Insights
Data Center Interconnects: Coherent or Direct Detect?
Article first published 15 June 2022, updated 18 May 2023.
With the increasing demand for cloud-based applications, datacom providers are expanding their distributed computing networks. Therefore, they and telecom provider partners are looking for data center interconnect (DCI) solutions that are faster and more affordable than before to ensure that connectivity between metro and regional facilities does not become a bottleneck.
As shown in the figure below, we can think about three categories of data center interconnects based on their reach
- Intra-data center interconnects (< 2km)
- Campus data center interconnects (<10km)
- Metro data center interconnects (<100km)
Coherent 400ZR now dominates the metro DCI space, but in the coming decade, coherent technology could also play a role in shorter ranges, such as campus and intra-data center interconnects. As interconnects upgrade to Terabit speeds, coherent technology might start coming closer to direct detect power consumption and cost.
Coherent Dominates in Metro DCIs
The advances in electronic and photonic integration allowed coherent technology for metro DCIs to be miniaturized into QSFP-DD and OSFP form factors. This progress allowed the Optical Internetworking Forum (OIF) to create a 400ZR multi-source agreement. With small enough modules to pack a router faceplate densely, the datacom sector could profit from a 400ZR solution for high-capacity data center interconnects of up to 80km. Operations teams found the simplicity of coherent pluggables very attractive. There was no need to install and maintain additional amplifiers and compensators as in direct detection: a single coherent transceiver plugged into a router could fulfill the requirements.
As an example of their success, Cignal AI forecasted that 400ZR shipments would dominate edge applications, as shown in Figure 2.
Campus Interconnect Are the Grey Area
The campus DCI segment, featuring distances below 10 kilometers, was squarely in the domain of direct detect products when the standard speed of these links was 100Gbps. No amplifiers nor compensators were needed for these shorter distances, so direct detect transceivers are as simple to deploy and maintain as coherent ones.
However, as link bandwidths increase into the Terabit space, these direct detect links will need more amplifiers to reach 10 kilometers, and their power consumption will approach that of coherent solutions. The industry initially predicted that coherent solutions would be able to match the power consumption of PAM4 direct detect solutions as early as 800G generation. However, PAM4 developers have proven resourceful and have borrowed some aspects of coherent solutions without fully implementing a coherent solution. For example, ahead of OFC 2023, semiconductor solutions provider Marvell announced a 1.6Tbps PAM4 platform that pushes the envelope on the cost and power per bit they could offer in the 10 km range.
Following the coming years and how the PAM-4 industry evolves will be interesting. How many (power-hungry) features of coherent solutions will they have to borrow if they want to keep up in upcoming generations and speeds of 3.2 Tbps and beyond? Lumentum’s Chief Technology Officer, Brandon Collings, has some interesting thoughts on the subject in this interview with Gazettabyte.
Direct Detect Dominates Intra Data Center Interconnects (For Now…)
Below Terabit speeds, direct detect technology (both NRZ and PAM-4) will likely dominate the intra-DCI space (also called data center fabric) in the coming years. In this space, links span less than 2 kilometers, and for particularly short links (< 300 meters), affordable multimode fiber (MMF) is frequently used.
Nevertheless, moving to larger, more centralized data centers (such as hyperscale) is lengthening intra-DCI links. Instead of transferring data directly from one data center building to another, new data centers move data to a central hub. So even if the building you want to connect to might be 200 meters away, the fiber runs to a hub that might be one or two kilometers away. In other words, intra-DCI links are becoming campus DCI links requiring their single-mode fiber solutions.
On top of these changes, the upgrades to Terabit speeds in the coming decade will also see coherent solutions more closely challenge the power consumption of direct detect transceivers. PAM-4 direct detect transceivers that fulfill the speed requirements require digital signal processors (DSPs) and more complex lasers that will be less efficient and affordable than previous generations of direct detect technology. With coherent technology scaling up in volume and having greater flexibility and performance, one can argue that it will also reach cost-competitiveness in this space.
Takeaways
Unsurprisingly, using coherent or direct detect technology for data center interconnects boils down to reach and capacity needs. 400ZR coherent is already established as the solution for metro DCIs. In campus interconnects of 10 km or less, PAM-4 products remain a robust solution up to 1.6 Tbps, but coherent technology is making a case for its use. Thus, it will be interesting to see how they compete in future generations and 3.2 Tbps.
Coherent solutions are also becoming more competitive as the intra-data center sector moves into higher Terabit speeds, like 3.2Tbps. Overall, the datacom sector is moving towards coherent technology, which is worth considering when upgrading data center links.