Large-scale Wireless Sensors Networks present a specific challenge when used in monitoring applications, at the Medium Access Control layer of the network stack, due to the interfering nature of radio links. While there has been TDMA-based MAC standards and algorithms catered to data collecting WSNs, these mechanisms only focus on solving the problem of interference-free transmissions scheduling for single cluster networks, and have already shown significant complexity. When the network employs routing strategies involving multiple clusters, one question of the MAC layer scalability becomes inherent: how to coordinate TDMA scheduling for these clusters in an efficient manner while nodes present at their physical borders will be subjected to interference from multiple clusters? We propose a new solution to this problem, where discovery and resolution of cross-cluster interference are done first, and internal scheduling at each cluster follows, taking into account the former resolution. This scheme ultimately allows all clusters to schedule their transmissions to operate in parallel, free of both cross-cluster and in-cluster radio interference. Our first rounds results shows data packets latency stability and the system’s scalability when applied to networks of increasingly larger sizes.