IEEE 802.15.4 MAC protocol is the basis of many wireless sensor networks. Several studies have focused on analyzing the MAC layer by means of mathematical models such as Markov chain in order to be able to estimate the protocol performance parameters. Normally, simulation is used in order to validate the accuracy of the model. Unfortunately, real life is not always as easy as we would expect and extra considerations must be taken into account when considering real scenarios. In this paper our objective is to determine the existing gap between theoretical models and measurement analysis in real scenarios. We compare results obtained by an a priori accurate mathematical framework modeling the slotted IEEE 802.15.4 MAC protocol with experimental results ob- tained in Telosb motes and an implementation of the protocol over TinyOS. Comparision was made in terms of average delay. We also present some implementation considerations that needs to be taken into account when designin