All-you-can-eat data downloads seemed like a good way to sell fancy smart phones at first, but now wireless carriers AT&T, Verizon and T-Mobile are slowing down the buffet line. A minority of customers who keep refilling their digital dinner plates (while others are still working on the first course) have helped the wireless providers realize that offering unlimited data plans doesn't add up. One exception appears to be Sprint, which still touts an $80 per month unlimited plan. Perhaps their customers are better at portion control.

AT&T on Friday announced that some smart-phone customers with unlimited data plans may experience reduced speeds once their data usage puts them among the top 5 percent of heaviest users. The company says that these most prolific users consume on average 12 times more than the normal smart-phone customer over its cellular network (AT&T isn't counting data consumed via WiFi).

"To rank among the top 5 percent, you have to use an extraordinary amount of data in a single billing period," the company said in a statement. AT&T pointed out that throttling data access—which will begin October 1—will not affect its 15 million smart-phone customers on a tiered data plan or even the vast majority of smart-phone customers with unlimited data plans.

AT&T isn't the first to see that unlimited data download packages are going to create problems for their networks. Nearly one-third of mobile operators worldwide have already implemented an application-aware charging model to even out smart-phone access to bandwidth clogging sites such as YouTube, according to a report recently released by researcher Allot Communications. YouTube is the single most popular mobile Internet destination, accounting for 22 percent of mobile data bandwidth usage and 52 percent of total video streaming.

Mobile providers have been looking for ways to reduce the volume of data transferred over their cellular networks rather than invest in expanding their core infrastructures.  This includes promoting portable base stations as a way to create a mini infrastructure that diverts large multimedia downloads from the main cellular network.

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