GSM 1900 vs. HSDPA 1900? What's the difference?

[Nexus4Buyer]

In my area I can get HSPA+ on the 1900mhz frequency from T-Mobile. I want to make sure the phone I purchase can connect to their HSPA+ network.

Comparing specs, I noticed that some phones list 1900 but only under GSM 2G and not under HSDPA 3G (example). Other phones, like the Nexus 4 have GMS and HSDPA 1900 listed. What is the difference? I was under the assumption that a phone simply supports the 1900 band or it doesn't. Can a phone only support 1900 for 2G and not HSPA+? I don't really understand how this all works out.

 

[IAmSixNine]

GSM and HSDPA / HSPA+ are different protocols.
The frequency they use can be the same but the sofware used to interpert them is not.
So you need support for the frequency and the protocol in order for this stuff to work properly.
Kind of like a CDMA phone not working on a GSM network. They can use the same band but cant work on one anothers network due to the protocol used to decode the info.

 

[Nexus4Buyer]

GSM and HSDPA / HSPA+ are different protocols.
The frequency they use can be the same but the sofware used to interpert them is not.
So you need support for the frequency and the protocol in order for this stuff to work properly.
Kind of like a CDMA phone not working on a GSM network. They can use the same band but cant work on one anothers network due to the protocol used to decode the info.

Thanks, and that leads me to a follow-up question. One of the phones I'm looking at is the Sony Xperia Ion, and it has two versions the AT&T LTE version and an HSPA version. They both support HSDPA 850, 1700, 1900, and 2100 yet under "Speed" lower on the page for the LTE version it says HSDPA 21.1 Mbps while the HSPA version says HSDPA 42 Mbps. I assume the LTE version is referring to AT&T's HSPA+ speeds which aren't as fast as T-Mobile's HSPA speeds probably represented by the HSPA version. Although if I get the LTE version unlocked, shouldn't I be able to get T-Mobile's fastest speeds because it supports the right HSDPA protocols? (I'm asking this because the LTE version sells for around $100 less)

 

[IAmSixNine]

I have yet to notice a difference on my Nexus 4, GS3 or Note 2 on TMO / vs the ATT One X and One X+ unlocked.
The One x has similar specs for HSDPA 21. The big thing you want to look for is 1700mhz frequency band. With that you will get the faster back haul that TMO has.
HSDPA 21 and 41 are not realistic speeds, but maximum speeds in a controlled environment. In the real world ive gotten between 2-20 megs down on TMO.. And in a few rare instances 23 or so..
You should be fine with a device that supports HSDPA 21 but if someone else can either debunk me or back me up on this id like to hear that as well.

Heres food for thought, if the LTE devices supports TMOs LTE then i might spring for that device and hope your area gets LTE one day.
Theres also this article by TMONews that points to a Sony Xperia Z coming to the carrier. Is Sony Set To Finally Return To T-Mobile's Lineup? Magic 8-Ball Says "Signs Point To Yes" | TmoNewsTmoNews