This is normally Brian's beat but with him busy putting the finishing touches on his review of HTC's One, I thought I'd help out. We're still seeing (and hearing) a lot of confusion about what T-Mobile announced earlier this week with regards to existing and future iPhone support on its network. Brian already went through all of this in his excellent article on the topic, but seeing continued confusion I thought I'd whip up a few diagrams to help explain.

For the purposes of this article I'm focusing on compatibility for the current AT&T iPhone 5 (hardware model A1428) as well as the new unlocked iPhone 5 (also hardware model A1428) that will be shipping start April 12.

The easiest question to answer is will existing AT&T iPhone 5s that have been unlocked work on T-Mobile's recently deployed LTE network. The answer is an emphatic yes. The original AT&T iPhone 5 was designed to work on LTE band 17 (700MHz) and band 4 (1700MHz), a superset of T-Mobile's LTE deployment (band 4). If you're in one of the few areas with T-Mobile LTE service and an existing unlocked AT&T iPhone 5, the combination will work just fine. Apple will need to release an updated carrier bundle (.ipcc file) for the phones, which I assume is coming soon - but there's no hardware change required.

The new unlocked iPhone 5 that will be available via T-Mobile doesn't add any additional functionality in this case. As you can see, both A1428 revisions support the same LTE bands.

Where it gets somewhat complicated is in the 3G WCDMA discussion. I emphasize somewhat because it's really not that hard to understand. The complexity comes from the fact that there are a number of names and acronyms here that aren't well understood by most who aren't of Klug-descent. If we focus on the frequency bands themselves and ignore their common names, things are a bit easier to understand.

The original AT&T iPhone 5 supported 3G operation on band 5 (850MHz) and band 2 (1900MHz). Only band 2 overlaps with T-Mobile's network. The problem with 1900MHz on T-Mobile is that the majority of that spectrum is used for 2G and hasn't yet been migrated over to 3G. The bulk of T-Mobile's 3G currently exists in band 4 (1700MHz uplink, 2100MHz downlink), which isn't supported on the existing AT&T iPhone 5.

After April 12th, the new unlocked A1428 iPhone 5 with band 4 WCDMA support will begin rolling out and should have much better coverage on T-Mobile's 3G network as a result. The diagram and toggles below help illustrate this:

The original A1428 iPhone 5 lacks band 4 support, which means it'll only support WCDMA on band 2. The only problem here, as I mentioned above, is that T-Mobile's 3G deployment on band 2 isn't ubiquitous - so in many cases you'll fall back to 2G/EDGE speeds. The new iPhone 5 simply enables band 4 WCDMA support.

There's one other benefit to the new iPhone 5. DC-HSPA+ (42Mbps max downlink) is now supported on all bands as well. Although it was never (and likely will never ever be) used by AT&T, DC-HSPA+ was a feature of the iPhone 5. T-Mobile on the other hand does use carrier aggregation on WCDMA in some markets and the new A1428 will benefit from higher speeds in those situations. 

That's it.

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  • DryCty - Friday, March 29, 2013 - link

    The problem is this does nothing to address the real question: Why is Apple not allowing existing A1428's to receive a modem firmware update to support AWS?
  • danstek - Friday, March 29, 2013 - link

    Since all original A1428 models (both unlocked and At&t units) are being phased out and replaced with the new A1428, I would agree with Brian's assumption that there's some technical issue involved that has to take place at the factory. I honestly doubt that Apple would do something so nonsensical as to alienate the same hardware model in this manner if there wasn't a technical issue. I do agree that it definitely sucks for anyone trying to distinguish the new/old A1428 if they're buying second-hand off eBay or Criagslist. We just have to hope that Apple will provide a way to clear up the ambiguity rather than retcon the ordeal.
  • darwinosx - Friday, March 29, 2013 - link

    Apple isn't doing this to sell more iPhones. Its a hardware issue.
  • danstek - Friday, March 29, 2013 - link

    That's what I just said...?
  • darwinosx - Friday, March 29, 2013 - link

    The real question is why you are uninformed as to think this is possible.
  • DryCty - Friday, March 29, 2013 - link

    darwinosx: Why don't you explain it to us? You seem to have the time to post offensive comments on my site so please do share your infinite knowledge.
  • FalcomPSX - Friday, March 29, 2013 - link

    if they are really so different, why not release it as a new model # to avoid the confusion?
  • ATimson - Sunday, March 31, 2013 - link

    Other devices allow their broadband firmware to be updated - if iPhones can't, then they seem to be unique in that poor design.
  • Devfarce - Friday, March 29, 2013 - link

    The reason that Apple isn't enabling these features on the existing handsets is a licensing/IP issue. If you think back to the Samsung/Apple lawsuits, Samsung would countersue based on 3G patents. Every radio mode that is actually used in the phone is patented many times over has to be licensed and Apple simply didn't pay for the unused technology that it didn't need even though the hardware was technically capable of these modes. And they aren't going to back pay for every iPhone 5 already in the market so that's how that works. I wish Anand would touch in on this a little more in depth so more people knew why they won't be enabling these features on existing phones.
  • DryCty - Friday, March 29, 2013 - link

    Devfarce: Your theory is thoughtful, which I appreciate. However, I am dubious about your arguments validity in this case. Apple was/is utilizing nearly every conceivable frequency and cellular technology supported by the Qualcomm MDM9615 in the current A1428 and A1429. Therefore it seems improbable that a company with Apple's legal department and scale would leave anything uncovered from a licensing standpoint.

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