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In this letter we investigate the placement of nodes, based on their residual energies, when both the forward and the backward directions of a data flow are considered. First we extend the monodirectional flow model and determine the nodes' positions that achieve the longest path-lifetime, then we propose a new model that allows us to find the optimal placement depending on the quota of energy that we are willing to spend for the backward or the forward direction of data flow. Furthermore the new factor introduced by the second model can be matched with some parameter of the data flow, such as the load, the delay or the jitter in order to provide QoS in the transmission.