Jun 14

Yeah, yeah, you get it: The new iPhone uses AT&T's best-in class (in NYC at least), 3G high-speed network, one that's getting faster and more spread out all the time. But there's a lot you probably didn't know about the technology involved: How fast can you really go on the thing? Why did AT&T feel the need to cap the iPhone's speed? If you want answers to these and other questions, you came to the right place.

Hokay, the iPhone 3Gness makes browsing a whole 2.4x faster than EDGE in Apple's test. (One thing that we can't explain: Why did Apple chose lonelyplanet.com for its performance benchmarks?) The 3G goodness is real: We've been conducting our own testing of AT&T's HSDPA in the New York area (including suburbs) and it really is faster and more readily available than Verizon's EV-DO 3G network. (For a quick primer on different kinds of 3G like HSDPA vs. EV-DO and other mobile terms, click here.) But the iPhone 3G is rated for 1.4Mbps, a nice clip but not the 3.6Mbps downstream that AT&T's HSDPA is capable of. (The carrier loves to brag that it'll have 7.2Mbps by the end of the year.) So why not crank up the iPhone to those better data rates? Turns out, according to AT&T people we talked to, 1.4Mbps is the capped bandwidth for all mobile smartphones on the network for a few reasons.

(UPDATE: AT&T is saying they're not capping the phone at 1.4mbps, but that's what its capable of doing now, due to factors below. There's no difference except intent, and AT&T is careful around words like "Cap" these days.)

A major one is battery life the faster you burn, the faster your battery dies, so going full steam at 3.6Mbps would cut you well short of that nice round five hours. A second one is cell site congestion and backhaul (carrier-speak for size of the wired dataline that connects cell sites to the actual telecom infrastructure). While everyone at AT&T, from the top down, is adamant that AT&T is "comfortable" with their ability to meet the huge data draw once 3G iPhones hit the streets, it's not like the pipe is unlimited.

AT&T wasn't able to give a breakdown as to how many of their towers have fiberoptic pipes as opposed to slower copper T-1 lines. Nor could they say how quickly they could add capacity to a site that is pummeling their demand expectations, since it varies from site to site. Ones in dense urban areas are loaded up with more backhaul and can handle more users than one closer to the edge of their 3G footprint. Still, generally speaking, more users on a site means more congestion, so if you're slurping from a site that's really slammed, it will be slower. As with all radio technologies, proximity also matters. (Hint: For the absolute fastest speeds, wait until 3am and then go sit right next to your favorite cell site.)

Something we missed, or you still wanna know? Send any questions about 3G, GPS, G-spots or anything else to tips@gizmodo.com, with "Giz Explains" in the subject line.