May 24, 1999
The cable modem vs. DSL performance race
By Bob O'Donnell
Though many areas of the United States are still stuck with the paltry transfer rates
of ISDN lines, affordable broadband Internet connections via cable modems and some flavors
of Digital Subscriber Line (DSL) technology have begun to make their presence felt. In
fact, in some areas of the country, including the insulated world of Silicon Valley, where
I (and InfoWorld) reside, many communities are starting to get access to both. As a
result, some lucky small business and home users actually have a choice between cable and
DSL.
Inevitably, this has led to discussions about which one is better and which is faster.
From an end-user perspective, both services provide a fast, always-on connection that
differs only in their physical connections (although even those are remarkably similar).
Technologically, however, there are some important differences in how the two types of
connections handle the data traffic. Cable modems use a shared access system, like an
office network, where all the traffic in a particular area (or neighborhood) travels
across a common connection. DSL modems, on the other hand, provide a dedicated connection
to the phone company's central office, but then share a single pipe from that central
point to the Internet itself.
Much has been made of these differences, with arguments being put forth that even
though cable modem systems are theoretically capable of higher transfer rates than
DSL-based systems, cable connections are at a distinct disadvantage because of their
network topology. And anecdotal evidence from cable modem users who claimed that their
connection speeds sometimes slowed to a crawl as the service has become more popular
certainly seemed to suggest that the problem was real.
But until recently, there hasn't really been any hard evidence that could support these
conclusions. Last week, however, Keynote Systems, a
company that tracks overall Internet and Web site performance through its Keynote Business
40 and other services, released an intriguing set of performance test results that
compares the performance of cable modem lines, DSL lines, and T1 connections.
The results of Keynote's month-long test show that -- at least for the specific
TCI/@Home cable modem connection vs. Pacific Bell DSL modem connection it tested -- in the
prime-time evening hours of home-based Internet usage, a 384Kbps-guaranteed DSL line
(which costs $49 a month, including the ISP charges) was approximately 11 percent faster
than a cable modem connection (which costs $39 a month, including the ISP charges). More
important, the reason for the difference was because of the bandwidth problems associated
with a shared cable modem connection.
During the day, cable modems were actually 15 percent faster than DSL lines, but in the
evening, the cable modem rates slipped by 8 percent as more users got onto the network.
Interestingly, DSL lines improved by 17 percent during that same time period, because the
phone line networks over which DSL-based data is carried have less traffic in the
evenings.
Another interesting point that the Keynote numbers confirmed is that although both
cable modems and DSL are often touted as having T1-like speeds, the reality is a bit more
sobering. The T1 connection consistently outperformed both cable and DSL connections and,
in some instances, was nearly twice as fast.
The battle for broadband Internet service is, of course, far from over. But DSL's
victory in this particular speed race confirms that the cable industry is nowhere near the
complete domination of this market that it was once expected to hold. This one's going to
be fun to watch.
©
Copyright 1999, by InfoWorld Publishing Corp., a
subsidiary of IDG Communications, Inc. Reprinted from InfoWorld,
155 Bovet Road, San Mateo, CA 94402. Further reproduction is prohibited.
