Summary Box: AT&T customers surprised by use limit

SLOW DOWN: AT&T has started cutting the data speeds of some users with “unlimited” data plans as part of an attempt to manage data usage on its network.

ABOUT THE PLANS: AT&T no longer offers unlimited data plans to new customers, but those who already had them could keep them. AT&T has some 17 million customers with such plans, representing just under half of its smartphone users.

THE TOP 5 PERCENT: AT&T has been slowing down speeds of the heaviest data users. AT&T says it only throttles users in areas where the wireless network is congested in a given month.