1000Base-BX WDM Bi-Directional SFP Module Up to 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 Work with TL-SM321B as a pair