PCM Modem Standard: ITU draft recommendation V.90

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PCM Modem Standard ITU draft recommendation V.90

• Joe Decuir, Microsoft, Windows Operating Systems
  Division

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ITU V.90 presentation, outline

• Phone lines and their limits
• How traditional modems work, up to V.34
• How PCM modems get so fast
• Why PCM modems arent faster.
• Some details of V.90
• Where we go from here

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Summary of phone system

• The 2-wire pair to your house is analog.
• The analog pair is DC feed plus duplex AC.
• AC includes 20Hz 86Vac ringing and loads
• The inside is digital, limited by the codecs
• 4kHz bandwidth, useful from 200-3700 Hz
• 8kHz sample rate
• 8 bit companded A/D/A (ITU G.711)

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G.711 PCM coding

• 8 bits/value 1 sign, 3 exponent, 4 mantissa
• 16 segments, each with 16 linear steps
• Step size at full-scale is 128 times step size at
  origin.
• Monotonic, but not linear
• 13 bit total dynamic range
• Two variations mu-Law, A-Law

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G.711 Transfer curve
codes
Low precision at high levels.
High precision near the origin
Analog values
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Network Impairments

• Tandem Encoding digital-analog-digital..
• ADPCM encoding (G.721, G.726)
• Loaded loop
• Robbed Bit Signaling (RBS)
• Digital Loss Pads
• A-Law/mu-Law conversion

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More Network Impairments

• D/A converter non-linearity's
• Frequency dependent non-linearity
• Talker echo
• Residual loop noise
• Residual loop distortion

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How modems work

• Establish channels by some means
• frequency division multiplex (FDM)
• time division multiplex (TDM)
• echo cancellation multiplex (ECM)
• Map data into symbols
• Frequency shift key (FSK) (1 bit/symbol)
• Phase shift key (PSK) (1-3 bits/symbol)
• Quadrature amplitude modulation (QAM)
• Bit rate symbol rate x bits/symbol

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Legacy analog modems

• Bell 103 (1960s) FDM, FSK, 0-300 bit/s
• Bell 212 (1970s) FDM, PSK, 1200 bit/s
• V.22bis (1984) FDM, QAM, 2400 bit/s
• V.32 (1986) ECM, QAM, 9600 bit/s
• V.32bis (1991) ECM, QAM, 14400 bit/s
• V.34 (1994) ECM, QAM, 28800 bit/s
• V.34 (1996) ECM, QAM, 33600 bit/s

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How V.34 got so fast, 1 of 3

• Probes the line at startup for maximum bandwidth
  (V.32 uses 2400 Hz symbol rate V.34 max is 3429,
  10/7ths larger)
• Probes the line attenuation characteristics, and
  pre-emphasizes (like Dolby).
• Advanced forward error control, 16 state 4D Wei
  codes, 20 overhead.

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How V.34 got so fast, 2 of 3

• Adaptively adjusting equalization and precoding.
• Nonlinear Encoding introduce distortion to
  compensate for PCM encoding.
• Shell Mapping map data bits to signal points in
  a multidimensional constellation, partitioning a
  2-D signal constellation into rings containing an
  equal number of points.

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How V.34 got so fast, 3 of 3

• Given all the improved S/N ratio from the
  preceding tricks, using very dense symbol
  constellations
• V.32 (1986) 32 points - 41 bits/symbol
• V.32bis (1991) 128 points - 61 bits/symbol
• V.34 (1994) 960 points - net 8.4 bits/symbol
• V.34 (1996) 1664 points - net 9.8 bits/symbol

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How can you go faster?

• Change the system go digital on one side, with
  ISDN or T1 lines
• Only one analog loop - halve the impairments.
• No echo from digital side.
• No quantization noise in downlink direction.
• Direct access to the G.711 PCM clock 8kHz
• Direct access to the G.711 PCM DAC 8 bits

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Traditional Modem Model
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PCM Digital Modem Model
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Why cant we go 64000 bit/s?

• Residual noise hurts close to the origin.
• Signal power limits clip peak code values.
• Codec filters hurt close to 4kHz.
• Line card transformers wont pass DC.
• Ring detectors are a big load below 100Hz.
• Other digital impairments, particularly RBS and
  Digital Loss Pads.

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Bit rate limiting envelope
max signal, from FCC
Signal power
-10dbm
minimum frequency, from DC and Ring detectors
maximum frequency, from codec filters
70Hz
3900 Hz
noise floor, from codec quantizer
-48dbm
frequency
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How do we get close to 56K?

• Adaptively probe the line, and determine the
  operational limits
• Mapping and framing multiple modulus conversion
  (3Com) - fractional bits/symbol
• Convolutional Spectral Shaping (Motorola)
• Digital Impairment Detection Mitigation.

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Digital Impairment Training

• The analog modem sends a DIL Descriptor (Table
  12/V.90) to the digital modem, which is used to
  generate a PCM signal sequence back to the analog
  modem.
• The analog modem continues this process until it
  has identified the digital network impairments,
  and informed the digital modem of how to
  compensate.

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Example DIL Parameters

• N, number of segments (0-255)
• Lsp, Ltp, lengths of sign training patterns
• SP, Single Sign Pattern bit map sequence.
• TP, Training Pattern bit map sequence.
• H1-8 signal length multipliers (L6(H1))
• REF1-8 reference symbol values.

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Uses for DIL sequences

• Identify and locate RBS
• Calibrate digital loss pads
• Calibrate the PCM codecs
• Recognize and compensate for A-Law to mu-law
  conversion.

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What do we get realistically?

• In Microsofts WHQL test lab, with average line
  simulators, X2 units typically ran at 50K,
  K56flex units typically ran 44K.
• Modem vendors contributed new tricks, hoping to
  bargain later on patents. V.90 products benefit,
  but the lawyers do, too.
• WHQL is tooling up to do V.90 inter-operability,
  but we dont have new data yet.

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Is this asymmetric?

• X2, K56Flex and V.90 are asymmetric, with a V.34
  uplink and a PCM downlink.
• Lucent has proposed means to calibrate the line
  delay from the user modem to the local office.
• This trick will allow a PCM uplink as well, maybe
  44K bit/s or higher.
• That may make IP telephony H.323
  video-conferencing usable on modems.

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What does this cost?

• Not much, actually!
• There is a lot less DSP work to do per symbol for
  PCM modems vs V.34.
• But, there are more symbols 8000 vs 3429
• The analog front ends are similar.
• Result the hardware is essentially the same.
  Private estimate are 20MIPS for both.
• There are at least 8 vendors planning to offer
  software based modems.

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The V.90 standards process

• Nobody wants to buy a modem that is obsolete in
  less than a year.
• ITU Study Group 16, fed by TIA TR-30.1, drove
  this aggressively, meeting monthly.
• They determined a draft in February.
• They plan to decide it next September.
• Anything that says V.90 today is beta.

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Where to go from here?

• The second version of V.90 (Issue 2) will
  address duplex PCM modulation.
• All the modem vendors are working on ITU
  G.lite, from Q4/15, also known as Universal
  Asymmetric Digital Subscriber Line (UADSL). It
  uses the same phone wire, but with much wider
  spectrum, and runs a lot faster.

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References (1 of 2)

• Modems
• Theory and Practice of Modem Design, Bingham,
  Wiley Interscience.
• Data Communication, Lee Messerschmidt, KAP,
  book on modem DSP
• V.34 and V.90
• The V.34 High Speed Modem Standard
• The Information Driveway
• IEEE Communications Magazine, vol34.12

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References (2 of 2)

• ITU-T V.34-1996
• ITU-T Q.23/16 TD70(Plen), Proposed text for
  V.pcm, February 4, 1998.