Cellular, WLAN convergence finds rough spots

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.

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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 WLANs (often dubbed 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."