When mobile users shift over from a cellular network to Wi-Fi, their traffic stops using the carrier's spectrum altogether. Wi-Fi is a big part of carriers' so-called offload strategies because it offers two fat bands of unlicensed spectrum and is built into nearly all smartphones and tablets. Many carriers have already built or bought extensive networks of Wi-Fi hotspots, and all encourage users to move their devices from the cellular network to their own Wi-Fi at home.

Continual advances in Wi-Fi technology, including the IEEE 802.11ac gear expected to be widely available next year, make wireless LANs even better for handling large numbers of users. Going from cellular to Wi-Fi may also become easier with new systems such as the Wi-Fi Alliance's standard. Efforts are now under way to let users roam automatically, among hotspots operated by different carriers and by aggregators such as Boingo.

The experience would be like roaming between cell networks and it could send a lot more mobile traffic off the scarce cellular frequencies, helping mobile operators to get by longer with the spectrum they already own. But carriers are expected to adopt the new roaming technology slowly. Consumers should note that unlike the access offered now on carrier-owned hotspots, Wi-Fi roaming for subscribers.