Software settings & tolerances can vary between CPE/CSU manufacturers also. Typically a DS1 pulse can be recovered/regenerated on a T1 span in the OSP every 3,000 to 6,000ft between repeaters(up to -28.0 -dbsx), depending on cable gauge. I have seen integrated CSU's on PRI equipment that would not achieve DS1 sync with the ILEC's NIU with only -15dbsx atten on the receive pulse, either because that CSU was not designed to accept an overly attenuated DS1 pulse or it was not software set to regen the pulse before converting it to unipolar, or it had no such setting. The CSU can also be incorrectly set to pad(LBO) the DS1 pulse toward the providers NIU. I have seen many tech's on both sides of the fence incorrectly set LBO based on cable footage between NIU's & CSU's not realizing that what they were doing was causing the problem.