Seamless Roaming Nears Reality

The 3GPP's voice-call-continuity standard aims to plug holes between cellular and Wi-Fi networks and drive availability of SIP-based dual-mode handsets. But VCC, at least in its first iteration, falls short

May 10, 2007

8 Min Read
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Be careful what you ask for. Enterprises have long wanted dual-mode Wi-Fi cellular phones to cut phone bills with the convenience of a single device. Carriers have had nothing to gain from that; in fact, North American wireless carriers have discouraged handset makers from developing Wi-Fi-enabled handsets and have gone out of their way to offer customized versions without Wi-Fi.

Now enterprises are going to get what they've asked for--sort of. A new plan will let enterprise users continue with a single device, but carriers won't lose those precious minutes. The VCC (voice-call continuity) initiative promises to provide seamless roaming between cellular and Wi-Fi networks, letting mobile phone users access the office network for mobile services.

This new standard arose within the carrier-centric 3GPP (Third Generation Partnership Project) and is built around SIP (Session Initiation Protocol). Due to be completed next month, VCC will drive the development and availability of carrier-grade, SIP-based dual-mode handsets, which third-party vendors can sell to enterprises.Unfortunately this near-term standard lacks support for conventional data services, such as SMS (Short Message Service) and MMS (Multimedia Message Service). VCC also lags behind the similarly consumer-focused UMA (Unlicensed Mobile Access) by at least 18 months. It inadequately addresses important enterprise features, such as call waiting and call transfer. And it has a simplistic roaming algorithm and no enterprise PBX integration.

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In Search Of Pervasive Communications

Despite the fact that carriers are revving their 3G networks and moving toward an all-IP infrastructure, enterprises are demanding pervasive communications even where the cellular signal is faint or nonexistent. While carriers promote cellular repeaters, DAS (Distributed Antenna System) and pico-cells, the mass adoption of Wi-Fi as the broadband wireless network of choice now provides an alternate means for these carriers to extend their presence-moving traffic away from their expensive and precious licensed spectrum onto unlicensed and almost unlimited Wi-Fi spectrum and increase usage (resulting in lower costs and more minutes because customers can use their phone more often with better coverage).

Rather than develop a standalone proprietary system, both wireless and wireline carriers focused on leveraging the IMS (IP Multimedia System) framework to build a standards-based platform that uses dual-mode handsets--which support both circuit-switched cellular voice and VoIP running over Wi-Fi. Wi-Fi is more than just an important element of their FMC (fixed-mobile convergence) strategy; it's also perceived by the carriers as a good medium-to-long-term strategic choice for delivery of services beyond mobile voice, such data, video and multimedia. Enterprises using Centrex (and possibly hosted VoIP) can continue to use their hosted voice for mobile voice services in and out of the office, with a single telephone number and voicemail box. Even though VCC is not the foundation upon which enterprise dual-mode PBX-oriented solutions will be built, its widespread consumer adoption would provide greater device choices.

VCC VS. UMAVCC is most often contrasted with UMA. Both promise seamless voice mobility, but UMA has some specific features that set it apart, such as the UNC (UMA Network Controller), which replicates the functionality of a BSC (Base Station Controller) in a GSM network. There are about half a dozen dual-mode handsets that support UMA, which has given a head start to carriers such as T-Mobile in the United States and BT in the United Kingdom.

VCC originated within the 3GPP community and is targeted for inclusion in Release 7. The main driver was developing a standards-based FMC plan built around IMS that maintained session continuity across diverse access technologies--wireless or wireline. Two architectures were proposed, but ICSA (IMS-Controlled with Static Anchoring) won out. As the diagram below shows, a dual-mode phone can communicate over either wireless network to a new core element, the CCCF (Call Continuity Control Function), which resides inside the IMS domain. This CCCF acts as the anchoring point between the circuit-switched cellular and packet-data networks and directs incoming calls over the appropriate link.

UMA started with the UMA Technology Group and was eventually folded into 3GPP, but VCC has its own champion with an industry body called MobileIgnite, led by BridgePort Networks. BridgePort Networks offers a product that fulfills the role of the CCCF. Last September MobileIgnite issued a functional specification for VCC that describes minimum expectations in terms of features and functionality. Although VCC is several months away from completion, vendors can build products and perform basic interoperability testing that puts them in a good position when the standard is approved.

VCC has several drawbacks in comparison to UMA as well on its own. From the carrier's perspective, it lags 18 months behind UMA and is still incomplete. While UMA is enjoying more than a dozen trials and deployments, VCC is just getting off the ground in terms of trials and handset availability. MobileIgnite claims 15 handsets that meet their functional specifications--which is different than VCC-compliancy. The existing dual-mode handsets on the market, whether UMA or VCC, are also handicapped with a short battery life.


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ArchitectureClick to enlarge in another window

Another significant carrier consideration is that VCC builds on an IMS core they may not have. Analysts have reported significant progress with IMS deployments, usually focused around specific applications, so this concern will erode over time; Sprint, Cingular, FT, Telefonica and Softbank Mobile have all rolled out IMS infrastructures.

Designed primarily for voice, VCC also lacks true data, multimedia and video support, including basic cellular telephony services such as SMS and MMS, while UMA supports the last two items and GPRS. Considering that, according to Forrester, wireless carriers are reporting that the data component of ARPU ranges from 12 percent to 16 percent, this feature gap cannot be ignored. The Internet still can be accessed over the Wi-Fi and cellular data (2.5/3G) links, but the data session continuity will be lost as the handset roams because data sessions are not tunneled through the CCCF.

There is also a question around the role of VCC once cellular networks support end-to-end IP. Third-party vendors have stated that EV-DO Rev. A provides the necessary bandwidth and low latency to support native IP voice services from the handset to the cellular core, and future advances will likely continue along this line. If it becomes "all IP" the requirement for an anchoring CCCF falls away.

Supplementary services, such as call hold, call waiting and call forwarding, aren't available or are handled differently by various operators. End-users expect feature consistency no matter the access network they happen to be on.Without VCC, any approach to seamless call handoff between multiple access domains would be proprietary. But the standard falls short in addressing key features, such as data-session maintenance and supplementary service support, and offers only a simplistic roaming algorithm. For enterprises that require PBX integration, VCC as an architecture is a nonstarter, but carrier support of an FMC standard means that there will be a multitude of SIP-savvy dual-mode handsets. Although UMA has just begun, sales of UMA-capable dual-mode handsets by Nokia and LG have blown by those of single-mode Wi-Fi only handsets. Handset vendors will compete to integrate lower-powered radios and extend battery life, all built around a SIP-based voice solution, and enterprises will benefit.

In the meantime, businesses looking for seamless voice solutions today will need to look toward proprietary solutions from the likes of DiVitas Networks, FirstHand and Siemens.

Frank Bulk is an Nwc Contributing Technology Editor and works for a midwest-based telecommunications company. Write to him at [email protected]

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