The explosive growth of traffic, generated by large amounts of data from cloud computing, AR/VR, AI, and 5G, puts a further strain on data center architectures and the underlying interconnects. With the increasing demand for high network bandwidth and its performance, the exploration of 800G optical modules begins to attract the attention of the industry. 800G Pluggable MSA released a white paper on 800G optical module technology which introduces the main application scenarios of 800G. The article will take you through the 8×100G solution for SR scenario.
For transmission at the class of 100m, many hyper-scale data center operators employ VCSEL multi-mode technology that likely allows for reaches of 30-50m, thus only partially covering the SR class. The MSA aims to develop a low-cost 8×100G module for SR applications, supporting 60-100m transmission. Particularly, the MSA is intended to specify a lower-cost transmitter technology with the potential to leverage sub-linear cost scaling with a high degree of integration. The low-cost 800G SR8 could also provide a low-cost serial 100G server interconnect to support the current data center trends of increasing switch ports and decreasing numbers of servers per rack.
Compared with the conventional SR modules, the 800G SR8 will not use VCSEL-based multi-mode technology but adopt PSM8, with PAM4 modulation formats and built-in DSP, to achieve 800G SR scenario requirements, as shown below.
To guarantee the advantages on the cost and power consumption of the SMF-based solution, the 800G Pluggable MSA established three principles for the definition of reasonable PMD standard requirements that are indispensable in 800G-SR8.
1. Diverse transmitter techniques, such as DML, EML, and silicon photonics (SiPh), can be applied in such scenarios.
2. All the potential of the components can be fully utilized to achieve the target link performance.
3. Key parameters in the PMD layer should be relaxed as much as possible while maintaining a reliable link performance.
The power budget of the SMF-based 800G-SR8 solution would be similar to that defined in IEEE 400G-SR8, with the only issue being the insertion loss of newly defined PSM8 SMF connectors. This means that currently mature optical and electronic components and DSP ASICs used in 400GE optical connections are sufficient to meet the power budget requirements of the 800G-SR8.
Besides, the definition of PMD parameters in 800G SR8 scenario also faces the challenge of finding out the suitable optical modulation amplitude (OMA), extinction ratio (ER), transmitter dispersion eye closure quaternary (TDECQ) of the transmitter, and sensitivity of receiver. The 800G Pluggable MSA compared the bit error rate (BER) performance of different transmitters to find the appropriate metrics for these parameters.
The above are three BER vs OMA curves of 100Gbps PAM4 signal based on commercial 400G DSP ASICs, which correspond to different transmitter technologies respectively. The BER performances of EML and SiPh for 100G per lane are well-known results. The BER performance of the DML shows that the OMA sensitivity, in this case, is comparable with that in the case of EML or SiPh. Considering the relatively low launching optical power of SiPh transmitter and sufficiently good sensitivity of all three solutions, the minimum OMA requirement in 800G-SR8 is recommended to be relaxed appropriately. The comparable sensitivity also indicates that the commercial DSP ASICs have much stronger equalization ability than the reference receiver IEEE defined in 400GE.
Meanwhile, considering the relatively low sensitivity requirement in SR scenario and limitation of the power consumption of the 800G module, a low complexity DSP mode is recommended in future modules. The ER is directly related to power consumption. Theoretically, a lower ER is desirable as long as it does not affect the reliability of the link. Based on the above analysis, the MSA believes that an SMF-based solution with low cost and low power consumption is feasible and promising in 800G-SR8 scenario.