BARCELONA -- Mobile World Congress -- T-Mobile US Inc. is once again trying to outdo GSM rival AT&T Inc. (NYSE: T) in the 3G wireless data speed race in the U.S.

This morning -- here under overcast skies -- the fourth largest mobile carrier in the U.S. announced that it has completed the 7.2-Mbit/s High-Speed Packet Access (HSPA) 3G upgrade to its network and is now working on having "broad national deployment" of HSPA+, which offers top download speeds of up to 21 Mbit/s, by the end of 2010.

AT&T said recently that it has finished deploying 7.2-bit/s HSPA software across its network and is now building backhaul to support it. Verizon Wireless , meanwhile, is promising 5- to 12-Mbit/s downloads over its new Long Term Evolution (LTE) network in select markets by year's end, and Sprint Corp. (NYSE: S) is offering similar speeds through Clearwire LLC (Nasdaq: CLWR)'s mobile WiMax network. (See AT&T to Spend $2B More on Wireless in 2010 and MWC 2010: Verizon on Track for LTE in 2010.)

T-Mobile says its HSPA 7.2-capable network now covers 271 cities in the U.S. The carrier was the first U.S. operator to launch HSPA+ in Philadelphia in September last year. It is planning to launch markets "on both coasts" this year before the widespread deployment is completed.

This potentially means that T-Mobile could be the fastest 3G network in the U.S. toward the end of 2010. AT&T has so far said that it plans to move to LTE in 2011 and skip HSPA+ altogether, although it has acknowledged that things could change if LTE gets delayed for whatever reason.

Of course, the maximum speeds promised on all these new deployments are only likely to be approached when a user is on an unloaded network standing very close to the cellsite. In the real world, HSPA 7.2 users get 3- to 4-Mbit/s data throughput on the downlink while stationary, less when they are moving. (See WiMax & LTE Meet the Real World.)

There is one potential fly-in-the-ointment with all these new T-Mobile upgrades as well. The operator says that, like AT&T, it is currently building out backhaul to support the HSPA upgrades. We have already seen what faster data downloads from the iPhone have done to AT&T's network performance in New York and San Francisco before the operator started upgrades on the radios and the back-end.

T-Mobile, however, has far fewer users on its network -- not quite 33 million subscribers, compared to more than 78 million on AT&T Mobility. This may mean that the network won't get as horribly slammed by data downloads when HSPA+ arrives.

— Dan Jones, Site Editor, Light Reading Mobile