By Marina Lopes and Alina Selyukh WASHINGTON (Reuters) – Verizon Communications Inc defended its decision to slow data downloads for some customers using older unlimited data plans, telling U.S. regulators it was a widely accepted and lawful part of network management. In a letter dated Aug. 1, Verizon responded to U.S. Federal Communications Commission Chairman Tom Wheeler, who last week wrote Verizon Wireless Chief Executive Dan Mead to say he was deeply troubled by Verizons plans. In July, Verizon said high speed wireless customers who subscribe to the companys legacy unlimited data plans might experience slower speeds starting Oct. 1. The type of network optimization policy that we follow has been endorsed by the FCC as a narrowly targeted way to ensure a fair allocation of capacity during times of congestion, Kathleen Grillo, senior vice president of federal regulatory affairs at Verizon, said in the letter.