Brief ADSL2 Review

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Brief ADSL2 Review

• Ian C. Wong, Daifeng Wang, and
• Prof. Brian L. Evans
• Dept. of Electrical and Comp. Eng.The University
  of Texas at Austin
• http//signal.ece.utexas.edu

http//www.ece.utexas.edu/bevans/projects/adsl
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1ADSL2 and ADSL2 - the new standards

• ADSL2 (G.992.3 or G.dmt.bis, and G.992.4 or
  G.lite.bis)
• Completed in July 2002
• Minimum of 8 Mbps downstream and 800 kbps
  upstream
• Improvements on
• Data rate vs. reach performance
• Loop diagnostics
• Deployment from remote cabinets
• Spectrum and power control
• Robustness against loop impairments
• Operations and Maintenance
• ADSL2 (G.992.5)
• Completed in January 2003
• Doubles the bandwidth used for downstream data
  (20Mbps at 5000 ft)

1Figures and text are extensively referenced from
ADSL2 ADSL2white
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ATU-Functional Model
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A. Data rate vs. reach performance

• Improved particularly for long lines with
  narrowband interference
• Achieves 12 Mbps downstream and 1 Mbps upstream
• Accomplished through
• Improving Modulation Efficiency
• Reducing framing overhead
• Achieving higher coding gain
• Employing Loop bonding
• Improving initialization state machine
• Online reconfiguration

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1. Improved Modulation Efficiency

• Mandatory support of Trellis coding (G.992.3,
  8.6.2)
• Block processing of Wei's Wei87 16-state
  4-dimensional trellis code shall be supported to
  improve system performance
• An algorithmic constellation encoder shall be
  used to construct constellations with a maximum
  number of bits equal to BIMAXds
• BIMAXds - maximum number of bits per subcarrier
  supported by the downstream transmitter (8-15)
• Note There was a proposal in 1998 by Vocal to
  use a Parallel concatenated convolutional code
  (PCCC), but it wasnt included in the standard
  (http//www.vocal.com/white_paper/ab-120.pdf)
• Data modulated on pilot-tone (optional, 8.8.1.2)
• During initialization, the ATU-R receiver can set
  a bit to tell the ATU-C transmitter that it wants
  to use the pilot-tone for data
• The pilot-tone will then be treated as any other
  data-carrying tone

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1. Improved Modulation Efficiency (cont.)

• Mandatory support for one-bit constellations
  (8.6.3.2)
• With trellis coding, 2 1-bit constellations can
  be combined

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2. Reduced framing overhead

• Programmable number of overhead bits (7.6)
• Unlike ADSL where overhead bits are fixed and
  consume 32 kbps of actual payload data
• In ADSL2, it is programmable between 4-32 kbps
• In long lines where data rate is low, e.g. 128
  kbps,
• ADSL 32/128 25 is overhead
• ADSL2 as low as 4/128 3.125 is overhead

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3. Achieved higher coding gain

• On long lines where data rates are low, higher
  coding gain from the Reed-Solomon code can be
  achieved
• Flexible framing allows RS code to have
  (7.7.1.4)
• 0, 2, 4, 6, 8, 10, 12, 14, or 16 redundancy
  octets
• 0 redundancy implies no coding at all (for very
  good channels)
• 16 would achieve the highest coding gain at the
  expense of higher overhead (for very poor
  channels)

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4. Loop Bonding

• Supported through Inverse Multiplexing over ATM
  (IMA) standard (ftp//ftp.atmforum.com/pub/approve
  d-specs/af-phy-0086.001.pdf)
• Specifies a new sublayer (framing, protocols,
  management) between PHY and ATM layer IMA99

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4. Loop Bonding (cont.)

• IMA (cont.)
• Some modifications to PHY are needed also (e.g.
  discarding of idle (K.2.8.2) and errored cells
  (K.2.8.5) at receiver)
• Some IP vendors have IMA compliant cores for
  FPGA, e.g. Xilinx (http//www.xilinx.com/bvdocs/wh
  itepapers/wp107.pdf)

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5. Improved initialization state machine

1. Power cutback
2. Spectral Shaping
3. Receiver-determined pilot tones

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5.1 Power cutback I

• Nominal transmit Power Spectral Density (NOMPSD)
  level
• - dBm/Hz, the transmit PSD level in the
  passband at the start of initialization, relative
  to which power cut back is applied.
• - Depends on near-end transmitter capabilities
• Power Cutback (PCB)
• - Reduce Crosstalk
• - Reduction of the transmit PSD level (dB)
  in any one direction, relative to the nominal
  transmit PSD level.
• - The same transmit PSD level reduction is
  applied over the whole frequency band (i.e., flat
  cutback)
• - Tradeoff between data rates and PSD
• - Depends on the loop and local
  capabilities.
• Reference transmit PSD (REFPSD)
• - REFPSD NOMPSD PCB

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5.1 Power cutback II- Daifeng

• Handshake (G.994.1)
• Capabilities List (CL) Capabilities List
  Request (CLR) in G.994.1
• Spectrum Bounds Parameter Block in CL/CLR message
• CLR US and CL DS/US

b7 b6 b5 b4 b3 b2 b1 b0
byte 1 Nominal transmit (NOMPSD) Nominal transmit (NOMPSD) Nominal transmit (NOMPSD)
byte 2 PSD level PSD level PSD level Nominal transmit (NOMPSD) Nominal transmit (NOMPSD) Nominal transmit (NOMPSD)
byte 3 Maximum nominal (MAXNOMPSD) Maximum nominal (MAXNOMPSD) Maximum nominal (MAXNOMPSD)
byte 4 Transmit PSD level Transmit PSD level Transmit PSD level Maximum nominal (MAXNOMPSD) Maximum nominal (MAXNOMPSD) Maximum nominal (MAXNOMPSD)
byte 5 Maximum nominal (MAXNOMATP) Maximum nominal (MAXNOMATP) Maximum nominal (MAXNOMATP)
byte 6 aggregate tx power aggregate tx power aggregate tx power Maximum nominal (MAXNOMATP) Maximum nominal (MAXNOMATP) Maximum nominal (MAXNOMATP)
NOMPSD, MAXNOMPSD, MAXNOMATP 9 bits, 0.1dB
steps, -25.6 to 25.5 dB
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5.1 Power cutback III- Daifeng

1. Timing diagram of PCB

128 C-COMB (16 tones,4-QAM)
C-COMB1/2
256 R-COMB (tone indices that are multiples of 6)
R-COMB2
C-MSG-FMT C-MSG-PCB
ATU-C determined power cutback
R-MSG-FMT R-MSG-PCB
ATU-R determined power cutback and pilot tone
C-TREF1
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5.1 Power cutback IV

• ATU-C C-COMB12(8.13.3.1.24)
• - estimates minimum US/DS power cutback
  for ATU-R
• ATU-C C-MSG-PCB(8.13.3.1.11)
• - conveys the ATU-C determined power
  cutback
• Definition for C-MSG-PCB

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5.1 Power cutback V- Daifeng

• ATU-R R-COMB2(8.13.3.2.4)
• - estimates minimum US/DS power cutback
  for ATU-C
• ATU-R R-MSG-PCB(8.13.3.2.11)
• - observes C-COMB1
• - conveys the ATU-R determined power
  cutback
• 3. Definition for R-MSG-PCB

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5.2 Spectral Shaping I

• Why Spectral Shaping1?
• The general shape of the DSL channel is such that
  higher frequencies are attenuated more than lower
  frequencies.
• ADSL systems allocate higher frequencies to the
  downstream. To improve the performance of ADSL on
  long loops, it is typically necessary to improve
  the downstream data rate.
• The upstream power can be moved lower in
  frequency to avoid crosstalk.
• Improvement for training receiver TEQ
• How Spectral Shaping?
• flexibility in shaping it's transmit spectrum,
  based on DMT modulation
• Putting power where the channel is better (either
  by shrinking the range of the downstream
  frequencies and boosting the power) by ADSL2
• May use interpolations

1http//www.commsdesign.com/design_corner/OEG20030
717S0028
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5.2 Spectral Shaping II

• Spectrum Shaping Parameter Block in CL/CLR
  message
• CLR US and CL DS/US

b7 b6 b5 b4 b3 b2 b1 b0
byte 1 9 bits 1 to 2NSCus-1 9 bits 1 to 2NSCus-1 9 bits 1 to 2NSCus-1
byte 2 subcarrier index subcarrier index subcarrier index 9 bits 1 to 2NSCus-1 9 bits 1 to 2NSCus-1 9 bits 1 to 2NSCus-1
byte 3 indication 7 bits
byte 4 spectrum shaping log_tssi 0 dB to -62.5 dB spectrum shaping log_tssi 0 dB to -62.5 dB spectrum shaping log_tssi 0 dB to -62.5 dB spectrum shaping log_tssi 0 dB to -62.5 dB spectrum shaping log_tssi 0 dB to -62.5 dB 7 bits
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5.2 Spectral Shaping II

• Illustration of the interpolation of log_tssi and
  indications

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5.2 Spectral Shaping III
Spectral Shaping Equation
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5.2 Spectral Shaping IV - Daifeng
An example of the downstream tssi values as a
function of the subcarrier index i, for the case
that the SUPPORTEDset contains the subcarriers
with index i 40 to 200 and N 2 NSC 512
(oversampled IDFT). At frequencies i ?f, with
40 i 200 and ?f 4.3125 kHz, the tssi value
equals 1 (0 dB).
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5.2 Spectral Shaping V - Daifeng

• BLACKOUT bits (i.e., BLACKOUTi for i 1 to NSC
  1) during the Channel Discovery Phase by the
  receive PMD function
• MEDLEYset SUPPORTEDset (as indicated by the
  transmitter) - BLACKOUTset (as indicated by the
  receiver)
• - ATU-R shall select a C-TREF pilot subcarrier
  from the MEDLEYset

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5.2 Spectral Shaping VI

• Application of spectral shaping and blackout
  during initialization
• DS - 0 lt tssi for MEDLEYset lt1
• US - tssi 1
• Flowchart for the implementation of tssi (Figure
  8-25a, p126)

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5.3 Receiver-determined pilot tones I

• Pilot - only applies for downstream direction
• Pilot Selecting(8.8.1.2)
• - During initialization, the ATU-R receive
  PMD function selects the subcarrier index of the
  DS pilot tone.
• 3. R-MSG-FMT(8.13.3.2.10, before R-MSG-PCB) for
  ATU-R

Set 1 with a fixed 4-QAM constellation point as
the pilot tone Set 0 with the defined pilot tone
for each of the ATU-C initialization states
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5.3 Receiver-determined pilot tones II

• C-PILOT ( Table 8-23)
• - 8 bits in R-MSG-PCB( bit23 to bit 16)
• - Index of the C-TREF pilot subcarrier
• - From MEDLEYset
• C-TREF pilot subcarrier (8.13.3.2.11)
• - used by ATU-C for the timing reference
• - used by ATU-R during C-TREF1/2 for
  timing recovery
• C-TREF1(8.13.4.1.2)
• - Modulate the 4-QAM 0,0 constellation
  point
• - ATU-C reference transmit PSD level
  (REFPSDds)

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6. Online reconfiguration (OLR) (10.2)

• Autonomously maintain operation within limits set
  by control parameters
• Useful when line or environment conditions are
  changing
• Optimise ATU settings following initialization
• Useful when employing fast initialization
  sequence that requires making faster estimates
  during training
• Types of OLR
• Bit Swapping (BS)
• Dynamic Rate Repartitioning (DRR)
• Seamless Rate Adaptation (SRA).

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6. Online reconfiguration (OLR) (cont.)

• Bit Swapping (mandatory)
• Reallocates data and power among the allowed
  subcarriers without modification of the higher
  layer features of the physical layer
• Reconfigures the bits and fine gain (bi, gi)
  parameters without changing any other PMD or
  PMS-TC control parameters
• After a Bit Swapping reconfiguration the total
  data rate (SLp) is unchanged and that data rate
  on each latency path (Lp) is unchanged

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6. Online reconfiguration (OLR) (cont.)

• Dynamic Rate Repartitioning
• Reconfigure the data rate allocation between
  multiple latency paths by modifying the frame
  multiplexor control parameters (Lp).
• Can include modifications to the bits and fine
  gain (bi, gi) parameters, reallocating bits among
  the subcarriers.
• Does not modify the total data rate (SLp) but
  does modify the individual latency path data
  rates (Lp)
• Performed in response to higher layer commands,
  and is thus an application option

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6. Online reconfiguration (OLR) (cont.)

• Seamless Rate Adaptation (SRA)
• Used to reconfigure the total data rate (SLp) by
  modifying the frame multiplexor control
  parameters (Lp) and modifications to the bits and
  fine gains (bi, gi) parameters
• Since the total data rate is modified, at least
  one latency path (or more) will have a new data
  rate (Lp) after the SRA
• Allows modulation parameters to change without
  modifying framing parameters
• Prevents frame desynchronization which causes
  uncorrectable bit errors or system retraining
• Used in response to higher layer commands, thus
  is an application option
• Any ATU that implements the optional PMD short
  initialization procedure should implement SRA
  operations
• This ensures the ATU is able to adapt to the
  channel conditions which were not as accurately
  estimated

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6. Online reconfiguration (OLR) (cont.)

• Seamless Rate Adaptation (SRA) simplified
  protocol
• RX monitors SNR of channel and determines rate
  change is necessary
• RX sends message to intitiate rate change, which
  includes all necessary parameters, e.g. bits and
  gains info on each subchannel
• TX sends SYNC FLAG signal used as a marker to
  designate exact time where the new parameters
  will be used
• RX detects SYNC FLAG and both seamlessly and
  transparently transition to the data rate

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ADSL2 (G.992.5)

• Doubles the downstream bandwidth
• Significant increase in downstream data rates on
  shorter lines

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ADSL2 (G.992.5)

• Reduces crosstalk
• Provides capability to use only tones between
  1.1-2.2 MHz by masking downstream frequencies
  below 1.1 MHz (DSM)

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Backup Slides
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ADSL2 improvements over ADSL

• Application-related features
• Improved application support for an all digital
  mode of operation and voice over ADSL operation
• Packet TPS-TC1 function, in addition to the
  existing Synchronous Transfer Mode (STM) and
  Asynchronous TM (ATM)
• Mandatory support of 8 Mbit/s downstream and 800
  kbit/s upstream for TPS-TC function 0 and frame
  bearer 0
• Support for Inverse Multiplexing for ATM (IMA) in
  the ATM TPS-TC
• Improved configuration capability for each TPS-TC
  with configuration of latency, BER and minimum,
  maximum and reserved data rate.

1Transport Protocol Specific-Transmission
Convergence
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ADSL2 improvements over ADSL (cont.)

• PMS-TC1 related features
• A more flexible framing, including support for up
  to 4 frame bearers, 4 latency paths
• Parameters allowing enhanced configuration of the
  overhead channel
• Frame structure with
• Receiver selected coding parameters
• Optimized use of RS coding gain
• Configurable latency and bit error ratio
• OAM2 protocol to retrieve more detailed
  performance monitoring information
• Enhanced on-line reconfiguration capabilities
  including dynamic rate repartitioning.

1 Physical Media Specific-Transmission
Convergence 2 Operations, Administration, and
Maintenance
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ADSL2 improvements over ADSL (cont.)

• Physical Media Dependent (PMD) related features
• New line diagnostics procedures for both
  successful and unsuccessful initialization
  scenarios, loop characterization and
  troubleshooting
• Enhanced on-line reconfiguration capabilities
  including bitswaps and seamless rate adaptation
• Optional short initialization sequence for
  recovery from errors or fast resumption of
  operation
• Optional seamless rate adaptation with line rate
  changes during showtime
• Improved robustness against bridged taps with RX
  determined pilot
• Improved transceiver training with exchange of
  detailed transmit signal characteristics
• Improved SNR measurement during channel analysis
• Subcarrier blackout to allow RFI measurement
  during initialization and SHOWTIME
• Improved performance with mandatory support of
  trellis coding, one-bit constellations, and
  optional data modulated on the pilot-tone

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ADSL2 improvements over ADSL (cont.)

• PMD related features (cont.)
• Improved RFI robustness with receiver determined
  tone ordering
• Improved transmit power cutback possibilities
• Improved Initialization with RX/TX controlled
  duration of init. states
• Improved Initialization with RX-determined
  carriers for modulation of messages
• Improved channel identification capability with
  spectral shaping during Channel Discovery and
  Transceiver Training
• Mandatory transmit power reduction to minimize
  excess margin under management layer control
• Power saving feature with new L2 low power state
  and L3 idle state
• Spectrum control with individual tone masking
  under operator control through CO-Management
  Information Base
• Improved conformance testing including increase
  in data rates for many existing tests.

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Bibliography

• ADSL2 ITU-T Standard G.992.3, Asymmetric
  digital subscriber line transceivers 2 (ADSL2),
  Feb. 2004
• ADSL2white ADSL2 and ADSL2plus-The new ADSL
  standards. Online http//www.dslforum.org/aboutd
  sl/ADSL2_wp.pdf, Mar. 2003
• Wei87 L.-F.Wei, Trellis-coded modulation with
  multidimensional constellations, IEEE Trans.
  Inform. Theory, vol. IT-33, pp. 483-501, July
  1987.
• IMA99 ATM Forum Specification af.phy-0086.001,
  Inverse Multiplexing for ATM (IMA), Version 1.1.,
  Mar. 1999