Sprint's (NYSE: S) pledge of unlimited data is looking increasingly less sustainable, as the carrier maneuvers to rein in network traffic generated by its heaviest data users and ensure quality of service for the majority of its customers.

Postpaid as well as prepaid Sprint customers, including those on its Virgin Mobile USA and Boost Mobile sub-brands, have begun receiving notices alerting them to new data "prioritization management" the operator will employ as of next month to prevent network congestion.

For example, a text message sent to Virgin Mobile customers said: "Beginning 6/1/14, to provide more customers with a high quality data experience during heavy usage times, Virgin Mobile USA may manage prioritization of access to network resources in congested areas for customers within the top 5 percent of data users."

Prioritization can include a reduction of throughput or speed for those targeted users connected to congested sites.

The approach "will enable us to provide more customers with a high quality data experience during heavy usage times," Sprint said in a statement sent to FierceWirelessTech. "Once the customer is no longer connected to a congested cell site, or the site is no longer congested, speeds will return to normal."

Prioritization represents an evolution in Sprint's approach to handling a growing data onslaught. "The practice is not the result of a sudden spike; rather, data usage has been increasing on a steady and increasingly steep basis for years and we expect the trend to only continue in years to come as more customers use mobile applications and as those applications grow in complexity and capability," said Sprint spokesman Lloyd Karnes.

In duplicate FAQ pages on the Sprint, Virgin and Boost websites, the network operator contends the heaviest data users consume a disproportionate share of network resources. Sprint said it has employed "fairness algorithms" on its CDMA and LTE networks to "dynamically allocate available bandwidth in a way that is fair to all users."

Regarding its WIMAX network, Sprint said it may periodically measure a user's bandwidth usage as well as overall bandwidth usage for all users on a segment or sector and the make temporary adjustments in available network resources as needed.

One problem for customers under the Sprint's prioritization scheme is that there is no exact amount of data consumption that will place them within the top 5 percent of users in any given month, and the threshold will change monthly as demand changes. But Sprint said customers who typically use 5 GB or more in a given month will likely be in that uppermost tier.

A customer who slips into that group will be subject to network prioritization during the following month of service. "Customers that continue to fall within the top 5 percent of data users will continue to be subject to prioritization," Sprint said.

Boost and Virgin have throttled users' speeds down to 256 kbps since spring 2012 if they exceed 2.5 GB of data usage in a monthly cycle. In March, Virgin and Boost customers were told that starting this month those using more than 2.5 GB of data would have their data speeds reduced to 128 kbps.

Customers can top up to re-start their monthly billing cycle if they want to access faster 3G CDMA or LTE speeds. This month, Boost also unveiled a $60 monthly plan that includes 5 GB of data before throttling.

By throttling its heaviest users to control congestion, Sprint is following the lead of the other three U.S. national mobile carriers.

During September 2011, Verizon Wireless (NYSE: VZ) implemented what it termed a "network optimization" plan to limit the bandwidth for the operator's top 5 percent of 3G smartphone users who are on a grandfathered unlimited data plan.

One month later, AT&T Mobility (NYSE: T) instituted a similar plan, targeting the top 5 percent of users on unlimited plans in specific high-traffic locations. However, AT&T was forced to alter its approach in early 2012 after an outcry from users who were unprepared to have their speeds reduced, particularly in cases where some of them had only consumed 2 GB of data. AT&T's revised policy slowed speeds of unlimited data users who exceeded specific data thresholds.

T-Mobile US (NYSE:TMUS) also uses a form of prioritization, noting "certain T-Mobile plans may be prioritized" over service plans under its GoSmart Mobile prepaid brand.

For more:

- see this Sprint FAQ, Virgin FAQ and Boost FAQ

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