Spectrum is the lifeblood of mobile services. The planned purchase by Verizon Wireless of 20MHz of spectrum from a group of cable operators, which the U.S. Department of Justice , is the latest sign of how important this invisible resource is to mobile operators.

Any service on the airwaves needs frequencies it can use without being overwhelmed by interference, whether the frequency it uses comes from an exclusive license or from a sharing arrangement. The more packets of data are being exchanged over a network, the more spectrum will be needed to carry them -- unless something else is done.

Mobile operators, technology vendors and governments have been sounding alarms about mobile networks nearing capacity for years, and those alarms are getting louder. A study released last year by investment bank Credit Suisse said mobile networks worldwide were filled to 65 percent of capacity on average, while North American networks were running at 80 percent. The U.S. Federal Communications Commission said in 2009 that it expected mobile data traffic to grow by 35 times in the next five years. Equipment vendor Ericsson predicts 10x growth by 2016. Moreover, the growth in demand is unpredictable, because new applications arrive all the time. And an overloaded spectrum band can slow down users' mobile experience.

To meet that demand, regulators and carriers are trying to make more spectrum available for mobile data services. In the U.S., the FCC pledged in 2009 to make 500MHz of additional spectrum available for mobile broadband in the coming years. CTIA, the U.S. mobile industry group, had called for 800MHz.

However, if the predictions about traffic growth come true, there won't be enough "new" spectrum available to keep up with it. Supply and demand are on different orders of magnitude. For example, if the FCC auctioned off 300MHz tomorrow and the two biggest U.S. carriers split it, that will increase their spectrum in the average market by not much more than double, according to independent industry analyst Andrew Seybold. (Then, it would take three or four years to get that spectrum online, he said.)