To most people, Wi-Fi is something of a miracle. Within 150 feet of some hidden base station, your laptop, tablet or phone can get online at cable-modem speeds—wirelessly.

But Wi-Fi is also something of a mystery. So many readers ask me about Wi-Fi that I’ve hunted down the answers, once and for all, from the nation’s most expert experts.

Often my laptop detects a four-bar Wi-Fi hot spot, but I can’t get online. What gives?
In the mid-1990s Alex Hills built a huge wireless network at Carnegie Mellon University that became the prototype for modern Wi-Fi networks—a story he tells in his book Wi-Fi and the Bad Boys of Radio. I figured that he would be perfect for this one. His explanation:

“Two issues might cause this. First, radio problems. The bars are an indication of how strong the Wi-Fi signal is, but they don’t tell you anything about interference or other radio problems that can corrupt a strong signal.

“Second, most Wi-Fi systems connect to wired networks that connect you to the Internet. But there may be problems in these wired networks: problems with link speeds, switches, routers, servers, and the like. You have a good Internet connection only when all of the links in the chain are doing their jobs.”

Why do expensive hotels charge for Wi-Fi but inexpensive hotels don’t?
Don Millman’s company, Point of Presence Technologies, runs the Wi-Fi for 150 hotels. His answer:

“Expense accounts: higher-end hotels attract business travelers who expense their stays, so the fee matters less to them.”

We’re frequently warned about the hazards of using free open hot spots, like the ones at coffee shops. What, exactly, is the risk?
Glenn Fleishman has covered networking for more than a decade (currently on the Economist’s Babbage blog):

“A bad guy across the room might be running free software that sniffs every bit passing over the wire­less network and grabs passwords, credit card numbers, and the like.

“You don’t have to worry about banking and e-commerce Web sites; they’re protected by secure, encrypted connections.

“But without encrypting your e-mail and regular Web sessions, you never know if someone sitting within ‘earshot’ is slurping down your data for the purposes of identity theft or draining a bank account. My tip: always use a virtual private network (VPN) connection, which blocks anyone on the local network from seeing anything but scrambled data.”

What’s up with the “Free Public Wi-Fi” hot spot that sometimes shows up at hotels and airports—even on planes—­but that rarely yields any actual connection?
I’ll field this one: Don’t bother trying to connect to “Free Public Wi-Fi” (or “hpsetup” or “linksys”). It’s never a working Wi-Fi hot spot. It’s actually a viral “feature” of Windows XP running amok.

Whenever Windows XP connects to Wi-Fi, it also broadcasts that hot spot’s name to other computers as an “ad hoc” (PC-to-PC) network so that they can enjoy the connection, too. Someone, somewhere, once created a real hot spot called Free Public Wi-Fi, probably as a prank. Ever since, that name has been broadcast wirelessly from one Windows computer to another. (Macs see the phony hot spot, too, but don’t rebroadcast it.)

In public places, people try and fail to connect—but now their PCs start rebroadcasting this ad hoc network’s name, too, and on and on it goes. Best bet: don’t connect.

This article was published in print as "The Trouble with Wi-Fi."