ADSL2 , ADSL2+

ADSL2 has been specifically designed to improve the rate and
reach of ADSL largely by achieving better performance on long lines
in the presence of narrowband interference. ADSL2 accomplishes
this by improving modulation efficiency, reducing framing overhead,
achieving higher coding gain, improving the initialization state
machine, and providing enhanced signal processing algorithms. As a
result, ADSL2 mandates higher performance for all standard-compliant
devices.

ADSL systems, on long lines where the data rate is low (e. g. 128
kbps), a fixed 32 kbps (or 25% of the total data rate) is allocated to
overhead information. In ADSL2 systems, the overhead data rate
can be reduced to 4 kbps, which provides an additional 28 kbps for
payload data.
On long lines where data rates are lower, ADSL2 achieves higher
coding gain from the Reed-Solomon (RS) code. This is due to
improvements in the ADSL2 framers that improve flexibility
and programmability in the construction of the RS codewords.
Additionally, the initialization state machine has numerous improvements
that provide increased data rates in ADSL2 systems. Examples
include:

Power cutback capabilities at both ends of the line to reduce
near-end echo and the overall crosstalk levels in the binder.

Determination of the pilot tone location by the receiver
in order to avoid channel nulls from bridged taps or narrow
band interference from AM radio.
ADSL2 provides better modulation efficiency by mandating fourdimensional,
16-state trellis-coded and 1-bit quadrature amplitude
modulation (QAM) constellations, which provide higher data rates
on long lines where the signal-tonoise ratio (SNR) is low. In addition,
receiver-determined tone reordering enables the receiver to
spread out the non-stationary noise due to AM radio interference
to get better coding gain from the Viterbi decoder.
ADSL2 systems reduce framing overhead by providing a frame
with a programmable number of overhead bits. Therefore, unlike
the first-generation ADSL standards where the overhead bits per frame
are fixed and consume 32 kbps of actual payload data, in the ADSL2
standard the overhead bits per frame can be programmed from 4
to 32 kbps. In first-generation Control of certain key initialization
state lengths by the receiver and transmitter in order to
allow optimum training of receiver and transmitter signal
processing functions.

Determination of the carriers used for initialization messages
by the receiver in order to avoid channel nulls from
bridged taps or narrow band interference from AM radio.

Improvement in channel identification for training receiver
time domain equalizer with spectral shaping during
Channel Discovery and Transceiver Training phases
of initialization.

Tone blackout (disabling tones) during initialization to enable
radio frequency interference (RFI) cancellation schemes.

ADSL2+
ADSL2+ is a work in progress at the ITU to standardize a new
member in the family of ADSL2 standards.The ADSL2+ recommendation
doubles the downstream bandwidth, thereby increasing the
data rate on telephone lines shorter than 9 kilofeet. ADSL2+ is
expected to reach consent at the ITU in early 2003.
While the first two members of the ADSL2 standards family
G.992.3 (G.dmt.bis) and G.992.4 (G.lite.bis) specify a downstream
frequency band up to 1.1 MHz and 552 kHz respectively, ADSL2+
specifies a downstream frequency up to 2.2 MHzADSL2+ can also be used to
reduce crosstalk. ADSL2+ provides the capability to use only
tones between 1.1 MHz and 2.2 MHz by masking the downstream
frequencies below 1.1 MHz. This can be particularly useful when
ADSL services from both the central office (CO) and a remote terminal
(RT) are present in the same binder as they approach customers'
homes ADSL2+ can be used to correct this problem by using frequencies
below 1.1 MHz from the central office, and frequencies between
1.1 MHz and 2.2 MHz from the remote terminal to the customer
premise. This will eliminate most of the crosstalk between the services
and preserve data rates on the line from the central office.


Credit:: www.dslprime.com/