Beginning Saturday, telecom giant AT&T will start "throttling," or slowing down, the bandwidth of some of its most loyal "unlimited" mobile data subscribers, which the network blames for their service's overall poor performance.

The company announced its intent back in July, and accused their most frequent users of degrading service for other subscribers. AT&T called these subscribers the "top 5 percent" of data users, insisting that their new policies will not affect "the vast majority" of users.

Although AT&T has not offered their "unlimited" data plan to new customers for some time, many smartphone users still have contracts from 2009 or before, which stipulate that their "unlimited" subscription continue.

Other mobile phone providers moving to "throttle" their top users' bandwidth include T-Mobile, Verizon and Virgin Mobile.

T-Mobile and Sprint-based WiMAX mobile broadband provider Clearwire have both been sued in the last year for advertising their bandwidth subscription plans as "unlimited," then slowing down network speeds once a user has downloaded more than whatever the provider has chosen to allot them.

For AT&T "unlimited" mobile data users, a series of warning messages will be sent to users who are approaching their bandwidth cap. Once it is crossed, the network will slow download speeds to that account for up to a month. The company said there will be a "grace" period as well, but it's unclear how that will be applied.

The company suggested that users remedy the problem of restricted download speeds by turning their phones over to WiFi mode, relying on an Internet connection other than the one the subscriber is paying for. AT&T added that it has installed over 26,000 WiFi hotspots all over the country that smartphone users can take advantage of free of charge, but in order to use the service subscribers have to visit a specific location and log in through a special online portal.

Photo: Flickr user Nico Kaiser.