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Mobile networks are at last yielding to the lure of dual-mode phones and voice over Wi-Fi, as flat-rate tariffs turn the business of delivering a call into a cost instead of a revenue.

That's the claim of Chong-Jin Koh, the head of Nuvoiz, a wireless VoIP start-up. He said that, after years of jealously guarding their voice revenues, some carriers have now realised that as average revenue per user (ARPU) moves to a flat rate, it's cheaper to offload call delivery onto Wi-Fi.

The dual-mode approach is especially interesting for mobile carriers which also have Wi-Fi networks. An early Nuvoiz customer is Yozan, which Koh said operates the largest Wi-Fi/WiMax network in Japan, and he added that Orange and Telia-Sonera - both of which have extensive hotspot networks - also have plans for dual-mode.

The main target though is enterprise users - they already have wireless LANs in place, and they want to cut their call costs, Koh said.

However, while static voice-over-Wi-Fi is fine, if you move around you can get breaks in transmission, leading to poor call quality. The problem is the handover time, when the call jumps from one Wi-Fi access point to the next.

So Nuvoiz has developed firmware that pre-authenticates the handset to adjacent cells, so that when a voice call needs to move, it can do so much more quickly. Koh said that VoIP handover can take up to two seconds, but that his software can cut the delay to a barely noticeable 50ms.

There are snags though - in particular, the Nuvoiz software operates at the driver level so it has to be built into the handset and can't be added later. Some carriers will therefore look for VoIP routes that are compatible with today's handsets, hence O2's plan to put GSM nanocells into home broadband routers. ®

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