Some Recent Topics in Physical-Layer System Standards

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Some Recent Topics inPhysical-Layer System
Standards

• Felix Kapron
• Standards Engineering

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Outline

• Spectral Bands
• CWDM and DWDM
• New Broadband Fibre
• Chromatic Dispersion Limitations
• Issues with NRZ and RZ
• Transverse and Longitudinal Compatibility
• Conclusions

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Allocation of Spectral Bands - Sup.dsn
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Spectral Band Conditions

• The definition of bands is not for specification
  that is left to systems Recommendations.
• Not all fibres will use all bands for system
  operation or maintenance.
• The U-band
• for possible maintenance purposes only
• fibre operation is not ensured there
• must cause negligible interference to signals in
  other bands

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Course Wavelength Division Multiplexing

• To allow simultaneous transmission of several
  wavelengths with sufficient separation to permit
  the cost-effective use of
• uncooled sources, allowing some wavelength drift
  with temperature
• relaxed laser wavelength selection tolerances for
  higher yield
• wide pass-band filters
• Wavelength spacing no less than 20 nm is optimal.
  
• Applications are to broadband access and metro.

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CWDM Wavelength Grid - G.694.2
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DWDM Frequency Grid - G.694.1

• Moved out of obscure Annex A of G.692.
• Channel spacings (in GHz) of 12.5, 25, 50, 100
  and above.
• Example nominal central frequencies for 50 GHz
  spacing.Allowed channel frequencies (in THz)
• 193.1 n ? 0.05where n is a positive or
  negative integer including zero

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Advanced Fibres - G.scu

• For broadband optical transport over theS C
  U bands, 1460 - 1625 nm
• With chromatic dispersion coefficient (under
  study)
• positive or negative
• above zero in magnitude
• to suppress four-wave mixing etc. in DWDM
• not too large in magnitude
• to avoid excessive dispersion compensation
• With specified attributes for the fibre, cable,
  and link.

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Broadband Fibre G.scu Dispersion
Chromatic Dispersion Coefficient (ps/nm-km)
positive dispersion
Wavelength (nm)
1465
1625
negative dispersion
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Chromatic Dispersion Limitations - old approach

• Began with G.957 on SDH up to 2.5 Gbit/s
• Continues through G.693 on intra-office
  systemsup to 40 Gbit/s
• chromatic dispersion (ps/nm) worst-case fibre
  chromatic dispersion coefficient (ps/nm-km)
  ? optical path length (km)
• bit-rate ? CD ? source linewidth number
  depending on desired power penalty
• Allowed CD(?) determines the Tx wavelength window

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CD Limitations - problems

• Tied to fibre, not signal.
• Sets an artificial fibre CD limit often far below
  what the signal will actually tolerate.
• Can unnecessarily limit
• transmitter wavelength window and spectral width
• the added CDs of in-line components
• Fails when the high bit-rate modulation spectrum
  is wider than the narrow-line source spectrum.

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CD Limitations - new approach (Sup.dsn)

• (bit-rate ? wavelength)2 ? CD duty cycle
  ? number depending on desired power penalty
• duty cycle 1 for NRZ, ?1 for RZ
• leads to compensation requirements for longer 40G
  links (G.959.1) with tuning of residual
  dispersion.

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Minimum CD Required for Several NRZ and
RZBit-Rates and Power Penalties
1 10G NRZ, 1dB penalty 2 40G NRZ, 1dB
penalty 3 40G NRZ, 2dB penalty 4 40G RZ
(f1/3), 2dB penalty
1
3
2
4
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Issues with NRZ and RZ

• RZ advantages
• Lower energy per pulse reduces nonlinear effects.
• May reduce requirements for 1st-order PMD.
• RZ disadvantages
• Increases signal bandwidth
• lower tolerable chromatic dispersion of link
• higher bandwidth at the receiver
• more sensitive to 2nd-order PMD

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RZ Issues for Different Applications

• Optimal values of duty cycle
• Other formats, e.g., CRZ
• Maximum source linewidth
• Maximum spectral density
• Minimum contrast ratio
• Maximum CD deviation
• Maximum PMD
• Partitioning and measurement of path penalties

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MultiSpan Longitudinal Compatibility

• All network elements come from one vendor.
• Only the cable characteristics are specified
• attenuation, CD, PMD, reflections, ...

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Multi-Span Full Transverse Compatibility
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Multi-Span Single-InterfaceTransverse
Compatibility
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Conclusions

• Spectral bands and grids in wavelength
  frequency have been well defined.
• Work on a Recommendation on a new broadband fibre
  is beginning.
• 40G applications require a different method of
  specifying chromatic dispersion other
  applications may need corrections.
• New RZ and NRZ applications are being developed.
• Longitudinal and transverse compatibility is
  being actively discussed (with implications for a
  new IaDI Recommendation).

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Multi-Span Limited Transverse Compatibility
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