IMS-based FMC has been widely accepted by carriers and equipment vendors. And the dual-mode handover technology, which can realize the fixed / mobile voice continuity are considered as the first key step by many industry experts including members of FMCA. So what could dual-mode handover bring to carriers and consumers? How can we realize dual-mode handover?

Dual-Mode Handover: Who are Interested?

Considerations of Fixed Network Carriers
Mobile services are increasingly surpassing fixed services, especially in terms of voice services. Analysys, a research consultancy predicts that 50% of Western Europe's voice traffic will be borne over mobile networks by the year 2009, and statistics from Ovum shows that more than 30% of mobile calls are from indoors. Although new services, like multimedia services and IPTV, will be the major potential revenue source for the fixed carriers in the long run, their current revenues are mostly generated by voice services. Thus, innovations in voice services will be more efficient ways to increase profits.

Fixed carriers possess rich ADSL and Wi-Fi access resources, and 30% of mobile traffic derives from Wi-Fi hotspot areas. Therefore, to transfer mobile traffic in Wi-Fi-covered areas using Wi-Fi-enabled multi-mode mobile terminals to fixed networks became an increasingly attractive to fixed carriers, especially in Europe. Then, when subscribers are roaming within Wi-Fi covered areas, connection to a Wi-Fi network will take priority to get subscribers more economical voice and value-added services no matter there is a 2G mobile network or not. When roaming outside a Wi-Fi area, the subscribers will be connected to 2G mobile network to use voice services in the CS domain.

Considerations of Mobile Network Carriers
The 3G commercial plans of many carriers have been postponed for a number of reasons, while some mobile carriers' 2G networks lack a sufficient spectrum of resources in densely populated areas. So, how to expand the access capacity has become a very important issue for the mobile carriers to tackle before the commercial deployment of 3G networks.

Wi-Fi is in the unlicensed ISM frequency band, so many mobile carriers wish to enlarge coverage and expand access capacity by adopting WiFi broadband access resources. When a mobile subscriber roams into a Wi-Fi-covered area, this mode creates an automatic connection to the WiFi network and provides the consumer with cheaper voice services and richer, high-speed multimedia services. Once the subscriber roams outside the Wi-Fi-covered area, the CS domain is accessed. Not only does Wi-Fi offer a voice service equal to the mobile CS domain, but WiFi/GSM bi-directional handover is also supported.

Considerations of integrated network carriers
A survey reveals that integrated network carriers are facing high subscriber churn in the fixed network field. And most lost subscribers choose a new mobile carrier rather than other mobile services of the same carriers.

Dual-mode handover, however, allows these carriers to rapidly provide packaged services to residential or business subscribers, and to provide integrated solutions for the business subscriber market. Through differentiated services, this mode can increase subscriber loyalty and ARPU.

Response from subscribers
According to Heavy Reading, about 61% of subscribers prefer to use multi-mode phones, which provides FMC convenience and advantages by auto-selecting the best and most appropriate network.

In fact, FMC can enhance subscribers' experience in a number of ways. Integration of wireless, indoor Bluetooth and Wi-Fi network makes it possible to communicate any time, anywhere. The "one number, one address, one authorization" mode offers more personalized and convenient services. FMC expands service scope and diversity, lowers price and ensures a higher QoS.

Fixed carriers, mobile carriers, integrated carriers, as well as subscribers are all increasingly interested in the dual-mode handover service mode. Some leading carriers have launched the FMC services based on the dual-mode handover function, such as BT fusion.

UMA or VCC?
Currently, there are two main solutions for dual-mode handover: UMA (Unlicensed Mobile Access) and VCC (Voice Call Continuity).

UMA
UMA facilitates the integration of mobile and Wi-Fi networks as defined by 3GPP R6, and is mainly oriented to mobile and fixed network carriers possessing the MVNO qualification. The Fig.1 below shows that this solution adds a UNC (UMA Network Controller), which is connected to GSM/GPRS equipment through Gb and A interfaces. Via the Internet, WLAN & Bluetooth AP (Access Point), it integrates GSM/GPRS and wireless local area networks (Wi-Fi, Bluetooth) into a seamless network.

The UNC forms a key part of the UMA solution and performs a range of functions. Working as the SGW (Security Gateway), it provides secure IP interface to each connected terminal that performs secure and private communications over an open IP network. It performs detection, registration, and redirection, thus ensuring that each terminal can be connected to the proper UNC. In terms of circuit-switched and packet-switched services, it establishes or releases the UMA bearer channel connection and converts IP-based voice bearer channels into traditional PCM-based voice channels. For mobile access modes such as WLAN and Bluetooth, the UNC imitates the wireless access process such as paging and handover. In addition, the UNC can provide proper physical signaling and bearer interfaces for the A/Gb/ Radius interfaces.

The UMA solution barely has any impact on the mobile core network. In other words, the existing service support system, bearer network and service platform can support UMA without requiring any changes. For subscribers, seamless handover between GSM/GPRS and WLAN networks can be achieved with a dual-mode terminal (integrated with UMA client), which supports GSM/GPRS and Wi-Fi. This ensures the continuity of voice services.

However, the UMA solution also has some serious disadvantages: it does not ensure the QoS of multi-service bearer; the integrated charging solution is not mature and the handover between UMTS and Wi-Fi is not yet supported. Due to access capacity limitations, UMA is more suitable for home or SOHO subscribers than the more lucrative enterprise customers. In terms of development, the UMA solution does not adopt the SIP protocol, so it is generally regarded as a short or medium-term solution because IMS architecture is the commonly accepted long-term core network solution.

VCC
With the rapid development of VoIP and the Internet, 3GPP defines the relevant VCC specifications based on IMS in its R6 and R7 to meet both carriers' and subscribers' requirements for service continuity in different access modes. Fig.2 shows VCC architecture defined in 3GPP TS 23.206.


The key feature of this solution is the VCC Application Server (VCC AS), which offers the control to the VCC service subscribers' session and thus realizes Wi-Fi and GSM bi-directional handover. The gsmSCF (gsm Service Control Function) can be performed by the SCP (Service Control Point) of the existing network. In cooperation with the Handover AS, it performs the anchor procedure in the IMS domain when VCC subscriber handover from GSM network to Wi-Fi network, thereby ensuring that the VMSC (Visited Mobile Switching Center) routes the call to the appropriate HOAS. Fig.3 illustrates that the handover function in the service layer is controlled and implemented by the Handover AS (VCC Application Server).


The HOAS performs functions similar to B2B UA. It is the first point to be triggered when a VCC subscriber makes a call, and the last one to be triggered when a VCC subscriber receives a call, so that the handover latency is effectively reduced. HOAS also solves the CS/IMS domain selection problem. In addition, it provides interface interconnection with other NEs (Network Elements), achieving interaction of subscriber information, like iFC with S-CSCF, and facilitates redirection when the UE (User Equipment) goes unexpectedly off-line from the Wi-Fi network.

VCC wins the race
Subscribers get different experience through the two modes. The UMA solution realizes RAN and the Wi-Fi RAN handover in GSM access networks, and expands the CS domain access network. This gives subscribers the same experience as delivered by CS domain services. Although the UMA solution is simple and easy to launch, it will not be a long-term solution due to the disadvantages in its inherent QoS, capacity and non-SIP.

The VCC solution accomplishes CS - IMS domain handover. And IMS architecture has independent access, thus the IMS-bound VCC solution is able to realize seamless voice and multimedia session handover for various terminals such as mobile phones, telephones and PCs. It is also applicable to a range of networks such as CDMA, GSM, UMTS, PSTN, Fixed BB, WiFi, and WiMAX, and can be implemented by fixed, mobile and integrated carriers. Therefore, it is the best choice to implement the dual-mode handover service.

In the 3G era, FMC provides opportunities for carriers to greatly reduce network OPEX and CAPEX. Network convergence enables them to maintain the two networks through a unified platform, and service integration allows greater service provision, which raises ROI. FMC can expand the 3G network's service scope and promote 3G service development. It is believed that service continuity of cross-networks realized through dual-mode handover solution will mark the first step towards network and service.