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The paper presents EMACs, a medium access protocol especially designed for wireless sensor networks. The medium access protocol consists of a fully distributed and self-organizing TDMA scheme, in which each active node periodically listens to the channel and broadcasts a short control message. This control message is needed for medium access operation and is also used to piggy back various types of information at low energy costs. Information in the control message is used to create a maximal independent set of nodes. This set of nodes creates a connected network and nodes in the set are active, while other nodes are passive and save energy by exploiting the infrastructure created by the connected network. The presented approach is compared in simulation with the SMAC protocol (a medium access protocol with coordinated adaptive sleeping) in a realistic multi-hop network setup where sensor readings are transported to a specific node and routes are established using the dynamic source routing protocol. The EMACs protocol is able to extend the network lifetime by 30% to 55% compared to SMAC in a static network topology, and in a dynamic network topology, EMACs prolongs the network lifetime by a factor of 2.9 to 4.2.