Sprint today said it will throttle speeds for "unlimited data" customers after they use 23GB in a month. Sprint noted that what it calls "prioritization" will apply "only in times and locations where the network is constrained," so customers should still be able to use their phones at normal speeds most of the time.

The policy appears to affect only customers who sign up for unlimited data plans from now on and customers who "upgrade their handset on or after October 16 and remain on an existing unlimited data plan." The throttling will apply for the remainder of the monthly billing cycle after the customer uses 23GB. While customers may have their data rates slowed, paying for unlimited data allows them to escape overage charges.

Sprint CTO John Saw explained that the system "is intended to protect against a small minority of unlimited customers who use high volumes of data and unreasonably take up network resources during times when the network is constrained." About three percent of postpaid subscribers are using a disproportionate amount of data, he wrote.

Sprint says it measures cell site performance in real time, with "prioritization" being "applied or removed every 20 milliseconds."

"[P]erformance for the affected customer returns to normal as soon as traffic on the cell site also returns to normal, or the customer moves to a non-constrained site," Saw wrote.

Sprint had previously dropped a similar throttling policy to avoid potential violations of the Federal Communications Commission's net neutrality rules. Sprint also eliminated a policy that slowed video speeds for unlimited data customers. But while the FCC has pressured carriers to end throttling of unlimited data plans, throttling in general is allowed in cases of "reasonable network management." Even when Sprint previously backtracked on throttling, it said that it believed its policy would have been allowed under FCC rules.

T-Mobile also throttles unlimited data customers in congested areas after they use 23GB per month, while AT&T does so after 22GB per month. Verizon recently stopped throttling the few customers on its network who still have unlimited data.