ANALYSIS & OPINION: COHERENT OPTICS Figure 1: growth at the network edge will affect network core requirements
lower modulation-order modes (lower bits/ symbols) provide lower capacity with farther reach. Integer bits/symbol steps may result in sub-optimal capacity use due to link margin gaps. Fractional QAM modulation enables non- integer steps, supporting many small increments between the integer steps. With this feature, the link margin can be optimised with much greater resolution than traditional interconnect technology.
operators can maximise return-on-investment. To accommodate varying traffic from the
edge into the core, a network may require different optimisation requirements in different parts of the network. For example, a leased optical interconnect linking two data centres may require the highest raw capacity achievable for low cost-per-bit between sites. Another example requiring flexibility is in metro or long- haul core environments with disparate channel impairments spanning a metro and/or long-haul link that require filling up the available spectral channel with a flexible control mechanism to optimise fibre use. Tis optimisation is critical when dynamic network reconfiguration is required to reroute optical channels for capacity reallocation or node failover onto alternate routes. Finally, for a long-haul or submarine link, achieving highest spectral efficiency may be the primary network requirement. Coherent technology is well-suited to address
these different requirements. For example, recent advances in coherent technology enable soſtware control of the modulation mode and baud rate, which then allows the same hardware to be used in multiple parts of a network (edge, metro, long-haul and submarine) resulting in a cost-effective way for network operators to scale their networks. Control of channel width, enabled by flexible reconfigurable optical add-drop multiplexers (ROADMs), is also key to a flexible core network. Here, we focus on the coherent transmission element of the network.
Figure 2: three elements of 3D shaping
Te rise of transmission shaping technologies Another recent advancement that supports network optimisation is the use of coherent transmission-shaping technologies – essentially, taking the soſtware control of modulation mode and baud rate to a higher level of granular control. Tese technologies add an increased level of network flexibility and optimisation control, turning capacity gaps into usable bandwidth by adapting the optical transmission to the network. 3D shaping is one example of this approach. Tis enables flexible fine-tuning of the line-side optical transmission, allowing network operators to optimise capacity, reach, and spectral utilisation. It is a power-efficient technology solution that pushes optical transmission capacity closer to the Shannon limit. Tere are three benefits of 3D shaping:
increase of capacity by shaping constellation points’ probability with Fractional QAM; increase of reach by shaping constellation points’ location; and maximisation of channel passband usage by shaping spectral width with adaptive baud rate.
Shape the constellation, increase capacity Typical coherent modulation modes, such as QPSK, 8QAM and 16QAM use integer bits/ symbol steps (eg., QPSK = 2 bits/symbols, 16QAM = 4 bits/symbols). Higher modulation- order modes (higher bits/symbols) provide higher capacity at the expense of reach, while
Improved channel performance Greater resolution of transmission baud rate with a wide range provides the benefit of optimising spectral passband utilisation of an optical channel, while enabling the use of lower modulation order operation to improve channel performance. In previous generation implementations, the baud rate of the coherent interconnect can be selected among a small number of setings. Having flexible continuous adaptive baud rate control gives network operators greater control to optimise channel passband use, which is especially important when channel passbands vary between links in the same network. Advanced transmission shaping solutions,
such as 3D shaping, enable the optical transmission technology to adapt to the network characteristics, accommodating a range of capacity and reach requirements, while improving network utilisation.
Feeding into the core Coherent solutions are also being deployed in edge/access aggregation networks. Today, pluggable coherent solutions supporting 100 and 200G links are already being used for transporting aggregated traffic into the core. In some cases, these modules are plugged directly into switches and routers, enabling IP-over- DWDM architectures. Looking to 400G, edge DCI applications
are expected to use modules based on the 400ZR standard, currently under development at the Optical Internetworking Forum. Tis interoperability standard targets modules using 400G client optic form factors, such as QSFP-DD and OSFP, for reaches of 80km and above. Te CFP2-DCO form factor is expected to be used for access and metro applications that require higher performance than 400ZR. Edge/access network deployments to
accommodate growing bandwidth demands may lead to a wider adoption of 100G/200G coherent aggregation links – with 400ZR on the horizon – using cost-effective pluggable modules. As we highlighted, these links will need to feed into a flexible core network that can be reconfigured in response to varying link capacity requirements. Coherent multi-haul solutions shine in these core networks, because they provide performance, flexibility and network optimisation. Network operators are planning to leverage all of these solutions to scale networks in capacity from the edge to the core, in order to achieve terabit speeds and beyond. n
Eugene Park is senior manager, technical marketing at Acacia Communications
30 FiBRE SYSTEMS n Issue 24 n Summer 2019
www.fibre-systems.com @fibresystemsmag
Acacia Communications
Acacia Communications
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