  We present an hybrid fiber link combining effective optical frequency
transfer and evaluation of performances with a self-synchronized two-way
comparison. It enables us to detect the round-trip fiber noise and each of the
forward and backward one-way fiber noises simultaneously. The various signals
acquired with this setup allow us to study quantitatively several properties of
optical fiber links. We check the reciprocity of the accumulated noise forth
and back over a bi-directional fiber to the level of $3.1(\pm 3.9)\times
10^{-20}$ based on a 160000s continuous data. We also analyze the noise
correlation between two adjacent fibers and show the first experimental
evidence of interferometric noise at very low Fourier frequency. We estimate
redundantly and consistently the stability and accuracy of the transferred
optical frequency over 43~km at $4\times 10^{-21}$ level after 16 days of
integration and demonstrate that frequency comparison with instability as low
as $8\times 10^{-18}$ would be achievable with uni-directional fibers in urban
area.
