VoIP over Wi-Fi networks is still fraught with problems that will take time - probably years - to solve, according to execs from vendors selling gear to support the technology, who spoke at a panel Tuesday at Spring VON 2004.

Key among the problems is guaranteeing quality of service on calls, especially when the end users are moving around from access point to access point during the call. Latency for the entire connection between handsets cannot be greater than 50 milliseconds, and just switching a call from one access point to the next takes hundreds of milliseconds.

"The handoff is good enough for data, but it's not good enough for voice," said Alain Mouttham, CEO of SIPquest.

In addition, LAN switches and IP service providers need to get involved to support the same QoS scheme, said Bob Jordan, vice president of marketing for Strix Systems. "This cries out for a VoIP forum," he said.

If successfully upgraded the technology could be used for workers within a facility placing calls untethered from desktop phones. So far vendors have identified colleges, hospitals and large warehouses as places where users move around within a well defined area as likely places to adopt VoIP over Wi-Fi.

Beyond the QoS problems, each access point can handle a limited number of voice calls - about 30 - before it gets overloaded and quality suffers, said Ujjal Kohli, CEO of Meru Networks.

There are other Wi-Fi VoIP problems that are being worked on, including standards that will promote interoperability, but these will take time, vendors said.

The phones could be tuned for particular applications using the presence capabilities of SIP-based Wi-Fi handsets, Mouttham said. For example, the wireless network could locate the handsets of doctors within hospitals and block calls if doctors are in patient rooms, he said.

While many Wi-Fi VoIP users are content with using their handsets - either dedicated or PDAs with VoIP  software running on them - just for Wi-Fi, vendors are working on phones that support cellular wireless technologies CDMA and GPRS. The goal is to have a single phone hand off between cellular and Wi-Fi on the fly as users pass out of range of cells or access points.

Such dual function phones could be an advantage to service providers supporting users in shopping malls, arenas and other highly traveled areas. The cost of handling calls placed from a given area using Wi-Fi is a tenth the cost of using cellular, said Kohli. Wi-Fi could also be used to improve cellular coverage inside buildings that disrupt cellular service, he said.