Study: DSL speeds can surpass cable

But other experts cautioned that the results, while telling for the individual neighborhoods where the tests took place, don't necessarily mean that digital subscriber line (DSL) technology will always beat cable modems.

The study was done by Internet consulting firm Keynote Systems, which tracked the performance of a cable modem, a DSL access line rated for downloads of 384 kbps, and four high-speed T1 lines over the course of a month.

Keynote's findings tracked generally with the way the different services have been marketed. Cable modems, which have been marketed to consumers more heavily, performed at their peak during the day, and dropped in performance at night when home users typically use the Net. DSL systems, marketed more to businesses, performed better at night as employee use subsides afterhours.

Using a standard set of Web pages as a benchmark, Keynote found that the Pacific Bell DSL system took an average of 3.55 seconds to download a page between the hours of 5 p.m. and 11 p.m., and 4.30 seconds during the daytime hours.

The cable modem system took an average of 3.97 seconds to download the set of benchmark pages during the evening, and 3.68 seconds during the day. That means that DSL bettered cable modem performance during the evening by about 12 percent, while during the daytime DSL connections were about 17 percent slower than a cable modem connection.

The results were obtained by measuring the download speed of a specific set of Web pages continuously over the month of April, all from neighborhoods in the San Francisco Bay Area. Identically configured, high-end Pentium computers systems running Microsoft's NT operating system were used in the tests.

What do the tests mean?
Experts were quick to point out that even where DSL connections bettered cable modem download speeds, variations in performance could be attributable to each system's different architecture, rather than the superiority of one type of connection over another.

Cable modems are based on a shared network, in which each subscriber in a given neighborhood shares access to the same cable stream. Thus, if a number of subscribers try to access the system at the same time, it cuts into the limited amount of bandwidth available, slowing down performance at peak hours.

DSL systems, in contrast, give each subscriber a dedicated line to the telephone company's central office. From that point on, the signal can run into interference, however. According to industry insiders, telephone companies like Pacific Bell typically "oversubscribe" their consumer DSL systems, which can overburden the link between their offices and the backbone network. This means that if many users are on the system at once they can still run into traffic jams on the far side of the telco's central office.

Keynote said their results indicated broad usage patterns for the different technologies, but shouldn't necessarily be taken as specific benchmarks for the access systems.

"It depends on how much your neighborhood looks like our test neighborhood," said Gene Shklar, a spokesman for the company.

Analysts generally agreed that the test mirrored usage patterns, but cautioned that the results should not be taken as universal since they used just a single example of each technology.

"Results will really vary by geographical location and by service providers," said Michael Harris, an analyst with Kinetic Strategies. "There are a bunch of different variables that can affect performance."

Patti Reali, telecomunications analyst with Gartner Group, takes exception to the survey's findings, too.

"I don't think that this particular analysis can be judged of all [cable] systems across the board," said Reali. For instance, there are some issues with how TCI's (now a unit of AT&T) cable systems are built to handle data traffic, she said. "In general, many of the complaints that have been issued [about cable modem service] have come from TCI systems."

TCI is the main provider of @Home's cable modem service in the San Francisco Bay Area.

In short, cable operators build their networks differently, and some of the discrepancies in performance between DSL and cable modem service could evaporate if the test was conducted in different areas of the country.

However, the survey does point to an important issue when considering how AT&T, Comcast, and Time Warner will offer all of the advanced services being promised--high speed Internet access, telephony, video on demand, and others--over a cable network.

"Engineering and design of the network will be very important in delivering services that people will be willing to pay for," Reali said.