Verizon Wireless warned it will slow down the data connection of its most-active users when its network is stressed, a new policy that comes as the company prepares to add Apple Inc.'s iPhone 4 to its network.

The biggest U.S. carrier by subscribers hasn't previously taken such measures to manage demand on its network, a process known as throttling. It is one of several ways carriers are dealing with the dilemma of offering unlimited data while managing the costs associated with carrying that traffic on their networks.

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