Have you ever wondered why your mobile phone keeps working in the depths of a large building or in the middle of an underground tunnel? It wasn't always that way. Those of us who were using cell phones ten years ago will recall that indoor coverage in malls, offices and airports was an almost unknown luxury. No signal? That's life: you'll just have to step outside to make that call.

Today, says Lance Wilson, ABI Research's director of wireless communications, consumers' expectations are much higher. And thanks to "in-building wireless" - systems that re-transmit mobile signals inside large buildings and underground - those expectations are increasingly being met. "People take in-building wireless for granted," he says. "It's below the radar for most of us. And yet people ask me 'why is in-building wireless important, and is the market for it going to grow?'"

The answer to the first question, says Wilson, is that it's important because today's cellular users expect ubiquitous coverage. We spend a great deal of time indoors, and the assumption is that the mobile phone is going to work all the time, anywhere.

The second question's answer is contained within the first: yes, the in-building wireless market is virtually guaranteed to grow, since it must keep pace with the expanding reach and range of outdoor wireless communications. In fact, even today only a relatively small percentage of buildings in North America and Western Europe are equipped with in-building wireless, so the market will probably grow even faster as more buildings are retro-fitted with these systems.

"At ABI Research we're very bullish," Wilson concludes, "and think that in-building wireless has an excellent future that's almost guaranteed, because of the ubiquity of cellular."

The in-building wireless market is analyzed in detail in ABI Research's recent study, In-Building Wireless Systems, which includes a variety of market forecasts for both revenue and deployments.