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Some Recent Topics in Physical-Layer System Standards

Some Recent Topics in Physical-Layer System Standards. Felix Kapron Standards Engineering. 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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Some Recent Topics in Physical-Layer System Standards

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  1. Some Recent Topics inPhysical-Layer System Standards Felix Kapron Standards Engineering

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

  3. Allocation of Spectral Bands - Sup.dsn

  4. 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

  5. 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.

  6. CWDM Wavelength Grid - G.694.2

  7. 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

  8. 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.

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

  10. 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

  11. 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.

  12. 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’.

  13. 10,000 Chromatic Dispersion(ps/nm) 1,000 100 10 1 10 100 Source 20-dB Width (GHz) 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 (f=1/3), 2dB penalty 1 3 2 4

  14. 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

  15. 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

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

  17. Multi-Span Full Transverse Compatibility

  18. Multi-Span Single-InterfaceTransverse Compatibility

  19. 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).

  20. Multi-Span Limited Transverse Compatibility

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