The Boston Globe's Carolyn Johnson on the new 802.11-based location detecter in Apple's iPhone:
Skyhook's technology uses signals from Wi-Fi hot spots to triangulate and find a person's location, instead of using a chip that lets a mobile device communicate with the Global Positioning System.
They developed the technology using the fact that each router has a unique identifier—essentially a beacon that can be picked up by any Wi-Fi-enabled device. Today Skyhook's technology works in about 8,000 U.S. cities and towns, and the company is expanding its database by mapping Wi-Fi signals in Europe and Asia.
In this manner, wireless access points provide a positive externality. Of course, if your wireless network is open you already provide a sizable public good.
The technology behind Skyhook—based right here in Boston—is simple. The onerous and impressive part is assembling the database mapping MAC address to location.
Here is how it works: Every wireless network access point has a unique number associated with it, called its MAC or BSSID address. This address is encoded into every signal sent out over the wireless network, including special beacon signals sent about once every second. Your laptop and iPhone can see these signals, and thus the unique address, without having to connect to the wireless network (and without needing any otherwise-requisite security keys). Skyhook has built a database mapping these addresses to longitude/latitude pairs—either by getting users at known locations to run software that records base station addresses, which in bulk would slowly build the database, or by driving a vehicle around with a wireless interface and GPS and recording the output. Skyhook then provides software for use on your iPhone that looks at the access point addresses in sight and maps them to their prerecorded locations. The more access points in sight, the more accurate the estimate, as the intersection of overlapping access points shrinks your probable location. As said, the software bits are trivial, but building and maintaining the database is remarkable.
Elsewhere, I am reading Tim Harford's new book, The Logic of Life, which is sadly not officially available for The Kindle. The book, like his last, applies a nuanced view of rational choice theory to everyday and unexpected situations.
An early example analyzes condom use by Mexican prostitutes, who charge a 25% premium for service sans condom:
It's tempting to argue that the prostitutes do not understand the risks. That's patronizing: Even without the efforts of the health and development organizations, prostitutes probably know more about the risks of sexually transmitted infections than anyone who thinks of them as simpleminded victims. In fact, prostitutes know that while the risks are real, they are modest. Only one in eight hundred Mexicans carries HIV, and even among prostitutes it afflicts just one in three hundred. Even if a prostitute is unlucky enough that one of her unprotected jobs is with a man who is HIV-positive, the risk that she will catch it is less than two percent if one of them is carrying some other sexual infection and less than one percent otherwise. None of the prostitutes want to catch HIV, but the risks of catching it because of one instance of unprotected sex are small, while the pay is substantially higher.
The typical Morelian prostitute is acting as though she valued one extra year of healthy life at between fifteen thousand and fifty thousand dollars or up to five years income.
Such analysis may appear insensitive, but understanding the incentives behind the choices helps policy makers and social workers better tailor their approach.
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