Early products and services that shift voice calls seamlessly between wireless LAN and cellular networks are proving themselves to enterprise users.

Early adopters, including those still in pilot projects, say the handoff between two, different wireless connections is generally unnoticeable and almost always reliable. These convergence capabilities also give mobile phone users at least some of the features offered by the corporate PBX, such as transferring calls or dialing a four-digit extension.

Nevertheless there are two recurring issues: the need for a pervasive WLAN designed with voice in mind, and the fact that the 802.11 radio on these so-called dual-mode mobile phones depletes batteries far more quickly than cellular radios do.

The users interviewed for this story are using two kinds of solutions for what's often called fixed-mobile convergence (FMC). One is a behind-the-firewall FMC server or appliance from a third-party vendor -- from big companies like Siemens and NEC, as well as newer, smaller companies like DiVitas Networks and Agito Networks. The server typically coordinates with a corporate IP PBX and with a client application loaded onto a mobile phone that has cellular and 802.11 radios.

The other convergence solution is a carrier service, using the Unlicensed Mobile Access (UMA) standard from the Third Generation Partnership Project to shift calls between an unlicensed Wi-Fi WLAN and a GSM carrier's licensed cellular network. In effect, these services shift the FMC server functions to a UMA controller on the carrier's network. In the US, T-Mobile offers a UMA service to residential and business customers.

Wherever it's located, this server works with the client application to detect when a user is moving into and out of range of cellular or Wi-Fi networks. Basically, the server starts a parallel call over the alternate wireless network; when it's secured, the server mixes the audio from the two sessions and drops the first wireless connection.