In BiDi (Bidirectional) SFP (Small Form-factor Pluggable) transceivers, "U" and "D" typically refer to different wavelength configurations for upstream (U) and downstream (D) communication. BiDi optics use WDM (Wavelength Division Multiplexing) technology to transmit and receive data on a single fiber optic strand by using different wavelengths for each direction.

1. U (Upstream):

• The "U" designation in a BiDi SFP typically refers to the wavelength used for upstream communication. Upstream refers to the data transmitted from the client device (such as a switch or router) to the network.

2. D (Downstream):

• The "D" designation refers to the wavelength used for downstream communication. Downstream refers to the data transmitted from the network to the client device.

The specific wavelengths used for upstream and downstream communication depend on the BiDi SFP module's configuration and the wavelength pairing chosen. Common configurations include 1310/1490 nm and 1490/1310 nm, indicating the upstream and downstream wavelengths, respectively.

For example:

• A BiDi SFP with "U: 1310nm" and "D: 1490nm" means that the upstream data is transmitted at 1310 nm, and downstream data is received at 1490 nm.

• A BiDi SFP with "U: 1490nm" and "D: 1310nm" means the opposite configuration.

It's important to choose the correct BiDi SFP module with the appropriate wavelength pairing based on the network infrastructure and requirements. Always refer to the specifications provided by the manufacturer for accurate information on the wavelengths and configurations of a specific BiDi SFP module.