RE-ADSL2
High speed internet services have became almost a neccesaty among consumers, and carriers around the world continue to deploy ADSL infrastructures rapidly to satisfy them. However, there are still a lot of consumers living outside the range of current ADSL technology, which deny them of service. A new system is in need in order to allow those who live at the end of longer phone lines.
RE-ADSL2 to the rescue
The
Reach-E xtended mode of ADSL2 (aka
RE-ADSL2), not an entirely new standard but an Annex to the basic ADSL2 (Annex L
for ADSL2 g992.3 to be precise), has been introduced to the family
of ADSL2 by the ITU in October 2003. Its main purpose: a mean to extend
the reach of ADSL2.
How it is done
RE-ADSL2 maintains 2 downstream
PSD masks - one mandatory and one optional,
and 2 mandatory upstream PSD masks. Analysis shows that using a higher
PSD levels can extend the reach of the loop while using smaller bandwith
portion then the basic ADSL2 (reffered to as annex
A
). More precise, the downstream PSD of
annex A uses ~1104khz, while annex L's downstream PSD
(which is ~3db higher in optimal conditions) is approximatly half of
it - 552khz. similiar ratio is encountered in annex A's upstream PSD
which uses 112khz compared to annex L's upstream PSD (1.6db and 5.1db higher
then annex A) which uses 78khz and 35khz (mask #1 and #2 respectively).
Selecting the PSD
Selecting the PSD masks must
take into consideration various factors in order to gain optimal
performance. These factors include physical deployment of the
lines,loop topology, and crosstalk. The carrier should decide the best
configuration for the PSD, however it is expected from the standard that this
decision could be made automatically by the tranceivers upon
initialization.
It is also desirable that the RE option could be implemented
easily on an existing annex A system, so it could be turned ON only on the
small portion of the infrastructures where long lines are not suitable
for ADSL2-annex A (since shorter lines will sustain lower data rates then annex
A as described below)
The results
RE-ADSL2
delievers increased performance over long lines under various crosstalk
conditions. It can reach 768kbps of downstream over a line up to
19,000ft (~5.7km) . On normal length lines however, it is not wise to
use the RE feature because they use less overall bandwidth therfore reach lower data
rates then Annex A.
