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QSFP-DD is a new module and cage/connector system similar to current QSFP, but with an additional row of contacts providing for an eight lane electrical interface. It is being developed by the QSFP-DD MSA as a key part of the industry’s effort to enable high-speed solutions.
A: The QSFP-DD MSA Group has defined the next generation, high-density, high-speed pluggable module form factor that addresses the industry need for high-density, high-speed networking solutions in a backward compatible form factor. The QSFP-DD Specification has been developed and refined by many companies within the QSFP-DD MSA group and released to the public.
A: Until recently the most popular interfaces in the networking industry consisted of single or quad lanes. However, to accommodate expected demand for data bandwidth or channel capacity, eight lane interfaces are being defined in venues such as Ethernet. The currently available form factors that support eight lane interfaces do not have all the desired features or density necessary to support the next generation systems that plan to implement these higher rate interfaces.
A: A successful form factor must support the entire set of media and transceiver types prevalent in the networking industry. For media this includes passive direct attached copper cables (DAC), multi-mode fibers (MMF) and single-mode fibers (SMF). For transceivers and active copper or active optical cable assemblies, this includes those defined by Ethernet, Fibre Channel, and InfiniBand for 100 Gbps, 200 Gbps and 400 Gbps. In addition, port density must not be compromised from that of current practice. Further, backward compatibility with the popular QSFP form factor is essential for industry adoption.
A: Backwards compatibility is critically important to the industry. Since ASICs are designed to support multiple interface rates, it is critically important that the system can take advantage of this. End users can take advantage of the newer ASIC and system products with lower port costs and are able plug in a wide range of currently available QSFP modules to support their desired media and reach without needing to have separate system products. This greatly decreases the risks associated with implementing new equipment. System designers can build common products that support a multiple of pluggable variants while leveraging known technologies and designs. Module designers do not need to port their lower rate designs into new non-backwards compatible form factors lowering their overall costs. The economy of scale achieved due to backwards compatibility make it highly desirable.
A: The QSFP-DD Specification defines a new pluggable form factor that supports 8 high speed electrical interfaces connecting to the host. This will include a mechanical module, a single height 1×1 surface mount cage and connector and a press fit 2×1 cage with integrated connector, thermal, pinout and management specifications. The QSFP-DD can support both optical and copper interfaces, but those physical layer specifications will be defined outside of the MSA specification as is usual for form factor MSA specifications.
A: Due to industry need, most pluggable form factors eventually see developed a two-high stacked cage-connector system in addition to a one-high cage connector system. Often the one-high system is included in the initial MSA specification and the two-high is left to independent individual suppliers. To serve better the industry, the QSFP-DD MSA group chose to develop concurrently both the one-high and the two-high cage-connector systems.
A: The mechanical interface for QSFP-DD on the host board will be slightly deeper than for QSFP28 to accommodate the extra row of contacts. The height and width will be identical to the QSFP form factor enabling system designers to achieve identical system port count densities for QSFP28 or QSFP-DD based designs. Yes, you can plug any current QSFP module into the QSFP-DD 1×1 or the 2×1 cage / connector combinations.
A: As is usual with MSA specifications for module form factors, many other standards specifications are used for the electrical and optical interfaces. QSFP-DD will support both CAUI-4 and 400GAUI-8 electrical interfaces. CAUI-4 is already defined by IEEE and 400GAUI-8 is currently under definition in IEEE. The external optical or copper interfaces that could be supported span 50 GbE, 100 GbE, 200 GbE and 400 GbE which are well covered in completed or under development IEEE standards or by other MSAs defining these optical interfaces.
A: The QSFP-DD Specification does not constrain what type of interfaces that could be supported in it. It is anticipated that the dominant implementations of QSFP-DD products will be focused on high volume, cost sensitive client-side solutions.
A: The QSFP-DD Specification defines power classes up to 14 Watts as well as a >14 Watts class. Due to innovative thermal management techniques used in the module and cage designs, QSFP-DD modules support power levels of at least 12 Watts in a typical system design. The extensive knowledge and experience of system design with QSFP family form factors enables innovative systems solutions that could extend beyond that range. Thermal management features needed for the higher power consumption classes are relaxed for the lower power classes to avoid unnecessary costs.
A: The system port densities are identical between QSFP-DD and QSFP28 module specifications. However, since each QSFP-DD port can accommodate 8 lanes instead of 4, QSFP-DD doubles the number of ASIC ports it supports for existing interfaces such as CAUI-4.
A: It is anticipated that QSFP-DD will become a useful family of modules for the industry with applications at 400 GbE and beyond. At this time there are no efforts underway to define these new speeds but it is expected that QSFP-DD will evolve to support higher data rates for future applications.
A: The term “Double-Density” refers to the doubling of the number of high-speed electrical interfaces that the module supports compared to the regular QSFP28 module. The connector specification is ready for the new PAM4 electrical modulation format that supports 50Gb/s that provides another doubling of speed resulting is a 4x increased in port speed for the QSFP-DD compared to the QSFP28 module.Q: WHY IS IT CALLED “DOUBLE-DENSITY” WHEN IT APPEARS TO QUADRUPLE THE PORT SPEED COMPARED TO QSFP28?
