Several lawmakers and public-safety officials pushed hard for the spectrum after communication problems during the Sept. 11, 2001, terrorist attacks on the U.S. and later disasters. Public-safety agencies, using a wide variety of devices on different bands of spectrum, weren't able to communicate with each other.

The FCC didn't require that bidders build certain types of networks, except that a voice and data network is envisioned for the public-safety network. And customers taking advantage of the open-access rules on about a third of the spectrum are likely to connect a variety of devices to the network. Beyond that, the FCC has required geographic or population-based build-out requirements on much of the spectrum.

What's being auctioned?
For sale is 62MHz of spectrum in the 700MHz band. In late 2005, after a decade of debate, Congress passed a law requiring U.S. TV stations to move to all-digital broadcasts and abandon analog spectrum between channels 52 and 69. The deadline for TV stations to end broadcasts in the 700MHz band is February 2009.

The spectrum is broken up into five blocks. The C block, a 22MHz of spectrum that has the open-access rules, is broken up into 12 regional licenses across the United States. A bidder can win one or more of those regional licenses.

The A block is 12MHz, broken up into 176 smaller regions called economic areas, as is the 6MHz E block. The 12MHz B block is broken up into 734 local areas called cellular market areas. Again, bidders can win multiple regional or local licenses.

Finally, 10MHz of spectrum in the D block, paired with about 10MHz set aside for public safety, is a nationwide license.

Congress has budgeted the auctions to raise at least $10 billion, but many observers expect them to cost much more. The FCC set the reserve price for the C block of spectrum at $4.6 billion.