EXFO CableSHARK P3 VF/DSL Cable Qualifier - Manual (Page 315)

CableSHARK P3 User Guide
303
2B1Q Two Binary One Quaternary - A signaling method used across the 2-wire BRI (U reference point).
2B1Q is a four level line code associating 2 bits to each line signal.
U Interface
- The physical 2-wire echo canceling interface on the network side of the NT1.