To all: A new twist in the CAP vs. DMT story?
I've just read an Inter@ctive Week story by Carol Wilson (October 10, 1995) on the Orckit website (Orckit is a cutting-edge Israeli xDSL company). Orckit is the only company to date that has developed VDSL (for the drop to the home in FTTC, fibre to the curb) and its product gives 51 Megs/s downstream and 2 Megs/s upstream over a single twisted pair.
What is interesting here is that Orckit, a DMT ADSL manufacturer, has NOT used DMT modulation for their VDSL product. They've developed a form of QAM (quadrature amplitude modulation) for this. QAM, I believe, has been used to date mostly in coaxial cable for CATV. The author of the piece implies that it is related to CAP (carrierless amplitude phase) modulation.
The article also says that AT&T Paradyne is developing a CAP-16 VDSL product, but the Orckit spokesman is quoted as saying that his company's QAM product will have "almost 75 percent better performance than the CAP-16 system." The article does not mention the Amati effort (as yet without a tested prototype, I believe) to develop VDSL based on DMT technology.
Orckit also says that, despite their DMT product line, they are leaning toward QAM now (for their ADSL products as well?) AND THAT THEY HOPE THE STANDARDS BODIES WILL FOLLOW.
This is extremely interesting, for the simple reason that VDSL and FTTC (hybrid copper-fibre) is the technology that all xDSL systems, including ADSL, will eventually lead to. Orckit, fully established in the DMT camp, says that QAM is better in terms of BOTH cost AND performance. I'd be interested to hear from anyone with more info on QAM. Thanks.
DH |
