Unlicensed Mobile Access or UMA, is the commercial name of the 3GPP Generic Access Network, or GAN standard. GAN is a telecommunication system which extends mobile services voice and data applications over IP access networks.
The most common application of GAN is in a dual-mode handset service where subscribers can seamlessly roam and handover between WiFi access points and GSM/GPRS/UMTS network using a GSM/Wi-Fi dual-mode mobile phone. GAN enables the convergence of mobile, fixed and Internet telephony, sometimes called Fixed Mobile Convergence.
On the cellular network, the mobile handset communicates over the air with a base station, through a base station controller, to servers in the core network of the carrier. Under the GAN system, when the handset detects a WiFi network, it establishes a secure IP connection through a security gateway to a server called a GAN Controller (GANC) on the carrier's network. The GANC connects to core network as a base station controller(BSC). Thus, when a mobile moves from a GSM to an 802.11 network, it appears to the core network as if it is simply on a different BSC.
The system was initially called UMA and then renamed to GAN. It was developed by a group of operator and vendor companies. The initial specifications were published on 2nd September 2004. The companies then contributed the specifications to the 3rd Generation Partnership Project (3GPP) as part of 3GPP work item \"Generic Access to A/Gb interfaces\". On 8th April 2005, 3GPP approved specifications for Generic Access to A/Gb interfaces for 3GPP Release 6. TS 43.318 and TS 44.318, and renamed the system to GAN. But the term GAN is little known outside the 3GPP community, and the term UMA is more common in marketing.
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