QSFP-DD800 MSA group releases new hardware spec

By Gina Roos, editor-in-chief

The quad small-form-factor pluggable double-density 800 (QSFP-DD800 ) Multi Source Agreement (MSA) group has released a new hardware specification for the QSFP-DD800 transceiver form factor. The QSFP-DD800 interface expands on the QSFP-DD, a pluggable form factor with an eight-lane electrical interface, which has been widely adopted by the latest Ethernet switches.

The QSFP-DD800 MSA group, launched by eight founding members, was formed to work on technology advances for the high-speed, double-density QSFP modules, which support 800-Gbits/s connectivity. The group’s focus is to define the specifications for the 800-Gbits/s module and connector systems, including a stacked connector and a hybrid connector — a BiPass/Flyover variant — that can eliminate the signal losses on a traditional printed circuit board (PCB).

The eight founding members — Broadcom, Cisco, II-VI, Intel, Juniper Networks, Marvell, Molex, and Samtec — plan for the QSFP-DD800 spec to be backward-compatible with QSFP-DD, QSFP28, and QSFP+ modules and cables to address future demands for 25.6-Tbits/s scale systems that support 100-GbE or 400-GbE interfaces.

“The prime challenges with any pluggable module usually fall into the domains of signal integrity and thermal cooling. Since QSFP based designs are so ubiquitous, there is also the challenge of ensuring backward compatibility with lower speed variants of the module,” said Mark Nowell, Cisco Fellow, Engineering. “With QSFP-DD800 these challenges were fairly easy to address. We found that some optimization to the pad design on the connector improved the signal integrity so that 100 Gbits/s signaling was supported without affecting any backward compatibility. By reducing the pad dimensions slightly, it reduced the parasitics enough to not impact performance or backward compatibility.”

“While the system designs necessary to cool a QSFP-DD800 module is outside of the scope of the MSA, we know from our members that the experience taken from making systems capable of cooling QSFP-DD modules up to 20 W can be extended to further increase the cooling capabilities of QSFP-DD800 if needed. The QSFP family of modules, with their top and bottom flat surfaces, allow system designers significant flexibility in how they cool their modules. As an MSA, we do not know the target power levels that these 800G capable modules might have but with continual advances in the technology to reduce module powers we are not anticipating any issues.”

The new QSFP-DD800 1.0 specification is an incremental update to the existing QSFP-DD 5.0 specification. The transceiver pads have been optimized to improve signal integrity for 100-Gbits/s performance per lane without affecting backward-compatibility, said the MSA group.

Scott Sommers, director of industry standards, DataCom and Specialty Solutions, Molex,  said the QSFP-DD800 1.0 specification addresses the main challenges of improved signal integrity and heat dissipation for higher wattage modules.

“The QSFP-DD800 MSA Promoters released the 1.0 specification to address some of these issues. The contact pads on the transceiver/DAC have been narrowed, which improves signal integrity for operation at 112 Gbits/s,” he said. “The corresponding connector/cage will accept these new narrower pads as well as legacy structures. In addition, a cabled variant was added to replace typical PCB traces with twinax cable. Twinax cable has lower loss than traditional PCB geometry, again a step toward improving signal integrity. To help with thermal solutions, the bottom of the module has been flattened to allow heat transfer in this area giving system designers more flexibility.”

In addition, the new specification defines a novel 2 × 1 connector/cage, with cabled upper ports as an option to address signal loss issues associated with traditional PCBs.

“Unrelated to the SI and thermal challenges, the group knew that the industry was looking for alternative variations of cage/connector designs to meet their goals,” said Nowell. “Consequently, the QSFP-DD800 group developed a novel configuration where a 2×1 port configuration was developed using an SMT mounted connector for the lower port and uses a high-speed cabling for the upper port. With a novel manufacturing approach, this allows flexibility in system design while maximizing the re-use of common components to reduce costs.”

The MSA group expects to work on new connector/cage variants, including 2 × 1 SMT versions that operate at 100 Gbits/s per lane. It also announced that it will act as an incubator, collaborating with the QSFP-DD MSA, to provide specifications to that group.

The QSFP-DD800 group remains as a small group, called the incubator, and the QSFP-DD MSA takes on the ownership of refining and completing the QSFP-DD800 specifications, said Nowell. “The incubator group will continue to look at additional variations and is currently at work developing a 2×1 stacked SMT connector design. Once this is ready for review it will also be transferred into the QSFP-DD group for review, refinement, and publication.”