This paper deals with estimation of blocking probabilities for OBS switches with Fiber Delay Lines (FDLs) and full wavelength conversion. An incoming burst that finds the wavelengths occupied is temporarily stored in a FDL. Hence, contention will be sorted out successfully if the residual life of the system is smaller than the maximum FDL delay. In order to derive the blocking probability, the most accurate methodology to date is the use of
systems [1,2,3,4]. Even though the approach is accurate for very short lengths of the FDLs we identify the cases in which inaccuracy is detected. This happens precisely when the system works with low loss probabilities. Mainly for large number of wavelengths on the fibers and values of the FDL length at least in the vicinity of the burst service time.