The Federal Communications Commission (FCC) asked Verizon Wireless on Wednesday to explain its intentions to slow downloads for customers who are still using older unlimited data plans.

Verizon announced in a blog post that the top five percent of data users of the unlimited data plans would start experiencing slower speeds during high demand hours to optimize network traffic in the area.

"The top 5% with devices on unlimited data plans may experience managed data speeds when connected to a cell site experiencing high demand after reaching certain data-usage levels in a bill cycle," wrote Verizon.

The telecom giant added that it decided to implement the so-called "Network Optimization" to prevent congestion for the rest of the users. It also clarified its difference from "throttling" which reduces the data's download speed during the entire bill cycle; with the new policy, speed is reduced only during high demand hours. Those who would like to avoid the new policy have been advised to switch to a usage-based data plan.

But FCC chairman Tom Wheeler expressed that he is "deeply troubled" with this new policy, which will begin this coming October, Reuters reported. He wrote a letter to Verizon asking to explain the need to reduce the data speed of some users - a move that that reportedly violates existing FCC regulations.

"It is disturbing to me that Verizon Wireless would base its 'network management' on distinctions among its customers' data plans, rather than on network architecture or technology. ... I know of no past Commission statement that would treat as 'reasonable network management' a decision to slow traffic to a user who has paid, after all, for 'unlimited' service," Mr. Wheeler wrote in a letter to Verizon Wireless.

Verizon told the Wall Street Journal that it hasn't received the letter yet; therefore, it cannot comment on the issue at this time.

"We will officially respond to the Chairman's letter once we have received and reviewed it," Verizon Wireless said in a statement. "However, what we announced last week was a highly targeted and very limited network optimization effort, only targeting cell cites experiencing high demand. The purpose is to ensure there is capacity for everyone in those limited circumstances, and that high users don't limit capacity for others."