Consumer advocates are upset (they always are) about AT&T's planned buyout of T-Mobile, announced Sunday, saying it will reduce choice and lead to higher costs. AT&T is spinning the acquisition as a way for it to become more efficient and to roll out next-generation 4G networks that much faster (they always say that).
In practice, the buyout will have little real effect, other than reducing the number of ads for cell phone service once (if) the deal passes regulatory hurdles. The acquisition makes a lot of sense at one technical level, given that AT&T and T-Mobile use the same basic cellular technology, GSM, for their networks. Earlier rumors of a Sprint/T-Mobile tie-up made little sense because they use incompatible network technologies (CDMA and GSM, respectively).
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You might think that the shared use of GSM means that AT&T can more easily expand its reach in areas where T-Mobile has better service than AT&T or help augment AT&T's overloaded networks in places like New York and San Francisco. But that's not necessarily the case and not immediately for smartphone and tablet users.
Although both carriers use GSM, they have different radio frequencies in the spectrum they license from the U.S. government. In practice, this means that voice calls can roam between AT&T and T-Mobile (which they already do), but not 3G data connections. Their phones work on each other's voice bands, but not each other's data bands. That's why you can't use an iPhone's data services on the T-Mobile network, even if you have a T-Mobile SIM card on an unlocked iPhone. Ditto for an iPad or any Android or BlackBerry models designed for AT&T's network.
Over time, AT&T could use T-Mobile's cellular infrastructure -- the towers and other antenna locations that T-Mobile owns or leases -- to add 3G and 4G data coverage to areas that T-Mobile serves better than AT&T. And it could increase its coverage density by upgrading the hardware in some of those locations, augmenting nearby AT&T infrastructure.