Originally Posted by Stinsonddog
UMA - Generic Access Network - Wikipedia, the free encyclopedia
Generic Access Network (GAN), also known as Unlicensed Mobile Access (UMA), is a telecommunication system allowing seamless roaming and handover between local area networks and wide area networks using a dual-mode mobile phone. It lets mobile operators deliver voice, data and IP Multimedia Subsystem/Session Initiation Protocol (IMS/SIP) type applications to mobile phones. Its ultimate goal is the convergence of mobile, fixed and Internet telephony (Fixed Mobile Convergence).
I will leave it to the others to edit and fill in, but I thought I would start this.
UMA acts as a transport layer between the handset and core GSM network, instead of the calls flowing through the cell towers, they flow through the internet and connect the handset back to the core gsm network
UMA also offers seemsless handover between a call over GSM to Wi-Fi or visa versa. For example if a caller is at home making a call over Wi-Fi using UMA and he/she decides to drive to work, as soon as he/she leaves the home away from the wi-fi coverage area, the call can be transferred to GSM without it being dropped. A carrier can turn this feature on or off and the call might not hand over correctly in case the customer is roaming in a different carrier's coverage area.
UMA can work internationally but a carrier might or might not allow that to happen and they can restrict that using different methods not just by checking the IP address and seeing where it's located geographically.