155M (Fast Ethernet), STM-1 Bi-Directional Fiber Transceiver, BiDi SFP or WDM SFP, is used in bi-directional single fiber communications applications. With a variety of transmitter and receiver types, allowing users to select the appropriate transceiver for each link. The form factor and electrical interface are specified by a multi-source agreement (MSA). Digital diagnostics functions are available via an I2C interface.