QSFP 40G SR BD stands for Quad Small Form-factor Pluggable 40 Gigabit Short Range Bidirectional. This type of transceiver module is designed for high-speed data communication over short distances. The "BD" in the name indicates Bidirectional, meaning it supports transmission and reception over a single fiber strand, using different wavelengths for each direction.

The cable type associated with QSFP 40G SR BD is typically multimode fiber (MMF). Multimode fiber allows multiple light modes (or paths) to propagate through the optical fiber. The specific type of multimode fiber often used with QSFP 40G SR BD is OM3 or OM4 (Optical Multimode 3 or 4), as these provide sufficient bandwidth for 40 Gbps data rates over short distances.

Therefore, if you are deploying QSFP 40G SR BD, you would typically use a multimode fiber optic cable with OM3 or OM4 specifications to ensure proper performance and compatibility with the transceiver module. Always refer to the manufacturer's specifications and guidelines for the specific requirements of your networking equipment.