QSFP-DD

Operators are strongly interested in 100G pluggables that can house tunable coherent optics in compact, low-power form factors like QSFP28. A recently released Heavy Reading survey revealed that over 75% of operators surveyed believe that 100G coherent pluggable optics will be used extensively in their edge and access evolution strategy.

These new 100G coherent pluggables will have very low power consumption (less than six Watts) and will be deployed in uncontrolled environments, imposing new demands on coherent digital signal processors (DSPs). To enable this next generation of coherent pluggables in the network edge, the next generation of DSPs needs ultra-low power consumption, co-designing with the optical engine, and industrial hardening.

The Power Requirements of the Network Edge

Several applications in the network edge can benefit from upgrading their existing 10G DWDM, or 100G grey links into 100G DWDM, such as the aggregation of fixed and mobile access networks and 100G data center interconnects for enterprises. However, network providers have often chosen to stick to their 10G links because the existing 100G solutions do not check all the required boxes.

100G direct detect pluggables have a more limited reach and are not always compatible with DWDM systems. “Scaled down” coherent 400ZR solutions have the required reach and tunability, but they are too expensive and power-hungry for edge applications. Besides, the ports in small to medium IP routers used in edge deployments often do not support QSFP-DD modules commonly used in 400ZR but QSFP28 modules.

The QSFP28 form factor imposes tighter footprint and power consumption constraints on coherent technologies compared to QSFP-DD modules. QSFP28 is slightly smaller, and most importantly, it can handle at most a 6-Watt power consumption, in contrast with the typical 15-Watt consumption of QSFP-DD modules in 400ZR links. Fortunately, the industry is moving towards a proper 100ZR solution in the QSFP28 form factor that balances performance, footprint, and power consumption requirements for the network edge.

These power requirements also impact DSP power consumption. DSPs constitute roughly 50% of coherent transceiver power consumption, so a DSP optimized for the network edge 100G use cases should aim to consume at most 2.5 to 3 Watts of power.

Co-Designing and Adjusting for Power Efficiency

Achieving this ambitious power target will require scaling down performance in some areas and designing smartly in others. Let’s discuss a few examples below.

• Modulation: 400ZR transceivers use a more power-hungry 16QAM modulation. This modulation scheme uses sixteen different states that arise from combining four different intensity levels and four phases of light. The new generation of 100ZR transceivers might use some variant of a QPSK modulation, which only uses four states from four different phases of light.
• Forward Error Correction (FEC): DSPs in 400ZR transceivers use a more advanced concatenated FEC (CFEC) code, which combines inner and outer FEC codes to enhance the performance compared to a standard FEC code. The new 100ZR transceivers might use a more basic FEC type like GFEC. This is one of the earliest optical FEC algorithms and was adopted as part of the ITU G.709 specification.
• Co-Designing DSP and Optical Engine: As we explained in a previous article about fit-for-platform DSPs, a transceiver optical engine designed on the indium phosphide platform could be designed to run at a voltage compatible with the DSP’s signal output. This way, the optimized DSP could drive the PIC directly without needing a separate analog driver, doing away with a significant power conversion overhead compared to a silicon photonics setup, as shown in the figure below.

Industrial Hardening for DSPs

Traditionally, coherent devices have resided in the controlled settings of data center machine rooms or network provider equipment rooms. These rooms have active temperature control, cooling systems, dust and particle filters, airlocks, and humidity control. In such a setting, pluggable transceivers must operate within the so-called commercial temperature range (c-temp) from 0 to 70ºC.

On the other hand, the network edge often involves uncontrolled settings outdoors at the whims of Mother Nature. It might be at the top of an antenna, on mountain ranges, within traffic tunnels, or in Northern Europe’s severe winters. For these outdoor settings, transceivers should operate in the industrial temperature range (I-temp) from -40 to 85ºC. Higher altitude deployments provide additional challenges too. Because the air gets thinner, networking equipment cooling mechanisms become less effective, and the device cannot withstand casing temperatures as high as they can at sea level.

Takeaways

The network edge could benefit from switching their existing direct detect or grey links to 100G DWDM coherent. However, the industry needs more affordable and power-efficient transceivers and DSPs specifically designed for coherent 100G transmission in edge and access networks. By realizing DSPs co-designed with the optics, adjusted for reduced power consumption, and industrially hardened, the network edge will have coherent DSP and transceiver products adapted to their needs.