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HSUPA (also known as Enhanced Uplink) is a 3G protocol that provides uplink speeds up to 5.76 Mbps in the Category 6 flavor supported by iPhone 4. It joins HSDPA (High-Speed Download Packet Access), the enhanced download upgrade to UMTS 3G service that was supported by last year's iPhone 3GS, enabling up to 7.2 Mbps downloads. Because it lacked support for HSUPA, iPhone 3GS was limited to just 384 kbps uploads.
Support for both HSDPA and HSUPA in iPhone 4 makes the phone a "3.5G" device and means it can theoretically achieve 7.2 Mbps downloads and 5.8 Mbps uploads, but those capabilities are also dependent upon the mobile operator.
Jobs: That's 'theoretically' because the carriers don't support it yet
In the US, AT&T's 3G HSDPA primarily maxes out at 3.2 Mbps, with typical speeds ranging from 0.7 to 1.7 Mbps. The company is in the process of deploying faster 7.2 Mbps service, but this is currently limited to just a few cities: Charlotte, Chicago, Dallas, Houston, and Los Angeles and Miami.
In terms of uploads, AT&T's 3G HSUPA network operates with typical speeds ranging from 0.5 to 1.2 Mbps, roughly two to four times faster than 0.3 Mbps theoretical maximum of the non-HSUPA capable iPhone 3GS. Not all of AT&T's 3G network supports faster HSUPA service.
In actual practice, tests reported by Gizmodo which pitted the iPhone 3GS against iPhone 4 found that download speeds measured across several locations in New York City were largely the same, with both ranging from 0.5 to 2 Mbps on both phones.
However, upload speeds were dramatically faster with iPhone 4. Without support for HSUPA, iPhone 3GS was stuck pushing files out at around 0.1 Mbps, while iPhone 4 was able to achieve uploads better than 0.6 to 1.4 Mbps, an improvement of an order of magnitude.
AppleInsider found similar upload speeds available to iPhone 4 in San Francisco: around 1.5 Mbps down and 0.7 Mbps up. That's much faster than before, but nothing near 802.11n WiFi supplied cable broadband, which effortlessly delivers Internet service 8.4 Mbps down and 3.5 Mbps up.
By comparison, Verizon's EVDO Rev A 3G network claims download speeds of 0.6 to 1.4 Mbps, and upload speeds of 0.5 to 0.8 Mbps, with pockets of slower service comparable to AT&T's EDGE. AT&T claims a slight edge in download speeds but significantly faster uploads. T-Mobile's 3G network, although limited in scope, is 7.2 Mbps. Sprint's highly touted WiMAX "4G" network claims average speeds of 3 to 6 Mbps downloads and 1 Mbps uploads, but is similarly only available in a few locations.
Outside the US, several mobile operators in Canada, Europe, Asia, and Australia have been supporting 7.2 Mbps or faster HSDPA for more than a year (with some offering even faster 14.4 Mbps or better service) and 1.4 to 5.8 Mbps HSUPA speeds. Even faster HSPA+ networks have started offering 21.6 Mbps downloads over the past year.