Current AT&T iPhone 5 will work on T-Mobile LTE, HSPA+ 21 networks

With the news that T-Mobile is throwing its hat into the iPhone ring, current owners of the iPhone 5 may be wondering exactly what their options are in terms of jumping ship from their existing carrier.

Thanks to the alphanumeric soup that is wireless technology, it’s not necessarily a straightforward question to answer. Not only are there different broad speed categories—2G, 3G, and 4G—but there are different technologies within those, and different frequencies and bands within those.

So, here’s the skinny: If you own an iPhone 5 and you’re on AT&T, or any other carrier that uses the GSM-based A1428 model, you’re (mostly) in luck. Assuming that you can convince your carrier to unlock your phone—a tough proposition, given that most iPhone 5 customers are only about six months into a two-year commitment—Apple tells Macworld that you can take your iPhone 5 to T-Mobile and get full LTE speeds on the carrier’s network. The same goes for the unlocked iPhone 5 that Apple sells, which is the same A1428 model.

However, if you’re in an area where T-Mobile hasn’t currently deployed LTE service, the best your phone will be able to do is use T-Mobile’s HSPA+ 21 network, which offers a maximum download speed of 21 Mbps. T-Mobile also has a faster, 42Mbps DC-HSDPA network (which T-Mobile sometimes refers to as HSPA+ 42), but that network is incompatible with the current iPhone 5.

That said, the iPhone 5 model that will be released on April 12 will support the right frequency to use T-Mobile’s DC-HSDPA network; it appears that it will be a modified version of the A1428. So if you want to purchase an unlocked iPhone 5 that can work fully on either AT&T or T-Mobile, your best bet is to wait a couple weeks until the newest iPhone 5 arrives.

Meanwhile, Apple tells us that those customers using a model A1429 iPhone 5, which is the model supported on Verizon, Sprint, and the smaller CDMA-based regional carriers, will only work on T-Mobile’s 21Mbps HSPA+ network, not the DC-HSDPA or LTE networks.

One thing I think we can all agree on: There are way too many “standards” in the wireless industry, and the situation would probably better for consumers if the market was less fragmented. But despite the fact that the phone industry is undergoing some changes right now, it’s probably not worth holding your breath on that front.

Updated at 3:56 p.m. Pacific to correct compatibility for the Verizon iPhone 5.