[DMT Plumbing]
Peter -
Since your a plumber I thought you might enjoy reading this:
JW@KSC
January 20, 1997, Issue: 937
Section: Communications Design
ADSL modems speed infobahn access
By John M. Cioffi, Chief Technology Officer,
Amati Communications Corp., San Jose, Calif.
(Snip)
In a more typical phone line with a bridge tap, crosstalk noise (which
decreases with frequency) and AM radio interference, DMT also adapts for the best results that can be achieved.
T1.413 ADSL DMT continuously updates the bit-loading of the tones to correspond to continuously measured tone quality. For fixed data rates, the DMT modem computes a distribution of information across the tones that maximizes performance at a given data rate.
Another major advantage of DMT is its well-known immunity to impulse noise. Impulses are temporary noises caused by a variety of electronic devices, such as refrigerators, hair dryers, switching equipment and such.
DMT takes a typical impulse and smears its energy equally among the tones, allowing for maximum immunity. ADSL also adds forward error correction to DMT in a way that creates a robust performance in the presence of impulses-a characteristic that has been well-documented in lab tests.
DMT has been tested by several neutral testing facilities, including Nynex, Bellcore, GTE and NTSL, with publicly reported results. The 1993 results with DMT, in an "Olympics" held at Bellcore, showed DMT significantly outperforming QAM and CAP systems, which led to DMT's unanimous selection as a standard. A single group has continued to pursue a nonstandard CAP technology, but, at the time of this writing, some three years after they were promised, has been unable to provide modems at speeds above 1.5 to 2 Mbits/s (only 64 kbits/s upstream), despite a worldwide marketing campaign behind the technology. Test results show that the performance domination of DMT has increased, despite competitors' claims to the contrary.
techweb.com |
