Abstract-- Wireless sensor networks (WSNs) require energyef cient protocols to improve the network lifetime. In this work, we adopt a cross-layer strategy that considers routing and MAC layers jointly. At the routing layer, we propose balancing the traf c through the WSN. We show that sending the traf c generated by each sensor node through multiple paths instead of using a single path allows signi cant energy conservation. On the other hand, at the MAC layer, we propose to control the retry limit of retransmissions over each wireless link. We show that by ef ciently adjusting the retry limit for each link, further energy conservation can be achieved, improving thus the network lifetime. A new analytical model for the joint optimization system is complemented by simulations in order to quantitatively evaluate the bene ts of our proposal.