T-Mobile said today it would upgrade its HSPA+ network to 42 Mbps in 2011: Everyone keeps upping the ante. T-Mobile wants to persuade customers that it has the fastest network out there, and doubling its raw speed for HSPA+ from 21 Mbps to 42 Mbps is a good way to do it. T-Mobile invested in bringing high-rate backhaul to its 3G network (which it wants to call 4G; whatever), and this is how it pays off.

AT&T yesterday said it has HSPA+ everywhere, but its backhaul won't be fully in place at those sites even this year: only 2/3rds of HSPA+ sites will have capable bandwidth in 12 months' time, according to yesterday's AT&T press release.

T-Mobile has never given guidance on the percentage of its HSPA/HSPA+ network that has the necessary backhaul, although it's consistency talked about its intent to build that infrastructure as it developed its green-field 3G network a couple of years ago. (Update: Analyst Charles Golvin of Forrester Research wrote in to say that T-Mobile provided such information in a briefing yesterday: 70 percent of T-Mobile's sites have Ethernet. Ethernet doesn't imply a specific speed, but my understanding is that it's all being installed as gigabit Ethernet. Since a single site can have multiple HSPA+ channels in use, more than 100 Mbps is necessary.)

T-Mobile's upgrades are a defensive move against LTE in 700 MHz. T-Mobile has a relatively small spectrum portfolio in the US, and LTE networks in this country will launch with much wider channels, allowing greater capacity and higher speeds. The 700 MHz frequency range also allows better in-building and in-home penetration than T-Mobile's mobile data frequency allocations.

Put simply: T-Mobile is installing the most advanced current-generation, off-the-shelf equipment that it can to compete with next-generation networks that are barely off the ground (Verizon) or not at all, and will take until 2013 to have a complete footprint. T-Mobile has that edge. But the AT&T and Verizon LTE networks will have substantive advantages over HSPA+ because T-Mobile will need to install equipment at a much higher density than either competitor to achieve the same coverage and capacity.

T-Mobile also still only reaches 200m people with its current mobile data network. AT&T reaches about 50m more, and Verizon has over 95 percent national coverage with its older 3G technology.