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UMA/GAN is a system that enables mobile phones to make seamless handovers between a cellular network (such as a GERAN--GSM/EDGE Radio Access network) and an IP access network (such as a wireless LAN) while carrying voice, data, both voice and data, or neither. A milestone on the road to convergence, UMA/GAN allows users of mobile phones to enjoy the benefits of a fixed broadband network. Originally called the Unlicensed Mobile Access (UMA) network, the system was renamed the Generic Access Network (GAN) by the 3GPP and here both technologies are referred to collectively as GAN1.
The GAN system architecture and protocols are documented in 3GPP Technical Specifications 43.318 and 44.3182, 3. A mobile phone that implements GAN must be shown to conform to these standards before the phone is released to the market. Prior to this demonstration, however, mobile phone developers have a number of opportunities to test their GAN implementations against the standards. Early verification of a phone's ability to handle data as well as voice greatly improves the probability of success during conformance testing.
Even for a relatively mature technology such as GAN there are opportunities for designers to differentiate their product through the performance provided. WLAN technology in mobile phones is already extensively used, both for VoIP (Voice over IP) and as a data modem. One of the key factors influencing GAN usage will be the seamless handover between an IP and a Cellular Network, for which standalone WLAN solutions cannot provide
Network Components
The GAN system adds a new architectural component--the GAN Controller or GANC--to the GERAN/UTRAN network. The GANC is equivalent in function to a Base Station Controller (BSC) in a typical GERAN network. Unlike a BSC, however, the front end of the GANC connects to an IP access network and communicates with the phone over this interface (known as the Up interface) using GAN-specific protocols. To allow signaling and user data to pass between the phone and core network, the GANC is responsible for converting Up interface messaging into existing BSC/Core Network interface protocols (BSSAP/BSSGP in the case of GSM).
To receive GAN service, mobile phones must be dual-mode, with the ability to detect when a user is roaming in and out of a Bluetooth or Wi-Fi network so that the phone can switch to and from GERAN/UTRAN mode. Furthermore, the phones are required to set up an Internet Protocol Security (IPSec)-based VPN tunnel to the serving GANC via an intermediate GAN-enabled security gateway (SEGW). Data, voice, and signaling traffic essential to mobile services is appropriately protected and transported over this tunnel.
Setting Up a Connection
In a GAN deployment several GANCs are typically used to divide responsibility for GAN services and provide load balancing. Each GANC in the network provides services for at least one of the following logical entities: Provisioning GANC (P-GANC), Default GANC (D-GANC), or Serving GANC (S-GANC).
A mobile subscriber with a GAN-enabled phone can make use of GAN only when the phone is within range of an unlicensed wireless network to which it has permission to connect. When the phone first attempts to make a connection, it needs to identify the Default GANC (D-GANC). The phone therefore initiates a discovery procedure to receive information about the D-GANC for use in the registration procedure.
In order to derive the address of the D-GANC and its associated Security Gateway (SEGW), the phone connects to a Provisioning GANC (P-GANC) in the phone's Home Public Land Mobile Network (HPLMN) via the P-GANC's associated SEGW. The addresses of the P-GANC and its associated SEGW may be supplied on a phone that has been pre-provisioned with Fully Qualified Domain Name (FQDNs) or IP addresses; otherwise the phone can derive the FQDNs based on information in the (U)SIM. Addressing requirements are covered in 3GPP TS 23.0035.
Next the phone establishes a secure tunnel to the SEGW of the D-GANC and attempts to register with the D-GANC. The D-GANC may become the S-GANC for that connection by accepting the registration or it may redirect the phone to a different S-GANC. A phone may indefinitely maintain a registration to an S-GANC without actively being in GAN mode and while simultaneously using GERAN/UTRAN services so as to allow for timely speech and data handovers to GAN. Once the phone has been explicitly handed over to GAN or has itself decided to switch to using GAN, the subscriber's current location information (which is stored in the core network) is updated by the phone and voice/data and signaling traffic can be routed to the phone via the GANC rather than the cellular network.
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