When the iPhone 3GS arrived last month, I explained the ins and outs of High Speed Packet Access (HSPA), the rubric that covers AT&T’s two highest-speed cellular data standards: HSDPA for downstream and HSUPA for upstream. It turns out that the iPhone 3GS is more limited than what I outlined in that article.

On the downstream side, all is well. The iPhone 3G handles up to 3.6 Mbps HSDPA, and the 3GS can use the newer 7.2 Mbps HSDPA flavour. AT&T is building out the 7.2 Mbps service, which will start being available in some metropolitan areas later this year.

I had supposed that Apple took the opportunity to build HSUPA on the upstream side, at either 1.4 or 1.9 Mbps speeds that are supported in many European networks that have already rolled out 7.2 Mbps HSDPA.

But it turns out, Apple didn’t. (As Jason Snell has written, Apple doesn’t like to offer much detail about the iPhone’s internal specs.)

After my HSPA article ran, reader Nick Dunklee pointed out in email that a teardown at RapidRepair of an iPhone 3GS shows that it has a UMTS/HSDPA chip. UMTS is the earliest 3G standard deployed on GSM networks, and it tops out at 384 Kbps.

It’s easy to test, if you have an iPhone 3GS. Go to any speed tester, like Testmyiphone when you’re outdoors with a good signal. Downstream, you might hit well over 1 Mbps; upstream, under 384 Kbps.

Dunklee examined the specs on a number of GSM network smartphones, and found none included HSUPA. It’s possible that there could be a firmware update from UMTS to HSUPA, but that’s unlikely. There’s usually a reason for using an older standard, which is related to power consumption, chip size, or cost.

In contrast, Dunklee noted, phones that handle EVDO Rev. A - the 3G standard used on CDMA networks like those operated by Sprint Nextel and Verizon Wireless - have the full high-data-rate upload speed. EVDO comes in multiple flavors (Rev. 0 and Rev. A), but phones that support Rev.

A for downstream access also support it for upstream. (Verizon and Sprint also have 3G more extensively deployed than AT&T due to network design as well as long-term infrastructure building that AT&T deferred at least in part until recently.)

It’s a shame that the iPhone won’t be able to send video and photos at the faster rates that laptops with 3G cards on the same network can. But I suppose this offers a new marketing bullet point for a future iPhone model - now, with HSUPA!

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