QSFP-DD stands for Quad Small Form-factor Pluggable Double Density. It is an evolution of the QSFP form factor, designed to support higher data rates and double the number of electrical lanes compared to QSFP28. QSFP-DD is developed to address the increasing bandwidth requirements in data centers and networking applications.

The key features of QSFP-DD include:

1. Double Density: QSFP-DD supports up to 8 electrical lanes, each operating at rates of 25 Gbps or 50 Gbps, resulting in a total aggregate data rate of up to 200 Gbps or 400 Gbps.

2. Backward Compatibility: Despite the increased density, QSFP-DD modules are designed to maintain backward compatibility with QSFP28 modules. This allows for a smooth transition and coexistence of both form factors in the same network.

3. High Port Density: The double-density design of QSFP-DD allows for increased port density, enabling data centers to achieve higher throughput without requiring additional physical space.

4. Hot-Pluggable: Like other QSFP form factors, QSFP-DD is hot-pluggable, meaning that modules can be replaced or added without powering down the entire system.

QSFP-DD is particularly relevant for high-performance computing, cloud data centers, and other environments where the demand for higher bandwidth continues to grow.