Bandwidth-intensive data services and applications have been driving continued demands for service providers to architect their networks for growth and flexibility. Client traffic from large bandwidth users such as hyperscale data centers has now reached 400G, thanks to recently introduced 400G port switches, routers and standardized 400G coherent optical modules. However, supporting 400G client traffic across multiple network segments can be a challenge if network operator legacy equipment is unable to address these data rates in an efficient, scalable, and cost-effective manner. Network operators surveying both terminal and line-system equipment to address this new 400G “currency” paradigm are looking ahead to ensure a certain level of cost-effective future proofing for higher capacity demands as client speeds increase beyond 400G. This article examines how recent advancements in terminal equipment coherent technology and line-system channel capabilities help them achieve that goal.
Coherent Optical Transmission
Today’s coherent optical transport terminal equipment provides channel capacities ranging from 100G to 1.2T. Currently available coherent digital signal processors (DSPs) enable network operators to optimize capacity and reach, while software controlled modulation and baud rate enabled by these DSPs allow the same hardware to be used in multiple parts of a network (data center interconnect edge, metro, long-haul, and submarine). This provides a cost-effective way to leverage common hardware for multi-haul applications. These DSPs also feature transmission shaping capabilities such as 3D Shaping, which includes shaping of constellations points as well as modulation and baud rate fine-tuning to optimize capacity, reach and spectral utilization for any line-system channel. Constellation shaping, including probabilistic constellation shaping, has helped to further close the gap to the Shannon Limit, the theoretical maximum capacity for a given channel.
Optical Line Systems and Wide Channels
Today’s line-systems enable light-path routing and channel bandwidth adjustment via software control, using reconfigurable optical add-drop multiplexers (ROADMs). Due to long upgrade intervals as a result of high capex and opex, careful consideration must be taken in the channel/passband requirements for next-generation line system deployments, given the various transmission options available. To ensure a meaningful return on investment (ROI), it is desirable to deploy a line system with not only flexible channel widths, but also with the widest channel capability in order to drive towards cost-efficiency as well as accommodate future higher capacity client rates and wider transmission spectrum.
A wide-channel-capable line system can also provide the network operator with the ability to accommodate multiple coherent transmission techniques. Even though today’s ROADMs can support a wide range of desired channel widths, it is preferable to utilize spacing based on regular multiples that can be combined without stranding spectrum due to fragmentation. Earlier generations of dense wavelength-division multiplexing (DWDM) networks have deployed 50, 75 and 100GHz channels. More recently, a focus on 150GHz-wide channels has been seen as the next evolutionary step for future-proofing the line-system for higher capacity network architectures, doubling the channel bandwidth of current 75GHz implementations.
Cost/Power/Size Benefits of Opto-electronic Integration
Advancements in opto-electronic integration within coherent modules have also progressed. Silicon photonics technology has enabled high levels of integration, low power consumption and high-volume manufacturing capabilities. These elements contributed to per-bit reductions of size, power, and cost of approximately 40% per year, enabling coherent applications to migrate towards shorter reach access and edge portions of the network. Recently, Acacia Communications introduced 400G coherent pluggable modules in high-density QSFP-DD and OSFP form factors. The benefit of these modules is that they can be plugged directly into switches/routers and offer the same density for both coherent DWDM and client optics in the same chassis.
With the ability to address multiple markets such as access, hyperscale, metro, and long-haul, these 400G integrated, high-volume solutions enable the alignment of component investments across a range of these applications.
Staying One Step Ahead of Demand
Service providers can stay ahead of the demand by building cost-effective, flexible, and scalable networks using coherent optical solutions and wide-channel capable line systems. This maximizes channel utilization and prepares for future client support beyond 400G, while achieving a desirable multi-generational ROI. In addition, a balanced long-term approach to technology roadmaps needs to consider not only the technology suited for maximizing channel capacity, but also the technology that leverages broader industry investments such as silicon photonics, opto-electronic integration, and volume manufacturing processes. All of these contribute to cost-effective scaling of the network.
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