IMS vs. UMA

By Richard “Zippy” Grigonis
One of the most-talked about convergence-related innovations in recent years is the ability for a suitably-equipped mobile phone or voice-enabled device to roam between unlicensed spectrum domains (such as WiFi or Bluetooth) and cellular phone environments while a call is in progress. Seamless handover of the call is the goal, with this type of “make-before-break” service often being called Dual Mode Service, a subset of the concept of Fixed Mobile Convergence (News - Alert) (FMC). Carriers generally deploy such a service either on a GSM network using UMA (Unlicensed Mobile Access) technology, or else a more elaborate IMS-based network that can handoff calls to GSM, CDMA2000 and other kinds of cellular networks.

It was Mark Powell and his colleagues at Kineto Wireless (News - Alert) (www.kineto.com) who were the principal developers (and boosters) of the revolutionary UMA ecosystem. (Though, to be specific, a full roster of companies involved in the UMA specifications includes Alcatel, AT&T Wireless, British Telecom, Cingular (News - Alert), Ericsson, Motorola, Nokia, Nortel, O2, Research-in-Motion, Rogers Wireless, Siemens, Sony-Ericsson and T-Mobile U.S.)

UMA is essentially an access technique; it allows cost-effective access to a GSM operator’s 2.5G core network via some WLAN (e.g., WiFi, Bluetooth) systems. UMA-enabled mobile devices access circuit-switched services via the “A” interface with GSM’s MSC (News - Alert) (Mobile Switching Center) and GPRS (General Packet Radio Service) or more evolved packet services via the “Gb” interface with the SGSN (Serving GPRS Support Node; which is the gateway between the RNC and the core network in a GPRS/UMTS network).

The key UMA component to achieve dual-mode access is a gateway called a UMA Network Controller (UNC) which sits at the boundary between the mobile core network and the IP network. When the dual-mode phone is communicating via a corporate WiFi network, the voice packets containing GSM format voice and SS7 signaling data travel via an IPsec tunnel originating in the handset and running across the Internet (or other IP access network) to terminate on the UNC where the encapsulating IP shell is stripped off and the voice data is sent as conventional GSM traffic into the mobile core network. The UNC is paired with either a MSC or SGSN in the cellular operator’s core network. These elements maintain call control, even when the phone has roamed onto the unlicensed spectrum network. Therefore, the call in a sense stays on the GSM cellular network even though it is embedded in IP packets – it isn’t actually a full-blown VoIP call. In accordance with this idea, the UNC is designed to masquerade as a BSC to the mobile network, so when handover occurs between WiFi and GSM cellular (or vice versa), the core network perceives it as a seemingly ordinary BSC-to-BSC handover (or “handoff” as we in the U.S. prefer to call it). UNC functionality can be integrated into the existing BSCs, thus relieving the core network from handling signaling and multiple resources related to users switching between wireless LANs and GSM cellular in the same geographic area.

Since the UNC also provides authentication, encryption and data integrity for signaling, voice and data traffic, and acts as a conventional cellular base station, this one little box certainly can be used as a short-term “quick fix” to quickly and inexpensively enable carriers to provide dual-mode services, and it was expected that mobile operators would naturally employ such a system as an adjunct to their GSM/GPRS networks. Indeed, British Telecom, Orange/France Telecom, Telecom Italia (News - Alert) and TeliaSonera Denmark have offered UMA-based services at one time or other.

Thus, UMA has mostly been a system to handover voice calls from Wireless LANs to a GSM/GPRS/EDGE cellular environment, although Kineto Wireless in 2006 first announced client software for mobile/WiFi handoffs that could support 3G UMTS.

Aside from the ubiquitous Kineto IP-based UMA Network Controller (IP-UNC), which underlies and/or works with UMA solutions from Motorola and Nokia (such as the Nokia 6136 and 6301 handsets), other UMA network solutions include the following:

• Alcatel Lucent (www.alcatel-lucent.com) offers a standalone or a more elaborate combined NGN-UMA architecturefor those mobile operators who have implemented Alcatel’s NGN solution. In both platforms, UNC functionality is provided by the Alcatel 5020 Wireless Call Server (WCS) for voice signaling support; Alcatel Wireless Media Gateway (WMG) (either 7540 or 7570) for voice bearer support; Alcatel 1000 Wireless IP Network Controller (WNC), also known as the GPRS Gateway, for packet signaling and bearer support; and the Security Gateway (an OEM product). The Alcatel 1430 Home Subscriber Server (HSS) serves as the UMA database and the 3GPP AAA server, and the Alcatel 1300 Operation & Maintenance Center - Core Network (OMC-CN) acts as the Operation and Maintenance Center for the UMA network and for Alcatel Core Mobile Network solutions.

• Ericsson’s (www.ericsson.com) solution for UMA is Mobile@ Home™ - also known under its 3GPP name Generic Access Network (GAN). In 2005, Ericsson delivered the world’s first commercial UMA/GAN network for British Telecom to launch their Fusion service and is now the main supplier of the Orange UMA/GAN solution in several countries. The Ericsson Mobile@Home solution consists of three main components, the Home Base Station Controller (HBSC); Mobile@Home™ Support Node (HSN); and Security Gateway (SEGW). Supporting nodes include the AAA Server and DHCP/DNS Server as well as the overall element management solution for the UMA/GAN relevant nodes.

• Motorola (www.mot.com) has an end-to-end UMA solution including a UNC, access control, billing integration, network management, WiFi access points, and UMA handsets such as the dual-mode Motorola A910.

• NEC (News - Alert) Corporation (www.nec.com) has partnered with Kineto Wireless to integrate Kineto’s UNC into NEC’s Home Gateway Solution, enabling deployment of a complete end-to-end femtocell solution, comprised of the NEC Home Gateway Network Controller (HGNC). The UMA-enabled HGNC interfaces to a mobile operator’s existing core network with standard IuCS/IuPS interfaces, enabling a fast time-to-market as well as full service transparency for the end user.

IMS – A Broader Canvas?

Deployed as they are by facilities-based MNOs, the classic formulation of UMA works only with GSM cellular networks. Since the voice traffic is in GSM format and is delivered to the mobile core network, UMA can’t completely leverage the “toll bypass” cost advantages of IP telephony, even though the voice traffic is encapsulated in IP packets. It doesn’t bring any benefits to traditional wireline operators, VoIP providers or Mobile Virtual Network Operators (MVNOs). Although it works specifically and very well for dual-mode WiFi/cellular access, UMA doesn’t support other FMC services and cannot extend FMC services to wired handsets, softphones or other kinds of devices.

UMA aficionados will say that UMA is the 3GPP standard for enabling all mobile services accessed over broadband IP and WiFi: voice, data and IMS, and that UMA, being an access technology, actually increases the number of locations where IMS services can be delivered. WiFi access in homes, workplaces and hotspots can now be added to the list of places where one can find cellular coverage and use SIP-based IMS services at full broadband capacity.

But whereas SIP or any IP-based protocol will run over UMA access technology, when a dual-phone is removed from a WiFi hotspot, a broadband SIP session would now have to be made to run over low-bit rate GPRS (or slightly higher EDGE). And handing off a SIP call to a GSM network would be a neat trick. That’s why UMA implements its own signaling and RTP (Real-Time Protocol) channel - all real-time traffic, such as audio or video, of the circuit-switched domain user plane is received at the UNC via the “Up” interface and conforms to the standard RTP framing format defined in the IETF specifications RFC 3267 and RFC 3551. (To enable downlink quality measurements in the mobile station, the UNC must send at least one RTP frame each 480 milliseconds.) And what about roaming from one storefront hotspot to another? Still, despite these mobility constraints, many operators today do deliver SIP applications to UMA-enabled devices.

IMS, though more expensive to install in the infrastructure, allows for many services to be deployed in carriers using a common service architecture. And IMS is access layer agnostic, which means it can use any IP connection to deliver applications over GSM, UMTS, WiFi, UMA, or DSL/cable. The type of WLAN/cellular handover associated with IMS is VCC (Voice Call Continuity), a more complicated “dual-service” approach that presumes mobile handsets access a fixed core voice network when connected via WiFi and a mobile core network when connected to GSM, something which UMA proponents claim will result in a different end-user service experience. Even so, the idea behind IMS is to go beyond the limited access convergence capabilities of UMA to deliver true service convergence, enabling the consistent delivery of many possible services (not just dual-mode) across all types of access networks and user devices.

Both sets of UMA and IMS “true believers” will no doubt be arguing with each other for years to come. The general trend, however, is that the world’s telecom infrastructure is slowly moving toward the flexibility and scalability offered by IMS.

Richard Grigonis is Executive Editor of TMC’s IP Communications Group.

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