You may think of sunsets as something nice to look at, but if you have an older cell phone or a home alarm system, there's one coming up on Monday that may not be so pretty.

That day, the U.S. Federal Communications Commission will let mobile operators shut down their analog networks. It's called the "analog sunset" because those AMPS (Advanced Mobile Phone System) networks, which were first deployed in the 1980s and brought cellular service to millions of Americans, will finally disappear behind the digital networks that serve almost all mobile phones in use today.

The biggest U.S. mobile operators, AT&T Wireless and Verizon Wireless, will close down their analog networks that day. At the same time, AT&T will turn off its first digital network, which uses TDMA (Time-Division Multiple Access) technology. (Sprint Nextel and T-Mobile USA don't have analog networks.) Calls to some small, rural mobile operators indicated that most of them plan to shut down AMPS, too.

There aren't many mobile phones out there that will go dark after the analog sunset, according to the big carriers, which have been warning subscribers about the change for months and offering them incentives to switch over.

"We're talking about a very, very small number of customers here," said AT&T spokesman Mark Siegel. He estimated that 99.9 percent of AT&T's traffic is carried on GSM (Global System for Mobile Communications). Verizon spokeswoman Debra Lewis estimated that less than 1 percent of that carrier's subscribers were on analog even before it started a big effort to reach them last year. Neither gave exact numbers of subscribers. But given that those operators have about 60 million subscribers each, the number might still be in the hundreds of thousands.

However, AMPS isn't only used for cell phones. Many alarm companies use the system to alert police or fire departments to emergencies at homes or businesses. About three years ago, the Alarm Industry Communications Committee (AICC) industry group took a survey which revealed that just under 1 million of the approximately 30 million monitored home and business alarm systems used an analog cellular network, said AICC chairman Louis Fiore. About 850,000 of them used the system only as a backup in case the phone line was cut, he said.

Alarm manufacturers are now replacing many of those analog systems with digital ones, Fiore said. About six months ago, the manufacturers believed there were about 400,000 AMPS systems still in the field, he said.

"There are some small companies out there that probably have not made the conversion yet," Fiore said.

One problem is that, except for a few high-end CDMA (Code Division Multiple Access) monitoring systems, all digital cellular alarms today rely on GSM, Fiore said. That creates a problem in areas that have good CDMA coverage but poor GSM, and Fiore has heard from at least one alarm company in Colorado that has customers outside of GSM's reach. Until now, they have been relying on analog cellular.

Some users of wireless roadside assistance have also been left behind in the transition. General Motors launched its OnStar system in 1996 on AMPS and later switched to CDMA. The automaker didn't wait for the Feb. 18 deadline but instead shut down its analog service on Jan. 1. In a statement on the transition last year, GM said about 90 percent of its subscribers' cars had CDMA or could be converted to use it. Others would lose their OnStar service. The wholly owned subsidiary of GM said last October it had about 5 million subscribers.

Last March, two OnStar customers in Pennsylvania, Robert and Sarah Gordon, sued GM for leaving analog subscribers behind. They are seeking damages and an injunction to force OnStar and GM to provide repairs or upgrades, and they want to turn the suit into a class action. It has been consolidated with a handful of other actions in the U.S. District Court for the Eastern District of Michigan.

Among cell-phone subscribers, the analog sunset is most likely to hurt so-called "glovebox users," said IDC analyst Scott Ellison. These are users, often elderly, who just keep a cell phone in the glovebox in case their cars break down. They usually don't feel a need to update their handsets.

"If you know that you have some kind of wireless link or wireless communications device and you're unsure whether you are affected, call your service provider," Ellison advised. A tip about phones: "If it has a color screen, you should be fine," he said.

AICC's Fiore gave similar advice. Some consumers have ignored potential problems with alarms because they confused the analog cellular shutdown with the end of analog TV, which won't happen until next January, he said. If you notify your alarm provider and they are prepared to go digital, all a repair person will have to do is come into your home and replace the radio, possibly moving it to another part of the house with better GSM coverage, Fiore said.

The perils of the analog shutdown point to a mismatch between technology lifecycles, IDC's Ellison said. Cars and home appliances often stick around for many years, while wireless technology changes more quickly. In fact, Illinois Valley Cellular, in rural Marseilles, Illinois, serves few analog phone users but plans to keep its analog network running after Feb. 18. That's because wind turbines that generate electricity in its service area still use AMPS radios to exchange operating data, according to Data Routing Manager Pam Craig. Replacing those radios would be difficult and expensive.

But as new technology comes along, such as cellular networks that use scarce radio spectrum more efficiently, the old often has to give way, Ellison said. As technology rapidly advances, will it happen again to wireless networks we take for granted now? "It probably will," he said.