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To: Frank A. Coluccio who wrote (1918)	8/18/1998 8:36:00 PM
From: Frank A. Coluccio	Read Replies (1) \| Respond to of 12823

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. <<ELECTRONIC ENGINEERING TIMES -- 08-17-98, p. PG4>> [Copyright 1998, CMP Publications]