While the A1688 and A1687 are also marked as GSM or CDMA, the models are actually identical: they are activated for one or the other, but can be used on either kind of network. CDMA doesn’t rely on a SIM (Subscriber Identity Module) like GSM phones, and carriers have more control over activation. However, all new “CDMA” iPhones include a GSM SIM slot, and have since 2011.
Strike up the Band 30
The only difference between the AT&T and “everyone else” models is support for LTE Band 30, which occupies frequencies starting at 2300MHz (2.3GHz). (The cell industry numbers LTE bands in addition to the frequency range for clarity and succinctness, because of the many possible configurations of LTE.) This is a fresh swath of spectrum for AT&T that it plans to roll out over time. In early September, AT&T spoke to FierceWireless and other publications about its plans to add capacity in urban areas using this bandwidth range, which it acquired in 2012 and continues to purchase more of. (Macworld has a query out to AT&T for additional detail.)
As of early September, only two markets had Band 30 switched on, and AT&T hasn’t clarified how quickly they plan to add service in more regions. Rather than offering better LTE rates, Band 30 will reduce congestion, according to the company’s earlier remarks. AT&T previously deployed LTE over 700MHz, which was abundant after being retired from use in analog UHF television broadcasts. This lower-frequency band penetrates buildings and can span greater distances.
However, in cellular networks, having smaller coverage cells allows greater density. The higher-frequency 2.3GHz band works better among buildings outdoors, as well as in corporate, convention center, and airport indoor deployments.
Something on the order of 100 million phones in the U.S. are equipped with LTE and lack Band 30 support, so while it’s important for future models, AT&T is expanding into this new frequency. Having a phone capable of it on AT&T’s network may allow you to have consistently higher throughput. Only a handful of new phones support it, including the Samsung Galaxy S6 and S6 Edge.
If you purchase a SIM-free iPhone from Apple, you get the A1633 or A1634, and thus are future-proofed against using it on AT&T or networks in other countries that could obtain the same frequencies.
As to why Apple would manufacture two sets of ostensibly almost identical phones that they sell at the same price? There may be a very slight cost per unit difference in the radio baseband chips required to add Base 30 support, but unless it’s significant, it would seem like more trouble to Apple to maintain two similar lines. (Apple declined to comment on that.)
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