Two smaller wireless companies, T-Mobile and MetroPCS, announced unlimited-data offerings this week as they try to compete with AT&T and Verizon. 

It marks a comeback for unlimited wireless data, after the two largest companies stopped signing up customers for such plans.

Here’s a look at unlimited data plans offered or once offered by the five largest U.S. wireless carriers:

— Verizon Wireless. The nation’s largest wireless carrier stopped offering unlimited plans to new customers last year, though those already on one can keep it. Verizon is now trying to wean those people off unlimited plans, as it introduced new plans this summer that let customers share a pool of data among multiple devices. Existing Verizon customers can keep older plans, but those with unlimited-data plans for their smartphones won’t be able to move those to new phones, unless they pay the full, unsubsidized price for those phones — typically hundreds of dollars more.

— AT&T Inc. The No. 2 carrier stopped offering unlimited plans to new customers two years ago, though those already on one can keep it. It, too, introduced shared data plans this summer. For those still on unlimited plans, AT&T sometimes slows down speeds drastically once a customer hits a certain level of usage in a month.

— Sprint Nextel Corp. The third-largest wireless company already has an unlimited data plan and credits that with helping it attract customers for its smartphones.

— T-Mobile USA. The nation’s fourth-largest cellphone company said Wednesday that it will start selling an unlimited-data plan again on Sept. 5, after stopping sales of such plans early last year. T-Mobile had been calling all its data plans “unlimited,” but once a customer hits a certain level of usage in a month, it slows down speeds drastically. Under T-Mobile’s new unlimited plans, all data will be at maximum speed.

— MetroPCS Communications Inc. The No. 5 cellphone company announced a price cut on its unlimited plan Tuesday. The plan will cost $55 per month if a customer signs up during a promotion period, down from $70. The plan includes unlimited texting and calls.