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GAN (UMA): Generic Access Network: Unlicensed Mobile Access (UMA)


How UMA Technology Works

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).

The local network may be based on private unlicensed spectrum technologies like Bluetooth or 802.11, while the wide network is alternatively GSM/GPRS or UMTS mobile services. 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 LAN, it establishes a secure IP connection through a gateway to a server called a GAN Controller (GANC) on the carrier's network. The GANC translates the signals coming from the handset to make it appear to be coming from another base station. 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 base station.

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.

Modes of operation

A typical UMA/GAN handset will have four modes of operation:

GERAN-only: uses only cellular networks
GERAN-preferred: uses cellular networks if available, otherwise the 802.11 radio
GAN-preferred: uses a 802.11 connection if an access point is in range, otherwise the cellular network
GAN-only: uses only the 802.11 connection

In all cases, the handset scans for GSM cells when it first turns on, to determine its location area. This allows the carrier to route the call to the nearest GANC, set the correct rate plan, and comply with existing roaming agreements.


For carriers:

  • Instead of erecting expensive base stations to cover every nook and cranny of a neighbourhood, GAN allows carriers to add coverage using low cost 802.11 access points. When at home, subscribers have very good coverage.
  • In addition, GAN relieves congestion on the GSM or UMTS spectrum by removing common types of calls and routing them to the operator via the relatively low cost Internet
  • GAN makes sense for network operators that also offer internet services. Operators can leverage sales of one to promote the other, and can bill both to each customer.
  • Some other operators also run networks of 802.11 hotspots, such as T-Mobile. They will be able to leverage these hotspots to create more capacity and better coverage in many populous areas.
  • Subscribers, not the network, pay directly for much of the costs associated with the service. They pay for a connection to the Internet, effectively paying the expensive part of the cost of routing calls from their location.

For subscribers:

  • Subscribers do not rely on their operator's ability to roll out towers and coverage, allowing them to fix some types of coverage blackspot themselves (such as in the home or office.)
  • The cheaper rates for 802.11 use, coupled with better coverage at home, make it more affordable and practical to use cellphones instead of land lines.
  • GAN is currently the only commercial technology available that combines GSM and 802.11 into a service that uses a single number, a single handset, single set of services and a single phone directory for all calls.


  • Handsets must support 802.11 network access which requires additional space, power and complexity and may affect the size, weight and battery performance of the phone. [citation needed]
  • GAN is designed around connecting directly to an existing 2/2.5G GSM network. As such it is only suitable for network operators who have an existing GSM network. For operators such as '3' in the UK this is not the case. To address the issue a development called EGAN is under way in 3GPP.


  • Generic Access Network

  • UMA

  • 3GPP GAN Specification 43.318

  • 3GPP GAN Specification 44.318

  • 3GPP Specifications Home Page

  • ETSI GSM UMTS 3GPP Numbering Cross Reference





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Last modified: September 20, 2023