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A team at the University of Michigan and Microsoft Research has uncovered, for the first time, the frequently suboptimal network practices of more than 100 cellular carriers.

By recruiting almost 400 volunteers to run an app on their phones that probes a carrier’s networks, the team discovered, for example, that one of the four major U.S. carriers is slowing its network performance by up to 50 percent. They also found carrier policies that drained users’ phone batteries at an accelerated rate, and security vulnerabilities that could leave devices open to complete takeover by hackers.

For decades, researchers have studied “middleboxes”—the network hardware that Internet service providers (ISPs) use to ferry packets of data from one endpoint to another. But the current work, by Zhaoguang Wang of the University of Michigan and colleagues, titled An Untold Story of Middleboxes in Cellular Networks, is the first significant attempt to apply this kind of research to cellular networks worldwide.

To gather data on so many networks, the researchers released their testing tool, NetPiculet, on the Android app marketplace. Volunteers downloaded the app, which ran a series of tests and sent the results back to the engineers who created it. It’s not the first time researchers have relied on everyday users to help gather data, but it’s one of the most elaborate testing suites ever used in an experiment of this kind. “We released NetPiculet on the Android Market in January 2011 and attracted 393 unique mobile users within merely two weeks,” says Z. Morley Mao, one of the University of Michigan researchers who participated in the work.

One of the first things the researchers discovered was the apparent handicap on network speed imposed by a major U.S. carrier. (For legal reasons, the team anonymized its data.) Surprisingly, packets of data sent across this network are buffered by the carrier itself. This means that when a packet of data fails to make it to its destination—a common occurrence on noisy wireless networks—it cannot be instantly retransmitted, as it would normally be on the Internet. Instead, the sending device must wait a long time—on the order of seconds—for a time-out to alert it to the failure.

On a one-megabyte download, this slows transmission rates by up to 50 percent, the researchers report. The team suspects that the carrier is doing this buffering so it can perform deep packet inspection on the data sent through its network, says Microsoft Research engineer Ming Zhang, who contributed to the paper. This would mean that the carrier is actually reconstructing the data it transmits, possibly for examination for malicious code. Zhang cautions, however, that the team found no direct evidence that the carrier is doing this inspection; it’s merely the most logical explanation.

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