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Consider cluster-based wireless sensor networks where sensor nodes are organized into groups to form coalitions for data aggregation and transport. We assume that each coalition head (CH) collects data from the sensors within the coalition and carries out data compression. Then, the data are transported from the coalitions to the sink. We assume that CSMA based random access is used by the CHs to reserve the channel. We examine three data transmission schemes. In scheme 1, a node in each coalition is selected randomly by the CH to transmit the data. In scheme 2, the node with the best channel condition in each coalition transmits the data, exploiting multiuser diversity gain. In Scheme 3, all the nodes within a coalition transmit in a cooperative manner, so the cooperative diversity gain could be achieved. We analyze the delay performance and energy consumption corresponding to the above three schemes. Finally, we investigate the optimal coalition size in the sense of maximizing the energy efficiency. Numerical examples are provided to illustrate the performance.