Mobile IP expert Birdstep has hired Symbian's porting expert, and set him the task of creating software to let data applications on smartphones roam between the cell network and Wi-Fi hotspots.

The software should be available by the end of the year, in time for an expected surge of dual-radio handsets, which can communicate across Wi-Fi and the cellular networks. Motorola, NEC, Nokia, Samsung, Sendo and Sony Ericsson have already pre-announced Wi-Fi-cellular handsets for 2005, and the 240 million smartphones are expected to sell in 2006.

Birdstep will open a new office in Cambridge UK, where Thomas Horsten, formerly head of base porting at Cambridge-based Symbian will head a team of four Symbian software experts porting the Birdstep client to the Symbian operating system. At Symbian, Horsten's job was helping phone manufacturers port the Symbian reference platforms to different phones.

Birdstep already has a client that allows Windows systems to roam seamlessly. The software (which costs around €40 (£27)) uses a standards-based approach built on the IETF's Mobile IP specification, and only requires a client, as long as the routers in the network have a mobile IP "home agent". Mobile IP is one of the main ways to roam between access points, and Cisco routers generally include the necessary software, as explained in this feature).

Other companies, such as Broadbeam (whose technology is in use at the London Ambulance Service) require a server-based system which may involve proprietary estensions.

"We want to do what GSM roaming did for voice on phones fifteen years ago," said Ketil Hoigaard, wireless account manager at Norwegian software company Birdstep. Email and other applications will continue in the background, using whatever bandwidth is available, he said.

The move to Symbian reflects the fact that few smartphones run Windows operating systems, and expresses a confidence in the long-term future of Symbian, despite the tussle for control of Symbian, in which Ericsson is determined to prevent Nokia taking over completely.

The move is intended for data applications, and does not relate to the idea of roaming voice calls between Wi-Fi and the cellular network, however. Although voice-over-IP could be used at a hotspot, the Birdstep client could only roam that application to GPRS data which is not good enough for voice. Voice roaming would require the network to switch the call to the switched cellular network - a more speculative technology, proposed by such as Longboard.

Two months ago, Calypso controversially claims to have patented the very idea of handing data connections off between radio technologies, despite what appears to be plenty of prior art from companies such as Birdstep. Very little has been heard of Calypso's patent claim since.

What do you think? How important is wireless roaming between GPRS and Wi-Fi. And is it important for data or voice? Join our Forum and tell us.