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The availability of low cost, multifunctional embedded devices have become ubiquitous due to their wide ranging application that includes monitoring Radio Frequency Identification (RFID) tagged objects to ambient conditions such as temperature and humidity. Also, these devices can be equipped with a camera and then deployed in hostile environments. A key characteristic of these devices is that they can communicate wirelessly and form an ad-hoc, Wireless Sensor Network (WSN). Devices in a WSN then collaboratively monitor environment factors or objects, and forward data back to one or more central nodes. A key challenge in WSN is ensuring nodes operate in the order of months as they have limited energy resource, and it is impractical to replace their batteries due to their large numbers, and they may be deployed in inaccessible terrains. Hence, it is critical that these devices or nodes employ energy efficient protocols. To this end, we present PairWise, a novel, low power, time division multiple access (TDMA) based protocol for use in WSNs. PairWise is easily deployable in large scale WSNs as nodes are not synchronized globally. Instead, they synchronize and establish a pair of channels with each of their neighbors independently. Each channel hops pseudo randomly in time according to a seed and maximum rendezvous period (MRP). Hence, nodes using PairWise experience very minimal to no collision during communications. Apart from that, higher layer protocols are able to control the MRP of each channel such that a node's duty cycle matches the observed traffic load. We have implemented PairWise in the ns-2 simulator, and compared it to Sensor Medium Access Control (S-MAC) and a TDMA MAC. Our results show PairWise to have very low power consumption whilst ensuring packets have minimal delays. Moreover, PairWise has a high goodput with increasing node density, where goodput is defined as the number of packets that are transmitted by each node pair successfully over a give- n total number of packets.