Sprint Nextel Corp. (NYSE: S) began turning on its new wireless data network Monday, providing a much-needed boost to the carrier, whose data speeds lag those of its largest competitors.

The news triggered a boost in Sprint’s stock price, too.

Shares in the third-largest U.S. wireless carrier rose to the highest level since September after Credit Suisse AG boosted its price target for the stock, citing its network upgrade and cost management. Sprint advanced 2.6 percent to $3.55 in trading early Tuesday after earlier touching $3.64, the highest price since Sept. 1.

The shares have risen 48 percent this year through Monday.

The Overland Park, Kansas-based company is upgrading its system to long-term evolution, or LTE, and said in a statement yesterday that it now has service in six cities and partial service in 15 additional areas.

“The company has built in some cushion in different areas on the cost side,” Jonathan Chaplin, a New York-based analyst for Credit Suisse, said in a telephone interview. The drag on costs from the Network Vision upgrades “will be less than everyone thought it would be,” Chaplin said.

Looking ahead to 2014, Chaplin estimates that Sprint will post earnings before interest, taxes, depreciation and amortization, or Ebitda, of $8.1 billion. He boosted his target price for the stock to $6 from $4.

Helping to reduce those costs, said Chaplin, is the falling prices of phones as Sprint moves away from the more expensive WiMax models toward LTE devices. Chaplin also said Sprint will see an increase in sales of iPhone 4S phones, which he expects will be priced lower when the iPhone 5 arrives later this year.

This will help “offset the higher subsidy costs” Sprint is facing with the new iPhone,’’ he said.

Sprint fired up 4G, network in Atlanta, Dallas, Houston, Kansas City, San Antonio and some smaller surrounding cities on Sunday. It uses LTE technology already in use by Verizon Wireless and AT&T Inc.

Four smartphones and one mobile-hotspot device are capable of using the network.

PC Magazine got early access to the network a month ago, and found that it’s not quite as fast as Verizon’s and AT&T’s equivalents, but a big step up from Sprint’s older 3G, or third-generation, network and the 4G access it rents from Clearwire Corp.

Sprin doesn’t have as much available spectrum, or space on the airwaves, as the larger carriers do. That holds back its LTE speeds somewhat.

Verizon and AT&T also have much wider coverage on their LTE networks. Verizon covers 304 markets, or two-thirds of the U.S. population.

Sprint is planning to expand LTE coverage this year and largely complete the network buildout next year. It’s planning to supplement its own LTE network with that of Clearwire, which has yet to fire up.

Sprint had said that Baltimore would also be among the first cities to get LTE, but it was missing from the initial lineup.

(The AP and Bloomberg contributed to this report.)