Go Small to Go Big with 400 Gig Line Side Pluggables

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Windstream Wholesale set an industry-leading benchmark of 400G using Acacia’s 400G ZR+ Coherent Pluggables over a live long-haul network.

Windstream’s ROADM-based network. For 400G line side settings, the OpenROADM standard calls for a 16 QAM modulation scheme with a symbol rate of 63 GBaud and uses the OFEC standard for soft decision forward error correction, which is set to a coding gain of 15 percent. This lower coding gain also provides a side benefit of delivering a lower latency relative to the 20 to 25+ percent FEC coding gains commonplace in the industry today.

The Windstream technology R&D team also concentrated on the digital signal processor (DSP) of the CFP2 module, homing in on its true capability to cope with physical layer impairments such as noise, CD, PMD, filter narrowing and non-linear effects. The team performed full coherent transceiver characterization, subjecting the 400G ZR+ transceiver to the same rigorous tests used for the higher performance SLED-based transceivers.

The testing and characterization were performed on Acacia’s pre-general availability CFP2 module plugged into Cisco NCS 1004s, and the results were very encouraging. The team was excited to move this out of the lab environment to truly test pre-GA performance with its size and power efficiency. In the field, we could confirm if our core understanding and testing of these modules would translate to successful transmission and validate if Windstream’s disaggregation strategy truly supports multi-vendor, multi-technology interoperation. We tested performance with variables from a real-world environment, including various vintages of fiber types, uneven span distances and coexistence with various other types and generations of coherent transceivers.

Field validation

The first field trial attempt did not generate the successful results we were looking for. This was attributed to a higher percentage of the fiber spans being NZ-DSF fiber type than was originally expected. As we were approaching the upper limit for noise tolerance, the additional sectors of NZ-DSF fiber introduced higher-than-expected overall span non-linear impairments, which manifest as additional noise. The combined ASE noise and noise due to non-linearities was just enough to close down any window of operation. This exercise, although disappointing, provided us with good real-world data, which we then used to sharpen up our networking modelling and make some additional adjustments to both the CFP2 and Cisco’s NCS1004 host platform.

Figure 2: Testing results, second trial
click to enlarge

The second field trial was an overwhelming success. Windstream Wholesale set an industry-leading benchmark of 400G using Acacia’s 400G ZR+ Coherent Pluggables over a live long-haul network. The successful trial deployed 400 gigabits per second single-wavelength transmission over 1,027 kilometers across Windstream’s fiber network between Phoenix and Los Angeles. Additionally, Windstream set another industry record by looping back the signal to achieve 300G transmission over 2,054 kilometers using the Acacia pluggables. The test results showed that there was enough additional margin for actual production use. In other words, 400G line side pluggable technology was viable for long-haul network applications.

Figure 2 illustrates the testing results of the successful second field trial. It can be seen that the window for operation (area above CFP2 limit line) is large enough to meet the production network’s operation margin.

Jimmy Yu, vice president of Dell’Oro Group’s Optical Transport research concurred. “Windstream’s live network trial demonstrates the real performance of 400G pluggable outside of a controlled environment. This is a significant development in the industry,