Heavy demand for upload capacity from the iPhone 4 has exposed a flaw in the software for Alcatel-Lucent's 3G network equipment, temporarily forcing lower upstream speeds for some AT&T subscribers.

Alcatel is working on fixing the bug and expects to know soon when it will be fixed, according to company spokeswoman Mary Ward. The flaw did not cause problems until the introduction of the iPhone 4, which comes with features such as high-definition video that can require a fast connection from the phone up to the network, she said. Downstream performance is not affected.

"It came to the forefront after the iPhone 4 launch because of the big increase in the uplink data," Ward said.

The problem affects devices that use HSUPA (High-Speed Uplink Packet Access) technology, the fastest upstream system AT&T uses. In addition to the iPhone 4, that includes AT&T's LaptopConnect cards for PCs and netbooks. However, because the problem only exists in areas where AT&T uses Alcatel equipment, it affects only about 2 percent of the carrier's mobile subscribers, said AT&T spokesman Mark Siegel.

While the bug is being fixed, AT&T is providing regular 3G upstream speeds instead of HSUPA, but the carrier will not shut down service in the course of solving the problem, he said.

iPhone 4 users commenting on some message boards have said their upstream speed in the past few days has fallen below 100K bps (bits per second) from more than 1M bps. Some accused AT&T of capping iPhone users' upload speeds.

The problem is not affecting other carriers that use Alcatel's HSUPA equipment, according to Ward.