Engineers see technical roadblocks for DSL, cable
modems
August 18, 1998
ELECTRONIC ENGINEERING TIMES via
NewsEdge Corporation : Palo Alto, Calif.- A wave of
cautionary words broke over a session on
high-bandwidth services to the home last week at
the sixth Hot Interconnects conference. In separate
papers, researchers warned of unsolved problems
looming for both digital subscriber line (DSL)
services and cable modems.
John Cioffi, associate professor at Stanford
University, described the work of his research team
on emerging challenges to DSL. Cioffi homed in on
two issues: the complexities of mixing plain old
telephone service (POTS) and DSL service without a
splitter and the problems of crosstalk.
On the POTS front, "there are many issues" with
splitterless DSL, said Cioffi, "including what
happens during a ring signal. But one of the most
interesting is the problem of insertion loss."
When a user picks up the telephone handset in a
splitterless service, the result is an immediate
change in load on the local loop, resulting in a loss
of one to two orders of magnitude in signal
amplitude, Cioffi said.
Recognition issue
"The modems are designed to recognize this," said
Cioffi, "and to interrupt the data service while they
retrain the modem for a lower data rate. When the
phone goes back on the hook, they interrupt again
and bring the rate back up. DMT [digital multitone]
is designed to allow for adaptation to slowly
changing line conditions, but not to abrupt changes
like a handset going off-hook. That requires
interrupting the data flow."
One potential issue is how tolerant system software
will be to these interruptions and changes in rate.
Cioffi described techniques in development at his
facility that would combine rate-loading and
margin-loading techniques to guarantee a minimum
data rate, then load for an achievable rate greater
than that.
Also, Cioffi added that crosstalk can occur when
several POTS twisted pairs in the same bundle are
used for DSL service. "Traditionally, the industry
has treated crosstalk as Gaussian noise and
attempted to filter it out," Cioffi said. "Our research
indicates that it is better to treat crosstalk as a
separate signal, and to try to detect both the
intended signal and the crosstalk signal."
The result is much better usable bandwidth for the
DSL. But the cost comes in computing power. "The
computing requirement for a DSL today is about 100
Mips," Cioffi said. "Crosstalk mitigation requires up
to several hundred more Mips. Consequently, we
won't see it implemented for a year or so. "
While Cioffi focused on DSL issues, Microsoft's
Chuck Thacker zeroed in on the unsolved problems
of cable modems. "I'm cautiously optimistic,"
Thacker said, "but I want to sound a cautionary
note." He warned the available raw bandwidth in a
branch of a hybrid fiber coax system-27
Mbits/second downstream and 10 Mbits/s
upstream-is "peanuts."
In a heavily loaded branch, he said, the downstream
bandwidth available to any one cable modem user is
about the same as it would be with a 33-kbit/s
modem.
But the critical bandwidth problems come when the
cable-modem user is not simply surfing the Web.
"What if you are a 'complex' customer, with, say,
two PCs and a printer," Thacker asked. "Cable
modems don't provide routing capability or any way
for you to talk to your own printer. Small businesses
will end up requiring a pocket router to mediate
between their own LAN and the cable system."
Copyright - 1998 CMP Media Inc.
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