Employ UMA for cellular/Wi-Fi convergence

With UMA, subscribers can move between mobile networks and WLANs with seamless voice and data session continuity as effortlessly and transparently as they move between cells within the mobile network. Seamless in-call handover between the WLAN (Wi-Fi in most cases) and mobile network ensures that the user's location and mobility don't affect the services delivered to the user. The subscriber experiences total service, location, and mobility transparency.

The UMA solution effectively creates a parallel radio access network, the UMA Network (UMAN), which interfaces to the mobile core network using existing mobility-enabled, standards-defined interfaces. The mobile core network remains unchanged. The common mobile core network makes it possible to deliver full service, feature, and operational transparency. The existing service provider Business Support Systems (BSS), service delivery systems, content services, regulatory compliance systems, and Operation Support Systems (OSS) will support the UMA network without change. Service enhancements and technology evolution of the mobile core network apply transparently to both the GSM access network and the UMA network.

UMA architecture for GSM/GPRS
The UMA service concept is an end-to-end solution involving subscriber owned or provided elements and new service provider network elements (Fig. 2). From the network equipment perspective, a UMA network controller (UNC) is needed to support UMA service. As far as the subscriber equipment is concerned, what's needed are UMA-enabled mobile stations (handsets), standard WLAN access points (APs), and standard broadband IP subscription. In addition, the UMA network elements interface to standard deployed network equipment in the GSM/GPRS network to enable seamless UMA-GSM/GPRS network mobility.

The UNC is the primary network entity of the UMA solution. The UNC interfaces to the GSM/GPRS core network as if it was a conventional GSM/EDGE (Enhanced Data for GSM Evolution) Radio Access Network (GERAN) basestation subsystem. The UNC also interfaces to public or private IP networks to communicate with UMA-enabled handsets. For the GSM/GPRS core network, the UNC provides the standard GSM A interface for circuit-switched voice services and the GPRS Gb interface for packet data services.

Within the UMAN, the interface between the UNC and the handsets is defined as the Up interface. The UNC uses standard IP transport to support the Up interfaces to each handset. The UNC maintains end-to-end communication with each mobile handset and relays GSM/GPRS control-plane and user-plane traffic through the A/Gb interface towards the mobile core network.

The UNC is responsible for making the IP-based UMAN access network appear as a conventional GERAN to the core network. Its primary functions are to:

• provide secure, private communications over open IP networks between handsets and the service provider core network.
• provide discovery, registration, and redirection services to allow the handsets to connect to the appropriate UNC.
• relay higher-layer mobile systetem and GSM/GPRS core network control signaling.
• set up and tear down UMAN bearer connections for circuit and packet services.
• transcode the voice bearer from VoIP transport to voice-over-circuit transport towards the conventional pulse-code modulation-based A interface.
• emulate paging, handover, and similar radio-access procedures for UMAN mobile access.
• provide standards-compliant A and Gb interfaces with appropriate physical, signaling, and bearer interfaces.

The UNC contains a security gateway (SeGW) function that implements a secure IP interface towards each handset, using an IPSec tunnel to provide data integrity and confidentiality and IPSec key exchange (IKEv2) for tunnel setup and mutual authentication. SeGW is authenticated using public key-based certificates, whereas subscribers are authenticated using subscriber identity module (SIM) credentials, via extensible authentication protocol (EAP)-SIM. The SeGW implements a Radius interface to the AAA server for EAP-SIM-based subscriber authentication.

UMA-enabled mobile stations
UMA-enabled mobile systems are WLAN-enabled cellular handsets running UMA client software (Fig. 3). This software enables the WLAN radio within a handset to appear as a parallel radio resource to the GSM radio. By integrating in this fashion, the higher-level services and applications within the handset remain entirely unchanged. The fact that a service may be delivered over the WLAN radio versus GSM radio is entirely abstracted from the higher-level service and control logic within the handset.

Standard WLAN access points
APs provide Wi-Fi coverage and a link to the broadband IP network for the UMA solution. The APs don't require any UMA-specific technology. The UMA mobile devices can use any generic off-the-shelf APs. The AP may be part of a residential wireless LAN, an enterprise wireless LAN, or a public wireless hotspot.

Standard broadband IP access
The broadband IP subscription provides the wide area backhaul of UMA traffic to the mobile core network. Any always-on, shared Internet access is sufficient. Enterprise subscribers can use the available enterprise IP network access and residential subscribers can use their residential broadband Internet access. The UMA traffic is simply a new, high-value application for the IP access subscription already in place.

UMA and UMTS
In recent years, a primary focus within the mobile community has been the implementation and market introduction of Universal Mobile Telecommunications System (UMTS) Radio Access Networks (UTRAN). Many operators have launched a UMTS network as a complement to an existing GSM network. There are also stand-alone UMTS operators. While the market drivers behind UMTS deployment are numerous, two primary drivers include the ability to deliver higher-speed mobile data services and to increase the voice capacity of macro radio networks.

As UMA was standardized in 3GPP TSG GERAN, operators want to know how UMA will work in concert with a UMTS network. One key aspect to consider is mobility between UMA/Wi-Fi and UMTS coverage areas. From an overall network perspective, seamless handover between UMA and UMTS networks is supported in the UMA standard today. From the GSM core network's perspective, the UNC is perceived as an additional basestation controller. Thus, when a call currently handled by the UNC is handed over to the UMTS radio network controller (RNC), the process is the same as for calls being handed over from a GSM basestation controller to the RNC.

Due to the radio characteristics of the 2-GHz spectrum, UMTS is susceptible to higher signal loss inside buildings, so providing acceptable UMTS coverage for high-performance mobile services within subscriber homes and offices is a significant challenge for operators. Because UMA enables the delivery of high-performance, low-cost mobile services within homes and offices, it provides an effective complement to UMTS network rollouts. As it leverages broadband access, UMA (like UMTS) can support high-performance 3G services. In addition, as it leverages existing in-building Wi-Fi networks, UMA lets operators address coverage and range issues of their UMTS network rollouts.

UMA support for UMTS core network interfaces
One question asked by operators is whether there's a need for 3G Iu support within the 3GPP UMA standard. As mentioned, Iu interface support isn't required to accommodate handover between UMA and UMTS networks. Moreover, operators with UMTS networks can support 3G services today through UMA using the existing A and Gb interfaces (Fig. 4).

However, as there are several UMTS-only operators, and operators with combined GSM/UMTS networks are looking to migrate more traffic to their 3G-serving GPRS support nodes and mobile switching centers, it's logical for the UMA standard to evolve to incorporate Iu interface support. Such a standardization effort is already in progress and supported by major mobile operators and vendors.

The UMA solution provides an unprecedented opportunity for mobile service providers to integrate Wi-Fi access with the mobile network. This integration provides an enhanced mobile experience for the subscriber, encouraging greater mobile voice and data service use and consequently greater revenues.

About the author
Steve Shaw is director of marketing for Kineto Wireless. For more information, visit www.kineto.com. Shaw can be reached at sshaw@kineto.com.