The lossy nature of wireless communication leads to many challenges in designing multi-hop sensor networks. As an integral part of routing protocols, effective link estimation is essential for reliable communication. In this paper, we conduct a detailed performance analysis of several representatives of link estimators in the interplay with Collection Tree Protocol. The evaluation process has been carried out a test-bed of real nodes running ContikiOS. We also improve the Direct Estimate estimator to show the importance of acknowledgement delivery in reliable communication. The resulting extend-Direct Estimate shows better performance. We final conclude some suggestions of using which links estimator for which kind of applications.