very widely used is because that requires a very low level of integration. Typically the radio and wifi processing code have to be made by the same company and in the same chip. Blackberries have used UMA for years where it is fantastic. But you can't just add it to a random phone because it is almost certain that there isn't that level of radio and wifi integration available.
I've been in Sri Lanka talking to someone in Philadelphia using UMA (which connects to Tmobile in Washington state) where the traffic was going half way around the world and back again. It works perfectly - the other party didn't even know until I told them.
The newer Kineto solution isn't doing UMA because it can't - the radio interface and handoff isn't there. It does still work, but their code is buggy.
I don't contest that UMA works, I'm sure it does - I claim that it seems complicated, and what you write seems to support that. UMA requires low-level hardware integration, SIP is trivially achievable in software.
The reason why it isn't very widely used is because that requires a very low level of integration. Typically the radio and wifi processing code have to be made by the same company and in the same chip. Blackberries have used UMA for years where it is fantastic. But you can't just add it to a random phone because it is almost certain that there isn't that level of radio and wifi integration available.
I've been in Sri Lanka talking to someone in Philadelphia using UMA (which connects to Tmobile in Washington state) where the traffic was going half way around the world and back again. It works perfectly - the other party didn't even know until I told them.
The newer Kineto solution isn't doing UMA because it can't - the radio interface and handoff isn't there. It does still work, but their code is buggy.