Verizon Wireless has told FCC Chairman Tom Wheeler that its policy of throttling unlimited data users on congested cell sites is perfectly legal and necessary to give heavy data users an incentive to stop using their phones so much.

Wheeler had sent a letter to Verizon accusing the company of throttling unlimited data users in order to make more money, presumably by encouraging users to purchase new data plans. "'Reasonable network management' concerns the technical management of your network; it is not a loophole designed to enhance your revenue streams," Wheeler wrote. Wheeler didn’t argue that throttling itself is never reasonable, but he called it “disturbing” 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," Wheeler added.

Verizon no longer sells unlimited data plans to new customers, though some customers still have them. Newer and pricier data plans with limits and overage charges aren’t throttled under this policy, even if customers are connected to congested cell sites and use just as much data as those with "unlimited" plans.

The policy, Verizon explained to Wheeler, “is narrowly tailored to apply (1) only at particular cell sites experiencing unusually high demand; (2) only for the duration of that high demand; and (3) only to a very small percentage of customers who are heavy data users and are on plans that do not limit the amount of data they may use during the month without incurring added data charges (and otherwise have no incentive to limit usage during times of unusually high demand)—and then only when the particular cell site serving those customers is subject to unusually high demand.”

The throttling is reasonable because it prevents heavy users from using so much data that other users get bad service, Verizon wrote. Verizon Senior VP Kathleen Grillo sent the company’s response (PDF) on Friday, and a Verizon spokesperson provided a copy to Ars today.

Verizon further explained that "a small percentage of the customers on these [unlimited] plans use disproportionately large amounts of data, and, unlike subscribers on usage-based plans, they have no incentive not to do so during times of unusually high demand. Rather than an effort to 'enhance [our] revenue streams,' our practice is a measured and fair step to ensure that this small group of customers do not disadvantage all others in the sharing of network resources during times of high demand."

They all do it, in various ways

The throttling policy applies to the top five percent of data users and is similar though not identical to policies implemented by AT&T, Sprint, and T-Mobile. T-Mobile says it throttles customers in the top five percent of users in each rate plan, while Sprint says it throttles the top five percent of all users.

AT&T is similar to Verizon in that it reserves its throttling for unlimited data users. But there’s one key difference: AT&T says that speeds are only affected for one billing cycle. Verizon applies its policy to the current “billing cycle and the following cycle." All of these policies apply only to congested cell sites.

Verizon has to follow different rules from the other carriers as a result of purchasing spectrum with special requirements. Wheeler asked Verizon how it can justify its throttling “consistent with its continuing obligation under the 700 MHz C Block open platform rules, under which Verizon Wireless may not deny, limit, or restrict the ability of end users to download and utilize applications of their choosing.”

Verizon’s response:

With network optimization, our customers continue to be free to go where they want on the Internet and to use the applications, services and devices of their choice. Although the policy may result in slowed throughput under the very limited circumstances described above, neither the C Block rules nor the Open Internet rules requires any particular minimum speeds, so long as providers are transparent with their customers. And here, Verizon Wireless is clearly apprising our customers that under certain circumstances, the speeds of a few heavy users may be temporarily slowed at congested cell sites in order to provide a great wireless experience to all of our customers.

Verizon also pointed out that the FCC's Open Internet Order "endorsed precisely this type of practice, using wireline cable modem service as an example." The FCC said in the order that "we agree that congestion management may be a legitimate network management practice. For example, broadband providers may need to take reasonable steps to ensure that heavy users do not crowd out others... For example, if cable modem subscribers in a particular neighborhood are experiencing congestion, it may be reasonable for a broadband provider to temporarily limit the bandwidth available to individual end users in that neighborhood who are using a substantially disproportionate amount of bandwidth."

The FCC isn’t necessarily limiting its scrutiny to Verizon. Wheeler “sent the letter to Verizon in light of their recent announcement, but our concerns are about the practice at issue, not one particular provider,” an FCC spokesperson told Ars. “We’re looking at whether other providers are engaging in similar practices.”

The FCC is considering a new set of net neutrality rules to replace its 2010 order, which was mostly struck down in court. But the FCC's current proposal largely exempts wireless carriers from rules that would apply to fixed broadband providers.