To use LTE, you need a 4G LTE wireless network. And not everyone will have one.
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Verizon has the largest LTE-only network -- meaning it's in more locations and more people potentially can use it: It claims 370 markets. AT&T is next, claiming 75 markets. Sprint for now is a distant third. All three are rapidly expanding the LTE coverage and capacity.
If you're an AT&T or Sprint (or even a Verizon) customer who is upgrading to an iPhone 5, you may not have LTE service in your area. In that case, your iPhone 5 can use high-performance HSPA+ if available or whatever other 3G connectivity the carrier has for your area.
One possible impact of the LTE iPhone: whether non-LTE subscribers at or near the end of their two-year contracts will switch cellular carriers to get LTE service in their area.
T-Mobile USA will start building its LTE network in 2013. In the meantime, it offers its expanding HSPA+ network (which some consider to be 4G). T-Mobile recently launched the "Unlocked & Unlimited" campaign, persuading users to buy a full-price unlocked AT&T iPhone and switch to T-Mobile's network. But some reports note that the iPhone doesn't support the larger part of T-Mobile's 3G network, so the connection options range from its 2G EDGE service to the HSPA+ service.
A new wide-ranging, real-world test of cellular data performance in the U.S., "The Need for Speed" by RootMetrics, found T-Mobile is delivering very good data performance: The percentage of connections running at over 5Gbps (an indication of "4G" capability) was 46.7% for T-Mobile, compared to 48.1% for AT&T and 77.4% for Verizon. A sign of its much broader LTE footprint, Verizon has a much higher percentage of connections over 15Mbps: 38% vs. 13% for AT&T and 6% for T-Mobile.
RootMetrics is one of several sites that offer tools to evaluate not just coverage but data rates in a given location, from different carriers.
LTE data rates