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78 SMT007 MAGAZINE I SEPTEMBER 2019 Celestica, Ciena, DellEMC, Fujitsu, Hewlett- Packard Enterprise, Nokia, and Sumitomo Electric. QSFP-DD: Ultimate Performance The QSFP-DD MSA has publicly released two specifications. The first is the QSFP-DD hard- ware specification, which defines the electrical connector, module cage, optical connectors, and module types. The other specification is the QSFP-DD Common Management Interface Specification (CMIS), which defines the man- agement interface protocol between hosts and modules. QSFP-DD is the industry's smallest 400-GbE module, providing exceptional port bandwidth density. The standard features an eight-lane electrical interface with each lane achieving up to 50-Gbps data rates and enabling at least 20 W of power dissipation. It also supports 3 m of passive copper cables, 100 m over parallel mul- timode fiber, 500 m over parallel single-mode fiber, and 2 km and 10 km over duplex single- mode fiber. The QSFP-DD connectors/cage are fully backward compatible with legacy QSFP modules and cables (Figure 1). Typical older data center interconnect solu- tions support single- or quad-lane data transfer; moreover, the few eight-lane form factors avail- able are not fully equipped for next-generation systems. QSFP-DD accommodates the band- width/channel capacity, electrical demands, and thermal requirements of high-speed net- working. Importantly, QSFP-DD intercon- nects can also deliver 400-Gbps performance for long-reach coherent interconnects via the help of innovative heat sinks. This is critical, as advanced ASICs consume more power and dissipate more heat. The QSFP-DD standard spans both software and hardware. CMIS describes the software methodology and registers required for the host to control QSFP-DD modules based on a two-wire serial interface. CMIS is intended to cover a wide range of possible module func- tionalities and applications from cable assem- blies to coherent dense wavelength division multiplexing (DWDM) modules. The QSFP-DD hardware specification, by contrast, defines the mechanical transceiver, connectors, and cages, including the electrical and optical inter- faces. It also defines the thermal properties of the QSFP-DD module, connector, and cage sys- tem. QSFP-DD connectors accept legacy QSFP cables and transceivers. QSFP-DD Performance Specifications While QSFP-DD supports both line-side and client-side interfaces, it's anticipated that the dominant implementations will be for high- volume, cost-sensitive, client-side use. A wide range of media and transceiver types are sup- ported, including direct-attached copper (DAC) cables, multi-mode fiber (MMF), single-mode fiber (SMF), wavelength division multiplexing (WDM), and coherent modules. Transceivers and active copper/active optical cable assem- blies will support 100, 200, and 400 Gbps for Ethernet, fiber channel, or InfiniBand (IB) applications. The QSFP-DD connector system is fully back- ward-compatible with all QSFP-based transceiv- ers and cables from 40 to 200 Gbps, including QSFP+ and QSFP28. Backward compatibil- ity is important, especially in this transitional era; many ASICs, for example, are designed for multiple interface rates. Having the ability to span multiple speeds allows engineers to stag- ger implementations across the data center and supports economies of scale in purchasing. Thermal management is an important part of the QSFP-DD system as well. A flat-top design allows vendor- and/or application-specific rid- ing heat sinks and/or heat pipes to accom- Figure 1: Molex QSFP-DD cage and cable.