VDSL has an odd technical resemblance
to ADSL. VDSL achieves data rates nearly ten times greater than
those of ADSL, as shown in Figure
, but ADSL is the more complex transmission technology, in large
part because ADSL must contend with much larger dynamic ranges than
VDSL. However, the two are essentially cut from the same cloth. ADSL
employs advanced transmission techniques and FEC to realize data
rates from 1.5 to 9 Mbps over twisted pair, ranging to 18,000 feet;
VDSL employs the same advanced transmission techniques and FEC to
realize data rates from 14 to 55 Mbps over twisted pair, ranging to
4500 feet. Indeed, the two can be considered a continuum, a set of
transmission tools that delivers about as much data as theoretically
possible over varying distances of existing telephone wiring.
VDSL is clearly a technology suitable
for a full-service network (assuming that full service does not
imply more than two HDTV channels over the highest-rate VDSL). It is
equally clear that telephone companies cannot deploy ONUs overnight,
even if all the technology were available. ADSL may not be a
full-service network technology, but it has the singular advantage
of offering service over lines that exist today, and ADSL products
are more widely available than VDSL. Many new services being contemplated
today--such as videoconferencing, Internet access, video on demand,
and remote LAN access--can be delivered at speeds at or below T1/E1
rates. For such services, ADSL/VDSL provides an ideal combination
for network evolution. On the longest lines, ADSL delivers a single
channel. As line length shrinks, either from natural proximity to a
central office or deployment of fiber-based access nodes, ADSL and
VDSL simply offer more channels and capacity for services that
require rates above T1/E1 (such as digital live television and
virtual CD-ROM access). Figure
outlines the differences between all flavors of xDSL.
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