QSFP56 and QSFP56 DD are both types of transceiver modules used for high-speed data communication, but they differ in their design and capabilities.

QSFP56 (Quad Small Form-factor Pluggable 56) is a transceiver module that supports data rates of up to 200 Gbps. It uses a traditional electrical interface and has 8 lanes of 25 Gbps each.

QSFP56 DD (Quad Small Form-factor Pluggable 56 Double Density) is a newer type of transceiver module that also supports data rates of up to 400 Gbps. It uses an upgraded electrical interface and has 8 lanes of 50 Gbps each, doubling the data capacity compared to QSFP56.

In summary, the main difference between QSFP56 and QSFP56 DD is the data capacity and electrical interface, with QSFP56 DD offering higher data rates and a more advanced interface compared to QSFP56.