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April 20, 1998

Do Windows users get more speed from x2 or K56flex?

A review showing that x2 modems far outperform K56flex modems has generated a great deal of interest among Windows users who surf the Internet.

The review appeared in Boardwatch Magazine's March issue, edited by Jack Rickard. He completed 10,484 calls to Internet service provider ports running x2 modems made by 3Com's U.S. Robotics unit. More than 90 percent of his downloads occurred at 42Kbps, plus or minus 5Kbps.

By contrast, he completed 10,325 calls to K56flex ports powered by Rockwell Semiconductors' chipsets -- but more than 90 percent of his downloads occurred at a mere 28.8Kbps, plus or minus 5Kbps.

Rickard estimates that fewer than 40 percent of all Internet users connect from work via LANs, while more than 60 percent connect from a dial-up modem at home. To these users, the speed of 56Kbps modems is a major concern.

Now that both x2 and K56flex modems can be upgraded to the new V.90 56Kbps standard, users want to know which kind of modem (after a firmware upgrade) is likely to perform best. I conducted extensive interviews with officials of Boardwatch, Rockwell, and 3Com to find the answers.

Rickard originally designed his test calls to determine how often ISP subscribers receive busy signals, not to test transfer speeds. It was only when he found enormous performance differences between x2 and K56flex that he published his findings.

Rickard has concluded that the differences were due to "digital impairments." Impairments are traits of digital lines between different telephone company central offices and ISPs. Digital impairments, which are not necessarily errors, include variations in hardware, conversions from American to international systems, and "robbed bits," where a circuit steals one or more bits every few bytes for its own call-control needs.

Rockwell's Dean Grumlose says the poor performance of K56flex modems in Boardwatch's tests is entirely due to a digital impairment at the central office near Littleton, Colo., where Rickard made his calls.

"K56flex is programmed only to look for certain impairments," Grumlose explains. "It's possible that if it detects an impairment it doesn't recognize, it will make an error in judgment."

John Powell, 3Com's manager of application engineering, says, "That exact impairment [at the Colorado central office] occurs in 25 to 30 percent of the lines in the United States."

3Com has more than 1,000 test sites running x2 modems. x2 modems compensate for tens of thousands of combinations of digital impairments, Powell says.

In my opinion, Rockwell's claim that its bad Boardwatch results are due to a digital impairment is persuasive. Rickard says he has identified the impairment as a digital "pad" (a kind of attenuator) at his Colorado central office. 3Com's assertion that K56flex modems are prone to digital impairments is also convincing, based on 3Com's thousands of test calls.

My advice? Owners of x2 modems should upgrade them to the new V.90 firmware today. There is no downside, and you may gain 7 percent to 10 percent in performance, according to 3Com. For a free upgrade, visit

Owners of K56flex modems may want to upgrade, too -- but I strongly advise you not to do so until you've read next week's column.

Brian Livingston is the co-author of several best-selling Windows books, including the most recent Windows 95 Secrets (IDG Books). Send comments to Unfortunately, he cannot answer individual questions.

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