Extend a Gigabit Ethernet connection across one single-mode fiber strand with a 1000BASE-BX10-U optical link. BX10-U places transmission at 1310 nm and reception at 1490 nm, allowing bidirectional traffic to share the same fiber. The SFP form factor provides a modular optical interface for a compatible Gigabit Ethernet port. The NOKIA 3FE25772AA is specified for a 10 km BX10 link. Its asymmetric wavelength plan is the defining deployment detail: the local interface transmits on 1310 nm and receives on 1490 nm. The far end of the fiber path must provide the opposite optical direction, transmitting at 1490 nm and receiving at 1310 nm, so that each endpoint can receive the other endpoint's signal. This transceiver fits fiber-constrained access, aggregation, and building-to-building links where one strand is available for a 1 Gbps connection. It differs from two-fiber Gigabit optics by carrying transmit and receive traffic over a single single-mode path, and it differs from opposite-direction BX optics through its 1310 nm transmit and 1490 nm receive arrangement. Select it where the installed fiber path, endpoint optical direction, SFP interface, and 10 km reach requirement all match.