Generic Access Network
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Generic Access Network or GAN is a telecommunication system that extends mobile voice, data and IP Multimedia Subsystem/Session Initiation Protocol (IMS/SIP) applications over IP networks. Unlicensed Mobile Access or UMA, is the commercial name used by mobile carriers for external IP access into their core networks.
The most common application of GAN is in a dual-mode handset service where subscribers can seamlessly handover connections between wireless LANs and wide area networks using a GSM/Wi-Fi dual-mode mobile phone. UMA technology has enabled the convergence of mobile, fixed and Internet telephony, sometimes called Fixed Mobile Convergence.
The local network may be based on private unlicensed spectrum technologies like 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 wireless 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 presents to the mobile core network as a standard cellular base station. The handset communicates with the GANC over the secure connection using existing GSM/UMTS protocols. 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.
UMA was developed by a group of operator and vendor companies. The initial specifications were published on 2 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 8 April 2005, 3GPP approved specifications for Generic Access to A/Gb interfaces for 3GPP Release 6.  and, 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 
The original Release 6 GAN specification supported a 2G (A/Gb) connection from the GANC into the mobile core network (MSC/GSN). Today all commercial GAN dual-mode handset deployments are based on a 2G connection and all GAN enabled devices are dual-mode 2G/Wi-Fi. The specification, though, defined support for multimode handset operation. Therefore, 3G/2G/Wi-Fi handsets are supported in the standard. The first 3G/UMA devices were announced in the second half of 2008.
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.
At the end of 2007, the GAN specification was enhanced to support 3G (Iu) interfaces from the GANC to the mobile core network (MSC/GSN). This native 3G interface can be used for dual-mode handset as well as 3G femtocell service delivery. The GAN release 8 documentation describes these new capabilities.
- Instead of erecting expensive base stations to cover dead zones, GAN allows carriers to add coverage using low cost 802.11 access points. Subscribers at home have very good coverage.
- In addition, GAN relieves congestion (meaning that networks can, through GAN, essentially piggyback on other infrastructure) 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 can leverage these hotspots to create more capacity and provide better coverage in populous areas.
- The carrier does not pay for much of the service, the party who provides the Internet and Wi-Fi connection pays for a connection to the Internet, effectively paying the expensive part of routing calls from the subscriber.
- Subscribers do not rely on their operator's ability to roll out towers and coverage, allowing them to fix some types of coverage dead zones (such as in the home or workplace) themselves.
- The cheaper rates for 802.11 use, coupled with better coverage at home, make more affordable and practical the use of cellphones instead of land lines.
- Using IP over 802.11 eliminates expensive charges when roaming outside of a carrier's network.
- 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, a single set of services and a single phone directory for all calls.
- GAN can migrate between IP and cellular coverage and is thus seamless; in contrast, calls via third-party VOIP plus a data phone are dropped when leaving high-volume data coverage.
- Subscribers must upgrade to Wi-Fi/UMA enabled handsets to take advantage of the service.
- Calls may be more prone to disconnect when the handset transitions from Wi-Fi to the standard wireless service and vice versa (because the handset moved out or within the Wi-Fi's range). How much this is a problem may vary based on which handset is used.
- The UMA may use different frequency that is more prone to some types of interference
- Some setup may be required to provide connection settings (such as authentication details) before advantages may be experienced. This may take time for subscribes and require additional support to be provided. The costs of support may be for more than the wireless phone company: network administrators may be asked to help a user enter appropriate settings into a phone (that the network administrator may know little about).
- The phones that support multiple signals (both the UMA/Wi-Fi and the type of signal used by the provider's towers) may be more expensive, particularly to manufacture, due to additional circuitry/components required
- This uses the resources of the network providing the Wi-Fi signal (and any indirect network that is then utilized when that network is used). Bandwidth is used up. Some types of network traffic (like DNS and IPsec-encrypted) need to be permitted by the network, so a decision to support this may impose some requirement(s) regarding the network's security (firewall) rules.
- Using GAN/UMA on a mobile requires the WiFi module to be enabled. This in turn drains the battery faster, and reduces both the talk time and standby time when compared to disabling GAN/UMA (and in turn WiFi).
Service deployments 
The first service launch was BT with BT Fusion in the autumn of 2005. The service is based on pre-3GPP GAN standard technology. Initially, BT Fusion used UMA over Bluetooth with phones from Motorola; since Jan 2007, it has used UMA over 802.11 with phones from Nokia, Motorola and Samsung  and is branded as a "Wi-Fi mobile service". BT has since discontinued the service.
On August 28, 2006, TeliaSonera was the first to launch a 802.11 based UMA service called "Home Free". The service started in Denmark and later expanded to Sweden and Norway.
On September 25, 2006 Orange announced its "Unik service", also known as Signal Boost in the UK. The announcement, the largest to date, covers more than 60m of Orange's mobile subscribers in the UK, France, Poland, Spain and the Netherlands.
Cincinnati Bell announced the first UMA deployment in the United States. The service, originally called CB Home Run, allows users to transfer seamlessly from the Cincinnati Bell cellular network to a home wireless network or to Cincinnati Bell's WiFi HotSpots. It has since been rebranded as Fusion WiFi.
This was followed shortly by T-Mobile US on June 27, 2007. T-Mobile's service, originally named "Hotspot Calling", and rebranded to "Wi-Fi Calling" in 2009, initially allowed users to seamlessly transfer from the T-Mobile cellular network to an 802.11x wireless network or T-Mobile HotSpot in the United States. However, the current version of WiFi Calling no longer supports handoff.
In Canada, both Fido and Rogers Wireless launched UMA plans under the names UNO and Rogers Home Calling Zone (later rebranded Talkspot, and subsequently rebranded again as Wi-Fi Calling), respectively, on May 6, 2008.
Industry organization UMA Today tracks all operator activities and handset development.
UMA is not implemented in Asia, Australia, Africa and some European countries.
UMA/GAN Beyond Dual-mode 
While UMA is nearly always associated with dual-mode GSM/Wi-Fi services, it is actually a ‘generic’ access network technology that provides a generic method for extending the services and applications in an operator's mobile core (voice, data, IMS) over IP and the public Internet.
GAN defines a secure, managed connection from the mobile core (GANC) to different devices/access points over IP.
Femtocells - The GAN standard is currently used to provide a secure, managed, standardized interface from a femtocell to the mobile core network. Recently[when?] Kineto, NEC and Motorola issued a joint proposal to the 3GPP work group studying femtocells (also known as ‘Home Node B's or HNB) to propose GAN as the basis for that standard.
Analog Terminal Adaptor – T-Mobile US once offered a fixed-line VoIP service called @Home. Similar to Vonage, consumers can port their fixed phone number to T-Mobile. Then T-Mobile associates that number with an ATA (analog telephone adapter). The consumer plugs the ATA into a home broadband network and begins receiving calls to the fixed number over the IP access network. The service was discontinued in 2010, however earlier subscribers were "grandfathered" in.
Mobile VoIP Client - Consumers have started to use telephony interfaces on their PCs. Applications offer a low cost, convenient way to access telephony services while traveling. Now mobile operators can offer a similar service with a UMA-enabled mobile VoIP client. Developed by Vitendo, the client provides a mirror interface to a subscriber's existing mobile service. For the mobile operator, services can now be extended to a PC/laptop, and they can give consumers another way to use their mobile service.
Similar technologies 
GAN/UMA is not the first system to allow the use of unlicensed spectrum to connect handsets to a GSM network. The GIP/IWP standard for DECT provides similar functionality, but requires a more direct connection to the GSM network from the base station. While dual-mode DECT/GSM phones have appeared, these have generally been functionally cordless phones with a GSM handset built-in (or vice versa, depending on your point of view), rather than phones implementing DECT/GIP, due to the lack of suitable infrastructure to hook DECT base-stations supporting GIP to GSM networks on an ad-hoc basis.
GAN/UMA's ability to use the Internet to provide the "last mile" connection to the GSM network solves the major issue that DECT/GIP has faced. Had GIP emerged as a practical standard, the low power usage of DECT technology when idle would have been an advantage compared to GAN.
There is nothing preventing an operator from deploying micro- and pico-cells that use towers that connect with the home network over the Internet. Several companies have developed so-called Femtocell systems that do precisely that, broadcasting a "real" GSM or UMTS signal, bypassing the need for special handsets that require 802.11 technology. In theory, such systems are more universal, and again require lower power than 802.11, but their legality will vary depending on the jurisdiction, and will require the cooperation of the operator. Further, users may be charged at higher cell phone rates, even though they are paying for the DSL or other network that ultimately carries their traffic; in contrast, GAN/UMA providers charge reduced rates when making calls off the providers cellular phone network.
- BlackBerry - Curve 8320, 8520, 8820, Curve 8900, Pearl 8120 and 8220, Bold 9700, Bold 9780, Torch 9800, Blackberry 9105, 9300, Blackberry Bold 9900 with OS 7.1 
- HTC - Touch 3G, T-Mobile Shadow 2009, T-Mobile myTouch 4G (sometimes called the myTouch HD), T-Mobile G2 (as of build 1.22.531.8 OTA update), Desire S, Wildfire S, Sensation 4G, Amaze 4G, HTC One, HTC One S
- Huawei - U8651T
- LG - KE 520, KF 757 (3G), GT505, Optimus One, LG Optimus Me
- Motorola - DEFY
- Nokia - 6300i, 6301, 6301b, 6086, 6136, 7510, E73 Mode, E5, C7 Astound, Lumia 521, Lumia 925
- Sagem - my419X
- Samsung - SGH-T339, SGH-T409, SGH-T709, SGH-T739 (Katalyst), T336, P250, P260, P270 (3G), T-Mobile's Galaxy S SGH-T959, Galaxy SII SGH-T989, Orange Galaxy SII with NFC (GT-i9100P), T-Mobile's Galaxy S4 SGH-M919
- Sony Ericsson - G705u (3G)
See also 
- UMA Today
- TS 43.318
- TS 44.318
- The Register:BT Fusion goes Wi-Fi
- Home Free
- Unik in France
- Unik in the UK
- CB Home Run Integrates Mobile Phone and Wireless Internet For Improved Indoor Reception
- T-Mobile Introduces Unlimited Calling Over Wi-Fi With the National Launch of T-Mobile HotSpot @Home
- CNW Group Fido's home and mobile calling are now 'UNO' with one phone, one number and one bill
- T Mobile @Home
- T Mobile @Home discontinued
- DECT Web DECT/GSM DUAL MODE and the advent of the ONEPHONE SERVICE
- Leaked BlackBerry 9900 ROM adds Wi-Fi Hotspot, Wi-Fi calling features
- T-Mobile Extends Wi-Fi Calling to Android Smartphones (Press Release)
- Orange Launch the HTC Desire S with Signal Boost (UMA)
- Orange launch the LG Optimus One with UMA
- T-Mobile Extends Wi-Fi Calling to Android Smartphones (Press Release)
- Orange Signal Boost (UMA)
- 3GPP GAN Specification 43.318, 3GPP GAN Specification 44.318
- Smart Wi-Fi (formerly UMAToday.com) — contains also a list of available GAN handsets
- UMA Technology
- T-Mobile USA Wi-Fi Calling FAQs