1 Some Recent Topics in PhysicalLayer System Standards

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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 3 Standards Engineering

Allocation of Spectral Bands - Sup. dsn 4 Standards Engineering

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 Standards Engineering

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 Standards Engineering

CWDM Wavelength Grid - G. 694. 2 7 Standards Engineering

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. 05 where n is a positive or negative integer including zero 8 Standards Engineering

Advanced Fibres - G. scu • For broadband optical transport over the S + 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 Standards Engineering

Broadband Fibre G. scu Dispersion Chromatic Dispersion Coefficient (ps/nm-km) positive dispersion 1465 1625 negative dispersion 10 Standards Engineering Wavelength (nm)

Chromatic Dispersion Limitations - old approach • Began with G. 957 on SDH up to 2. 5 Gbit/s • Continues through G. 693 on intra-office systems up 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 Standards Engineering

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 Standards Engineering

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 40 G links (G. 959. 1) with tuning of ‘residual dispersion’. 13 Standards Engineering

Minimum CD Required for Several NRZ and RZ Bit-Rates and Power Penalties 10, 000 1: 10 G NRZ, 1 d. B penalty Chromatic Dispersion (ps/nm) 1, 000 2: 40 G NRZ, 1 d. B penalty 3: 40 G NRZ, 2 d. B penalty 4: 40 G RZ (f=1/3), 2 d. B penalty 1 100 3 2 4 10 100 Source 20 -d. B Width (GHz) 14 Standards Engineering

Issues with NRZ and RZ • RZ advantages – Lower energy per pulse reduces nonlinear effects. – May reduce requirements for 1 st-order PMD. • RZ disadvantages – Increases signal bandwidth • lower tolerable chromatic dispersion of link • higher bandwidth at the receiver • more sensitive to 2 nd-order PMD 15 Standards Engineering

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 Standards Engineering

Multi. Span Longitudinal Compatibility • All network elements come from one vendor. • Only the cable characteristics are specified – attenuation, CD, PMD, reflections, . . . 17 Standards Engineering

Multi-Span Full Transverse Compatibility 18 Standards Engineering

Multi-Span Single-Interface Transverse Compatibility 19 Standards Engineering

Conclusions • Spectral bands and grids in wavelength & frequency have been well defined. • Work on a Recommendation on a new broadband fibre is beginning. • 40 G 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 Ia. DI Recommendation). 20 Standards Engineering

Multi-Span Limited Transverse Compatibility 21 Standards Engineering

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