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Mobile operators are embracing Wi-Fi technology as a low-cost, high-performance RAN (radio access network) technology. A recent Infonetics report, "FMC Equipment, Phones and Subscriber Market Outlook," projects the FMC market to grow to $46.3 billion in 2010. According to principal analyst Stphane Tral, "UMA, which was believed to have short legs just a year ago, is the predominant technology deployed today to implement seamless FMC between wireless and 2G cellular network--it can now support 3G, is backed by the 3GPP, and has a clear migration roadmap to IMS."
There are thirteen deployments in nine countries of dual-mode handset service based on Universal Mobile Access (UMA) technology. To meet increasing operator demand, phone manufactures are rapidly adding UMA and Wi-Fi to handsets to capitalize on this explosive growth opportunity.
Extending mobile services via UMA
Mobile operators and telecom equipment providers developed UMA technology as a way to extend existing mobile voice and packet services over a broadband access network. The most common application of UMA is in a dual-mode (circuit and packet) handset service. In this case, the operator installs a UMA Network Controller (UNC) in the core network, and the subscriber uses a UMA-enabled dual-mode handset.
The UNC functions much as a 2G base station controller (BSC), with standard A and Gb interfaces for circuit and packet services respectively. Work is currently underway at the 3GPP to add support for 3G Iu interfaces onto the UNC, and the Iu definition should be complete in the first half of 2008.
The dual-mode handset has a cellular (GSM, 3G) radio as well as a Wi-Fi radio. Because a UMA client is embedded into the baseband of the device and requires a modification to the GSM protocol stack, users cannot download a UMA client.
In a UMA-enabled handset, Wi-Fi becomes a RAN radio resource. The Wi-Fi radio and the traditional cellular radio operate in parallel. One of the key functions of the UMA client is to evaluate the Wi-Fi radio network, monitor Wi-Fi for appropriate triggers, establish hand in/out requests, and manage the transition process.
How UMA works
UMA-enabled handsets contain a profile of known Wi-Fi access points. The access points may be added to the profile by the user, or be pre-provisioned by the operator. Each profile will contain relevant information about the AP, including SSID and security keys.
Periodically, the Wi-Fi radio in the handset will scan to determine if the profiled Wi-Fi APs are within range. If an AP is present, and the handover thresholds are met, the hand-in process will begin.
First, the handset must establish a connection with the AP. This is outside the scope of the UMA protocol. There may be Wi-Fi layer security enabled, but it is not required. Most UMA devices support WEP and WPA, and occasionally WPA2. After the handset and access point are associated, the handset will have an IP connection.
At this point, the UMA protocol begins. Stored in the handset is a fully qualified domain name (FQDN), which abstracts the IP address of the UNC in the network. The handset does a domain name service (DNS) look-up on the FQDN to establish the address of the UNC's security gateway element.
The handset then uses the 3GPP-defined Internet Key Exchange version 2 (IKE V2) protocol to initiate an IPSec session with the security gateway. In UMA, the procedure to establish a tunnel is actually a combination of IKE V2 and the Extended Authentication Protocol for SIMs (EAP-SIM).
Per the IKE protocol, the handset exchanges a pre-loaded security certificate with the UNC's security gateway. Then the device is authenticated with the EAP-SIM protocol to the mobile core. Once the device is authenticated, the IPSec tunnel is established between the device and the mobile core network.
At this point, the UMA client notifies the network that a new, stronger signal is available from UMA/Wi-Fi. The network then transfers the current session from the RAN to the UNC in the same way it would move a call from BSC to BSC as a subscriber traveled down the road. Once completed, all circuit voice and packet data traffic to and from the device is sent through the UMA IPSec tunnel.
Inside a UMA Device
A UMA-enabled handset is a standard GSM handset with a new Wi-Fi radio resource. The UMA client is embedded into the baseband processor of a dual-mode handset. The client executes the UMA protocol and is also responsible for monitoring Wi-Fi threshold levels, establishing the IPSec tunnel, and handling VoIP aspects of a UMA circuit voice session. Consequently a UMA client consists of four logical elements.
The UMA Protocol Interface
The core UMA protocol resides within the baseband stack. This is the set of core messages responsible for managing and maintaining an active UMA session. The core protocol module is integrated into the platform's radio resource (RR) layer and tunnels all messages between the handset's Non Access Stratum (NAS) layers and the mobile core network. GSM has standardized a large number of supplementary services. The UMA architecture is transparent to the NAS layers (MM and above) in the handset and the network. Because the supplementary services logically reside in the NAS, they are inherently supported in UMA access mode.
The IP Interface
The IP Interface module manages the mobility of the device and interfaces with the Wi-Fi subsystem. It is responsible for monitoring handover triggers like RSSI and packet/link quality. If and when a handover event happens, the IP Interface module communicates with the Core Protocol module to execute the event.
IPSec
The IPSec module handles all aspects of establishing and maintaining the UMA IPSec tunnel. Some higher-end handsets may have an existing IPSec module that can be used for UMA.
For handset developers, it is important to realize that different operators have different IPSec tunnel termination products. While the standard is fairly well defined, there can be idiosyncrasies between clients and different security gateways.
Audio Processing
Transporting mobile voice traffic over a UMA tunnel is actually a VoIP operation. Therefore, the handset must have basic VoIP audio processing capabilities to overcome issues common to a VoIP session. An Audio Processing module will address jitter and buffering issues.
UMA relies on the handset's existing adaptive multi-rate (AMR) codec rather than a traditional VoIP codec, which offers a significant advantage compared to non-UMA dual-mode devices. The AMR codec is highly optimized on the device and therefore is a relatively low-power operation. The use of a more traditional VoIP codec, such as a G.723 or G.729 codec, would require more processing power and thus result in a higher power draw and much shorter battery life.
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