Evan Schuman's StorefrontBacktalk

Another concern speaks to consumers who shop with their mobile phone in airplane mode, effectively transmitting no signals at all. They would turn airplane mode off to make a purchase and then turn it back on. Few consumers do that today, because it prevents them from receiving phone calls or texts. But privacy-worried (or, for that matter, cancer-fearing) consumers who do go airplane mode could undermine this system.

A different hiccup for this tracking lies in the nature of the signals. A mobile phone has a unique number called an IMSI (International Mobile Subscriber Identity). When first connecting with a cell tower, the phone sends this number to the tower, which confirms that the phone is allowed on the network (for billing purposes), and then returns a similar (but random) temporary number called a TMSI (Temporary Mobile Subscriber Number). The TMSI is then used between the phone and the cell tower after that, until they get disconnected.

This is the number we’ve been discussing, and it has one catch: The TMSI changes every time the phone moves to a new cell tower and also is regularly changed by the cell tower just to make sure the phone is harder to track. That means if you’re tracking the TMSI within a store or mall, it may change. If your system is keeping a list, it may notice that a TMSI vanishes suddenly and a new TMSI appears to have taken its place. Maybe that just means the TMSI was changed by the network. Or maybe it means that one customer turned off her phone or left the store at the same time a different customer turned on her phone or entered the store. There’s no way of telling. But until the TMSI changes, it will consistently point to the same customer.

There actually is a way to deal with this TMSI change, and the software can deal with it. Unless that specific aisle at that specific moment is very crowded—a scenario that could certainly happen during a holiday busy period—the changeover will typically happen very quickly. If the system, therefore, sees 1234567890 and then that number vanishes but a new number—9292929292—suddenly materializes in the exact same spot, the system can tentatively assume that it’s the same person. In all probability, it is.

Even Wal-Mart is trying to figure out these issues. Venky Harinarayan, Walmart’s senior vice president of global E-Commerce and head of Walmart Labs, was on a panel at the GigaOm Roadmap conference in San Francisco last week when he addressed the mobile issue. “What we’ve got to figure out is, how do you make that customer experience something really worthwhile,” Harinarayan said, according to a PCWorld story. “These are not technology problems, they are customer experience problems.”

The Wal-Mart exec is quite correct. The beauty of the wireless tracking is that it is entirely transparent to the customer, which is crucial to maintain that experience. And yet it also confirms that the phone that paid for that product is the same phone approaching the exit right now.

Additional reporting by Frank Hayes.