Rate and Reach improvement - ADSL2 provides greater download stream (12Mbps compare
to 8Mbps in ADSL) due to improved modulation efficiency, and greater
upstream (1Mbps). Higher data rates are also gained due to improved
modulation technique.
ADSL2 also presents several improvments in the initialization state machine, which also helps avoiding line interference. This has also enabled to extend the reach of ADSL2 in regard to ADSL by approx 600 feet (~185m), and to increase data rates. Some of these improvements are listed in the next paragraph.
In addition, ADSL2 reduce framing overhead by allowing to determine the number of overhead bits in range 4-32 kbps (compared to ADSL frames which had a fixed 32kbps overhead per frame). Also, ADSL2 achieves higher coding gain due to new framing algorithms (the reed-solomon forward error correction code).
ADSL2 also presents several improvments in the initialization state machine, which also helps avoiding line interference. This has also enabled to extend the reach of ADSL2 in regard to ADSL by approx 600 feet (~185m), and to increase data rates. Some of these improvements are listed in the next paragraph.
In addition, ADSL2 reduce framing overhead by allowing to determine the number of overhead bits in range 4-32 kbps (compared to ADSL frames which had a fixed 32kbps overhead per frame). Also, ADSL2 achieves higher coding gain due to new framing algorithms (the reed-solomon forward error correction code).
Improved initialization-
ADSL2 improves the the line initialization
process, which is more automated than ADSL, thus
reducing error rate and increasing throughput. Improvements
includes:
- Reducing near-end echo and crosstalk interferences at the binder.
- Enabling RFI (radio frequency interference) cancellation techniques by disabling tones during
initialization.
- Receiver allocated pilot tone helps avoiding bridged taps and RFI.
- Receiver and Transmitter reach optimum in signal processing functions by controlling the length of
certain initializations states.
- Better equalization with spectrum shaped init signals to improve channel discovery.
- Reducing near-end echo and crosstalk interferences at the binder.
- Enabling RFI (radio frequency interference) cancellation techniques by disabling tones during
initialization.
- Receiver allocated pilot tone helps avoiding bridged taps and RFI.
- Receiver and Transmitter reach optimum in signal processing functions by controlling the length of
certain initializations states.
- Better equalization with spectrum shaped init signals to improve channel discovery.
–
Diagnostics - ADSL2's transceivers have
been enhanced with Real-time performance-monitoring capabilities to
provide measurements of line noise, loop attenuation, and signal-to-noise
ratio (SNR) at both ends of the line, even when connection can not be
achieved due to poor quality of the line. This is of great value to both
service providers which can use it to track and prevent failures, and to
carriers to determine if a customer qualifies for higher data-rate services.
Power consumption - ADSL2 introduces two
power management modes to help reduce power consumption: the first is called
"L2 low power mode" which enables power saving at the transceivers in the
central office (the ATU-C) by going into low power state whenever internet
traffic is decreased. The moment traffic has increased its
detected by the transceiver which goes back into full power mode (called
L0).
The other power saving technique is called "L3 low power mode", and is used on both CO's transceivers and the remote transceivers (the ATU-R), and basically enters the transceiver into sleep mode when no traffic is detected on the connection. Returning into full power mode requires re-initialization, but as we see shortly it is quite fast.
The other power saving technique is called "L3 low power mode", and is used on both CO's transceivers and the remote transceivers (the ATU-R), and basically enters the transceiver into sleep mode when no traffic is detected on the connection. Returning into full power mode requires re-initialization, but as we see shortly it is quite fast.
Fast startup - a fast start-up mode reduces initialization time from about
10 seconds (in ADSL) to approx 3 seconds.
Bonding for higher data rates - ADSL2 supports ATM Forum's inverse multiplexing
over ATM (IMA) standard. Through IMA, ADSL2 chipsets can bind two or
more copper pairs in an ADSL link to get fiber-like data rates over
existing copper lines. The IMA standard is integrated as a sublayer
between ADSL's physical layer and the ATM layer. See "Bonded
ADSL2" section for more details.
Channelization and Channelized Voice-Over-DSL (CVoDSL) - ADSL2 supports channelization, meaning it can split
its bandwidth into several channels, allowing each channel to serve a different
application using different characteristics. For example, a
voice or a gaming application can sustain low latency yet a higher error
rates, while data application require just the opposite - it can suffer from
high latency but require low error rates.
By allocating 64kbps channels of the bandwidth ADSL2 can transport digital signals representing voice (ds0) directly through the DSL's physical layer(much like plain pots) using PCM (Pulse Code Modulation) on the voice signals. This enables service providers to offer a complete voice and data solution in a relatively low cost.
By allocating 64kbps channels of the bandwidth ADSL2 can transport digital signals representing voice (ds0) directly through the DSL's physical layer(much like plain pots) using PCM (Pulse Code Modulation) on the voice signals. This enables service providers to offer a complete voice and data solution in a relatively low cost.
Figure
1. CVoDSL vs. Voice over ATM vs. Voice over IP
All digital mode - ADSL2 enables to put the bandwidth allocated for voice in use
for data transfer, thus adding 256kbps of upstream data rate. This
is an advantage for organizations which maintain an additional phone lines for
voice transmittion apart fron the lines used for data, winning some extra
upstream bandwidth.
Rate Adaptation - ADSL's lines are sensitive
to interferences which damage their performances. The most common type
of interference is called "crosstalk", an electro-magnetic interference caused on
one pair of cooper lines by another adjacent pair. Changes in the
Crosstalk can drop the ADSL line.
Other interferences can be caused by temperature, AM radio bands disturbances, and water gathered in the binder /shielding.
ADSL2 solves these problems by adapting the line's data rate in real time upon probing an interference, without any data lose or bit errors correction neccesary, transparently from the user. This is method called "SRA" - Seamless Rate Adaptation, a protocol which, by decoupling the modulation and framing layer, can change the transmission data rate parameters (applied by the modulation layer) without changing any of the frame layer parameters, so there is no need for restarting or resynchronizing the two end systems.
The SRA protocol is described in the figure below:
Other interferences can be caused by temperature, AM radio bands disturbances, and water gathered in the binder /shielding.
ADSL2 solves these problems by adapting the line's data rate in real time upon probing an interference, without any data lose or bit errors correction neccesary, transparently from the user. This is method called "SRA" - Seamless Rate Adaptation, a protocol which, by decoupling the modulation and framing layer, can change the transmission data rate parameters (applied by the modulation layer) without changing any of the frame layer parameters, so there is no need for restarting or resynchronizing the two end systems.
The SRA protocol is described in the figure below:
Figure
2. the SRA protocol in action
Support for packet-based services - ADSL2
includes a "packet mode transmission trans-convergence layer" (PTM-TC)
which supports operation of packet-based services (such as ethernet) over
ADSL2.
