While testing a project designed to let handheld users go between wireless LANs and cellular networks, Pradip Patel made a call and walked into a stairwell of a classroom building on the University of Michigan's Ann Arbor campus and was immediately thwarted.

His first frustration was that the IP voice call he was making over the school's 802.11b WLAN stopped, because the WLAN access points weren't set up to cover such marginal areas. That's not a problem for data users but it is for voice calls, as Patel discovered.

The second frustration for Patel, an engineer with the school's IT Central Services group, was that the HP iPAQ 6315 he was using to make the call didn't automatically switch over to a designated cellular carrier as it was supposed to. He had to input the number again, using the iPAQ's GSM/GPRS modem to link with the carrier network.

Welcome to the seamless world of converged Wi-Fi and cellular networks. Convergence has been talked about for more than a year, but the first dual-mode handhelds, combining a WLAN adapter and a cellular interface, emerged in late 2004. Today, there is a handful of products from companies such as HP, Nokia and Motorola, with more due later this year.

The idea is to give users one handheld device that makes use of whichever wireless connection is available. The device could support voice or data over IP-based WLANs. Outside the range of those LANs it could use the more pervasive, and much more costly, cell networks. But to switch between the two disparate networks is not a simple matter.

The University of Michigan is one of three schools, along with Northwestern University and Texas A&M University, that has started small trials with a gateway from VeriSign, the network operator that handles roaming and clearing among mobile network providers. VeriSign's Wireless IP Connect Service acts as a translator between the school's IP network and the carrier's complex cellular network.

At the Ann Arbor campus, three users carry iPAQ PDAs, each of which has a WLAN radio, a GSM/GPRS radio, a Bluetooth radio and software from VeriSign. Users can take and make VoIP calls when in range of any campus access point, or be switched to a cellular connection when outside the WLAN. The VeriSign client code lets cellular devices register as IP devices through the WLAN, and sets up a VPN connection, for security, to the operator's data center.

The iPAQ users have full access to PBX features such as five-digit calling on campus and call management. They also have access to carrier data services such as Short Message Service, ring tones, content offerings and voice mail.

The wireless future?
For Andrew Palms, the university's director of communications systems, the VeriSign trial is a starting point to explore the future of campus communications. The university has 35,000 traditional phone lines and about 1,500 VoIP lines. "Currently, VoIP doesn't buy us much," he says. "But it could buy us a lot if we could include mobility. The idea is transitioning our traditional phone service to a mobile VoIP service."

What he envisions is letting students and faculty choose whatever handheld device they want, and then provisioning that device with voice, data and video services from the university's network infrastructure so campus users can communicate.

He's not alone. A recent survey by Sage Research asked 166 IT respondents to rate their interest in being able to blend VoIP on internal WLANs with a mobile carrier's network. About 57 percent indicated some level of interest, with about 17 percent being extremely interested.

In June, Alexander Resources, a consulting company specialising in wireless, estimated that convergence of Wi-Fi and cellular networks will generate US$1.6 billion in new revenues for carriers by 2010, an indication of the willingness of users to pay considerable money for such a service.

Problems in spanning networks
But the University of Michigan trial already shows how hard it will be to realise Palms' vision. The problem lies not in the gateway functions, but in being able to span voice and data services over two very different networks.

"On the physical [network] level, it's actually quite easy," says Tom Kershaw, VeriSign's vice president of marketing. "We're emulating the mobile net on the IP side. The complexity is in making the two nets look the same to the subscriber."

Different tariff plans are an example. A student might have a flat rate for VoIP calls via the WLAN. But when he's seamlessly handed off to a cell network, the carrier charges by the minute. "You might need to somehow alert the subscribers to that change, and ask them 'do you want to pay this difference?'" Kershaw says.

Dialing plans are different, Michigan's Patel says. "On campus, you enter an access code and then have five-digit dialing to reach someone," he says. "But in the cellular net, these plans don't work. So how do we resolve that?"

Another issue for the university is improving WLAN coverage to eliminate dead spots, such as the stairwell that frustrated Patel. Enterprise WLANs will have to be pervasive to support mobile VoIP.

Handset trade-offs
Today's small crop of dual-mode devices forces a number of trade-offs on users. Until recently, most devices have been PDA-types, larger and heavier and more expensive than most voice cell phones. Newer devices such as Motorola's recently released CN620 handset are more "phone-like." But the CN620 and Motorola's SIP proxy server are part of a system jointly developed by Motorola, Avaya and Proxim. Avaya contributes the VoIP software and network hardware; Proxim has tuned its WLAN access points for voice traffic.

Nokia in June said it also was partnering with Avaya, in this case to create a dual-mode handset that would run a custom-built Avaya application enabling the handset to make and take calls over WLANs or cellular networks, via the server-based Avaya Communication Manager. The software will have an easy-to-use menu-driven user interface, so that cellular handset users can easily access a wide range of IP telephony features, such as four-digit dialing to a co-worker's extension and call transfers, says Fritz Ollom, a senior marketing manager with Avaya.

Hungry for power
Battery demand is a key issue, says Patel, who is managing the VeriSign trial on-campus. "If I use just basic phone service, that will be OK for a day," he says. "But if I start browsing [the Web], I use up more battery life." The WLAN adapter is always active, while the cellular link is activated only when needed, he says. The drain increases when other services, such as GPS, are used. Patel says an eight-hour battery life for a dual-mode device, being used for both voice and data, is probably a requirement.

That kind of battery life is found in the latest dual-mode devices, in large part because of advances by radio chip makers, says Frank Hanzlik, managing director for the Wi-Fi Alliance, a vendor group that tests and certifies interoperability of WLAN products. About a year ago, the Alliance formed a group to focus on testing for dual-mode devices. So far, about 20 devices have been tested, though most of them will be released later in 2005 or early in 2006.