The QSFP-DD (Quad Small Form Factor Pluggable Double Density) runs 400G (4x100G / 8x 50G) and it is backward compatible with QSFP+ and QSFP28 (40/100G). It supports 8 high-speed electrical interfaces connecting to the host and can accommodate both fiber optical and copper interfaces.
The “Double Density” term underlines that the number of high-speed electrical interfaces supported by the module is doubled compared to the QSFP28. Its 8-lane interface increases the bandwidth, channel capacity and port density.
The QSFP-DD’s mechanical interface on the host board is deeper than for QSFP28, in order to accommodate an additional line of contacts. As for the width and height, they are equivalent to the QSFP.