3.8 ADSL2 Summary

   


The following summary of the new aspects of the second generation ITU ADSL Recommendations G.992.3 and G.992.4 (2002) is adapted from information provided by Frank Van der Putten, the ITU Associate Rapporteur for G.992.3.

3.8.1 Improved Performance

ADSL2 increases the bit-rate and line-reach performance of ADSL, enabling up to 256 kb/s of additional performance on typical lines. A further increase in performance is possible when using the all-digital mode described in Section 3.8.7 in combination with the enhancements described here. The specifications now address bit rates up to 8 Mbit/s downstream and 800 kb/s upstream. An emerging ADSL standard can be scaled up to 15 Mbit/s downstream and 1.5 Mb/s upstream (on short lines). Trellis coding is now mandatory, whereas it was previously optional. 1-bit constellations enable additional capacity, using tones that would previously not have been used. FEC overhead support of 16 parity bytes is also mandated . Data can now be modulated on the pilot tone. Framing overhead has been reduced. The easily met performance requirements in G.992.1 have been replaced by far more complete and strict performance specifications in DSL Forum TR-048 for North America (annex A), and ETSI TS 101 388 V1.3.1 for Europe (annex A & B).

3.8.2 Loop Diagnostic Tools

A special diagnostic mode initialization has been added to help start-up on troublesome lines. Additional double-ended line testing has been specified for trouble resolution; this defines the following measurements to be performed by both the ATU-C and ATU-R: line attenuation, quiet line noise, and signal-to-noise ratio over the ADSL band . Messages are defined to convey these measurements to the other end of the line.

3.8.3 Improved Initialization

To improve robustness against bridged taps and RFI, the receiver can allocate the location of the pilot tone. RFI cancellation techniques are enabled by turning off transmission in selected tones when necessary, and a tone interleaving or shuffling option is included to disperse RFI distortion. Equalization is improved with spectrum shaped initialization signals. Rate negotiation is improved by replacing G.992.1's method of offering four rate options with the receiver choosing the bit-rate. To improve robustness, the receiver now determines the duration of initialization signals.

3.8.4 On-Line Reconfiguration (OLR)

Seamless Rate Adaptation (SRA) provides for in-service bit-rate changes to track application and BER requirements. The speed of the bit swap protocol has been improved. Dynamic rate repartitioning has been added for applications such as channelized VoDSL.

3.8.5 Power Management

The L2 low power mode has been added to enable statistical power savings based on user activity. Service is kept alive and full-rate operation is restored within 0.5 ms by maintaining low-bit-rate transmission during the L2 mode. Since signal energy is maintained during the L2 mode, non-stationary crosstalk behavior is minimized.

3.8.6 Framing

A new reduced overhead framing mode supports up to 4 frame bearers and up to 4 latency paths with delay and BER configurable per frame bearer. Framing now scales to support high data rates without using S = 1/2. The RS coding now scales better for long loop performance.

3.8.7 All-Digital Mode (No Underlying Analog Voice or ISDN Service)

The new all-digital mode extends ADSL transmission through the 0 to 25 kHz band to provide a total of 32 (Annex I) or 64 (Annex J) upstream tones. This enables an additional 256 kbit/s upstream data rate in addition to the other performance improvements described above, and is particularly important for improving performance on long lines.

3.8.8 Higher Layer Adaptation

Bonding of multi-pair operation via IMA for ATM-based ADSL is specified. Packet-based ADSL (e.g., Ethernet) support is described.

3.8.9 Home Installation

The specified architecture now includes in-line filters (splitterless), and a high impedance state is specified to permit multiple ATU-R to be connected to the line provided that only one ATU-R is active at a time.

3.8.10 Fast Start-Up (3 Seconds)

Fast start-up is provided from stand-by mode, sleep mode, and as an error recovery during Showtime mode.

3.8.11 Backwards Compatibility with First-Generation ADSL

G.handshake (G.994.1) will indicate support of multiple ITU-T ADSL (and other) Recommendations: G.992.1, G.992.2, G.992.3, G.992.4. ADSL modem implementations are expected (but not mandated) to support G.992.1/2 and G.992.3/4 for interoperability with existing deployments. G.992.1/2 equipment practice (e.g., DSLAM port density and power consumption) is expected not to be impacted by multimode support for G.992.3/4. PSD Masks are identical to G.992.1 for operation over POTS and ISDN, and spectrum management and deployment considerations are the same as for G.992.1.


   
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DSL Advances
DSL Advances
ISBN: 0130938106
EAN: 2147483647
Year: 2002
Pages: 154

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