Two vendors are separately releasing products that let enterprise networks do something even the carrier networks aren’t yet doing: bridge the gap between corporate wireless LANs and cellular nets.

Both Siemens Communications and startup DiVitas Networks can switch voice calls between cellular and Wi-Fi nets, without dropping the call. The goal in both cases is keep enterprise users tied into corporate VoIP services - both fixed and mobile - and data services, and let users both make and take calls over either wireless net.

Both products use a behind-the-firewall appliance, usually working via a Session Initiation Protocol (SIP) connection with a corporate PBX, and client software running on a handset, such as a so-called dual-mode phone, which has both cellular and Wi-Fi adapters.

There are differences. DiVitas says its customers can use a Wi-Fi hot spot, such as a Starbuck’s coffee shop WLAN, for connectivity. Siemens doesn’t support that initially but plans to in the future by certifying VPN clients on the handsets. DiVitas has a built-in open-source Asterix IP PBX, Siemens relies on an existing enterprise IP PBX.

This product class, often referred to as fixed-mobile convergence, is still in its infancy, though some analysts estimate as many as two dozen companies, from newly minted startups to network giants, are building products to bridge the cellular/Wi-Fi divide. The driving forces: productivity gains from having employees always reachable, and extending enterprise management and security to smartphones loaded with sensitive corporate data.

Avaya in cooperation with Motorola offers a product package to handle a similar cellular/Wi-Fi handoff via an Avaya PBX. Cisco and Nokia have done something similar for Nokia handsets and Cisco VoIP nets. Other vendors such as Alcatel-Lucent are working on gear that will enable carriers make the handoff within their nets.

Both the Siemens and DiVitas products free enterprises from reliance on the carriers for such services.

The Siemens HiPath Mobile Connect Appliance continuously monitors the mobile users handset, via the companion client application loaded on a dual-mode phone. The appliance keeps a SIP registry of all client handsets, such as dual-mode phones or Wi-Fi VoIP handsets, and it tracks the presence of the handsets. A built-in SIP server links the appliance with a SIP-based PBX. The PBX maintains its function as the clearinghouse for all incoming and outgoing enterprise calls.