We consider a Wireless Sensor Networks (WSN) in a planar structure with uniform distribution of the sensors and with a two-level hierarchical topology. At lower level, the clustering architecture is adopted in which the sensed information is transferred from nodes to a cluster head (CH). At CH level, CHs transfer information, hop-by-hop, towards to the sink located at the center of the sensed area. We propose a Markovian model to evaluate the end-toend transfer delay. The numerical results reveal that the traffic carried by CHs near the sink is higher than the traffic carried by CHs located near the perimeter of the sensed area, as it could be expected. Furthermore, for a given radial distance between the source and the sink, the transfer delay depends on the angular orientation between both. This asymmetric behavior is revealed by the model.