TP-Link 1000Base-BX WDM Bi-Directional SFP Module (TL-SM321B)

Up to 12.4 miles (20 km) transmission distance with 9/125 μm SMF (Single-Mode Fiber)
Bidirectional WDM communication on a single fiber
Compatible with Small Form Pluggable Multi-Source Agreement (SFP-MSA)
Supports Digital Diagnostic Monitoring (DDM)
Hot-swappable module design
Works with TL-SM321A as a pair