We study the problem of finding an efficient schedule for convergecast in a wireless sensor network, using the SINR model of interference, and a TDMA protocol for medium access. We compare two approaches to computing a minimum length schedule for the TDMA frame. In the first approach, called the TC-approach, a multi-set of transmission configurations that are interference-free and that cover the convergecast traffic is computed and the scheduling algorithm restricts itself to using these configurations. In the second approach, called the tree-based approach, first a routing tree or subgraph is computed, and next, sets of non-interfering links are scheduled in rounds, based on which links have available traffic in each round. In this paper, For the TC-based approach, we provide new column generation approach and several new scheduling heuristics. For the tree-based approach, we propose the construction of a new tree called TFM tree, which takes into account variable power assignment, as well as a new scheduling algorithm for the second phase, called the ROS algorithm. We performed extensive experimental evaluations of both approaches. Our results show that the tree-based approach using the TFM tree significantly outperforms the TC-approach.