Spurred by the introduction of 400ZR, the discussion on coherent pluggable optics is arguably the hottest topic in optical networking today. Combined with other major trends in open networking, disaggregation and automation, coherent pluggables are helping to usher in a new era in transport networks.

To understand the future of high speed coherent optics, Heavy Reading launched the Coherent Optics at 400G, 800G and Beyond Market Leadership Study with industry partners Cisco, Infinera and Juniper. Conducted in September, the survey attracted 81 qualified network operator responses worldwide.

This blog is the first of three highlighting the key findings from the coherent optics project. It focuses on 400ZR and 400G ZR+ pluggable optics and how these new optics will affect service provider network topologies and architectures.

Topologies for 400G pluggables

The 400ZR specification was defined by the Optical Internetworking Forum (OIF) with the goal of integrating 400G pluggable coherent DWDM modules within next-generation 400 Gigabit Ethernet routing and switching platforms. The hyperscalers see 400G coherent pluggables for massive point-to-point connectivity between data centers as their primary use case. However, one of the main themes to emerge from this Heavy Reading study is that coherent pluggable optics apply well beyond data center interconnect (DCI).

In the survey, 63% of respondents selected unamplified point-to-point links as the top expected topology for 400ZR — consistent with the dominant <80km DCI use case that drove 400ZR standardization. For 400G ZR+, amplified single-span topologies are the top choice, as communications service providers (CSPs) seek the higher performance of ZR+ to address longer DCI links in their networks.

More surprising is the strong interest in more complex topologies for both 400ZR and 400G ZR+. A majority of respondents (at 56%) anticipate 400G ZR+ in multi-span point-to-point networks and in ring/mesh topologies. At 53%, a majority of CSPs anticipate 400ZR pluggables in ring/mesh topologies and nearly half (46%) anticipate 400ZR pluggables in amplified multi-span networks.

We do expect 400ZR optics to dominate amplified single-span deployments, providing the lowest cost option in this use case. 400G ZR+ optics, with up to 0dBm transmitter output power, will likely provide a further performance boost for multi-span and ring/mesh topologies. Testing and validation will determine where 400ZR and 400G ZR+ are ultimately deployed, but CSPs clearly envision coherent pluggables addressing a broad spectrum of use cases in their metro and regional networks — well beyond point-to-point DCI.

Figure 1: What physical topologies are you considering for 400G pluggable optics? n=81
(Source: Heavy Reading, 2021)

System choices

Broad interest in coherent pluggable optics is also reflected in CSPs' choices of systems to house them, including fully integrated DWDM elements, disaggregated (or purpose-built) DWDM shelves and switches and routers. At 54% and 53%, respectively, fully integrated DWDM systems and switches/routers are statistically tied as the primary platforms of interest for coherent 400G pluggable optics. While still important for pluggables, disaggregated transponder shelves rank third, selected by 40%.

The survey results also confirm what Heavy Reading has been hearing from CSPs in presentations and in one-on-one discussions all year: coherent 400G pluggables are at the heart of a resurgence of interest in integrated DWDM optics on routers, historically known as IP over DWDM (IPoDWDM). In particular, CSPs are interested in 400G pluggables that fit in "client-side" form factors (i.e., QSFP-DD and OSFP) that can be swapped into switches/routers without sacrificing router capacity and faceplate density. These IPoDWDM pluggable optics include 400ZR and longer reach 400G ZR+ pluggables.

Figure 2: Where do you plan to physically place coherent 400G pluggable optics? n=81
(Source: Heavy Reading, 2021)

Feature requirements

Looking at feature requirements for coherent pluggables, two trends stand out: the importance of technology flexibility and the ability to operate in brownfield networks. Nx100G multiplexing and ROADM and filter compatibility top the list of important coherent pluggable features, selected as "critical" by 33% and 29% of respondents, respectively. Heavy Reading believes that 400G will become the new data rate currency for future builds, but 100G links will remain an important data rate in networks for a long time — thus requiring 100G multiplexing to pack 400G wavelengths efficiently. Similarly, ROADMs are widely adopted in metro and regional networks and must be accommodated.

Also important for coherent pluggable optics is Precision Timing Protocol (PTP) support to meet network synchronization requirements for 5G transport, selected as critical by 25% of survey takers. Layer 1 encryption for data centers is rated as highly relevant as well, although we expect this will more often be addressed through MACsec encryption when 400ZR and 400G ZR+ optics are used inside switches and routers.

Figure 3: Which of the following features are important for your coherent pluggable optics networks? n=81
(Source: Heavy Reading, 2021)

The Heavy Reading study shows that coherent pluggable optics are relevant in a wide range of use cases and are a key technology development for many CSPs. The industry is responding with further standardization efforts, such as OpenZR+, to drive more interoperability. Together with more suppliers releasing products, this will create a healthy and competitive ecosystem for a new era in transport networking.

Looking for more information? Coherent Optics at 400G, 800G and Beyond

This blog is sponsored by Juniper Networks.

— Sterling Perrin, Principal Analyst, Heavy Reading