conference & convention enabling the next generation of networks & services Optical Layer Concatenation of Long- Span Amplified Systems Span Amplified Systems J Schwartz¹, S Ediridinghe¹, W Wong¹ Rodolpho Acebo Jr², Gregg Palinski³ ¹Xtera Communications, ²Pacnet, ³Global Crossing
conference & convention enabling the next generation of networks & services Presenter Profile Since joining the former Azea Networks, now Xtera, Joerg has managed the NXT systems definition and developed the company’s systems engineering competency, providing network solution design, field and lab trials, sales support, design, field and lab trials, sales support, and systems research. Previous experience includes System Design for Ericsson, submarine terminal development for Alcatel, and founding optical components supplier Quarterwave. Joerg Schwartz VP System Engineering Email: Joerg.Schwartz@xtera.com Tel: +44 1708 335408 Mobile Tel: +44 7817 394626
conference & convention enabling the next generation of networks & services Why link concatenations? ��� ��� New York • Avoids terminals at intermediate POP stations needed where no traffic ��� ��� is dropped Long Island ���� ���� USA • Allows extension of submarine line to Point of Presence • This means simpler, more reliable networks ��� ���� • Easiest on a per fibre pair basis Sylt CLS ��� ���� Germany • Possible also for single/bands of channels ���� Holland CLS ��� • Can be re-configured if demand changes (unlike Branching ��� ���� Units) Amsterdam ��� POP
conference & convention enabling the next generation of networks & services Concatenation Options • Type of links to be combined – Repeatered submarine with repeatered submarine – Repeatered submarine with unrepeatered submarine – Repeatered submarine with terrestrial – Unrepeatered submarine with unrepeatered submarine – Unrepeatered submarine with unrepeatered submarine → see Poster by Philippe Perrier – Combinations of these • Location of interconnect – At SLTE client interface – trivial but costly (only saves SDH equipment) – At cable station interface – most common – At beach manhole
conference & convention enabling the next generation of networks & services Potential Issues and Challenges • Implementation – Compatibility of different links – Compatibility of line currents (if PFE is shared) • Network management – Removal of network elements requires re-configuration – Removal of network elements requires re-configuration – Line performance monitoring – Management access to remote pass-through amplifiers • Transmission – Impact on operating margins – Impact on maximum link capacity • A range of transmission issues are possible and require a closer look…
conference & convention enabling the next generation of networks & services Loss and Link Attenuation • If an existing segment is extended by a unrepeatered terrestrial or subsea link, this reduces the power at the input of the receiver, or the first repeater • Linking amplified segments a high-loss amplification span at the interconnection has to be avoided– this would reduce the Optical Signal to Noise Ratio (OSNR) and performance 12.00 Amplifier 11.00 11.00 no Amplifier OSNR (dB/nm) 10.00 Concatenation of 9.00 two similar 1800 km 8.00 segments with long 7.00 shore end spans 6.00 1545 1550 1555 1560 1565 Wavelength (nm) • Insertion of a optical ‘pass-through’ amplifier at the interconnection point fixes the problem
conference & convention enabling the next generation of networks & services Optical Signal to Noise Ratio • As for single span line design, adequate receive OSNR performance is first priority • Most submarine systems – independent of length – are designed to meet a target OSNR of ~7 dB (in 1 nm) – Cascading two segments reduces this by 3 dB, i.e. to ~4 dB/nm 20 Original Link Original Link OSNR (dB/nm) Extension with same characteristics 15 Extension with different characteristics 10 - 3 dB 5 Performance Limit 0 0 2 4 6 8 10 12 14 Distance (1000 km) • Reduced OSNR leads to degraded Q and/or less channels being supported
conference & convention enabling the next generation of networks & services Capacity Impact Reduced OSNR means that margins are reduced by ∆ Q • • To recover this and maintain the capacity, SLTE with improved performance (improved FEC gain, less photons per bit) is needed • Alternatively, the number of wavelengths sharing the (fixed) repeater power has to be reduced by a factor of ~10 ( ∆ Q/10) , e.g. 0.8 for -1 dB 5 5 Observed Q Penalty (dB) Nonlinear Region 4 3 6200+6200 km (10G) 2 2000+2000 km (40G) 2000+1000 km (10G) 1 2300+2300 km (20G) 7000+500+100 km (20G) 0 0 1 2 3 4 5 Distance Imposed OSNR Degradation (dB) • If Q penalty is higher than OSNR degradation in dB, this suggests that additional degradations apply, such as nonlinearities
conference & convention enabling the next generation of networks & services Nonlinearities • Nonlinear propagation impairments increase with transmission distance, hence they are more pronounced for extended links • The lower nonlinear threshold reduces the achieve able Q 15 Original link (7000 km) 14 Extended link (7600 km) 13 13 12 11 Q(dB) 10 9 8 0 1 2 3 4 5 6 7 8 OSNR (dB/nm) • Detailed computer simulations help understanding and mitigating the impact of nonlinearities, e.g. via dispersion management
conference & convention enabling the next generation of networks & services Dispersion • Repeatered links are usually fully dispersion managed – However links can be differently designed, e.g. different dispersion slope or zero accumulated dispersion wavelength • Terrestrial/unrepeatered links use G.652/G.654 fibres. If added to a link, these off-set the dispersion pre-/post-compensation S2 S3 S2 S3 S1 S1 2000 2000 Dispersion (ps/nm) 1500 1000 500 0 -500 -1000 -1500 -2000 0 1000 2000 3000 4000 5000 6000 7000 8000 Distance (km) • Dispersion management needs to checked for entire bandwidth and adjusted at intermediate location as required, e.g. via DCF+amplifiers
conference & convention enabling the next generation of networks & services Gain Shape • Combining different generation wetplant or links made by different original suppliers often means mixing very different gain characteristics • Mismatch leads to distorted dispersion maps and different gain characteristics in the two different directions, reducing the bandwidth Segment 5 only Segment 5 only Segment 5+6 Segment 5+6 Segment 6 only Segment 6+5 • Intermediate addition of filters or loading waves reduce the impact
conference & convention enabling the next generation of networks & services Pass-Through Optimisation • The impact of the gain mismatch can be mitigated by intermediate elements • Gain shaping filters can be used to re-adjust the channel powers before re-launch of the signal into the next segment • In the example below loading waves have been added at the concatenation location Segment A Segment B Segment A+B, loaders added Segment A+B 1530 1540 1550 1560 1570 1580 • This avoids build-up of 1538 nm gain peak and optimises OSNR
conference & convention enabling the next generation of networks & services Conclusions • Serial concatenation of several subsea links, or extending them with unrepeatered/terrestrial spans helps optimising submarine network topologies • The capacity impact of increasing the transmission distance can usually be minimised by using advanced distance can usually be minimised by using advanced terminal equipment • If the links to be combined are very dissimilar, intermediate elements such as dispersion compensation, amplification, filtering, or loading must be considered • Careful system design, validated by computer simulations or practical tests are important for successful implementation
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