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Reliable data transfer (RDT) is one of the key issues in wireless sensor networks (WSNs) and can be achieved by using link-level re-transmissions and multi-path routing. Another key issue is the scalability of WSNs. In this paper, we try to better understand and characterize/quantify the relationships between reliability and scalability, and identify possible design options for the future RDT protocols in large-scale WSNs. With this in mind, we first conducted actual experiments to characterize link reliability measures in an actual sensor network setting. We then used these measures and analyze how commonly used RDT mechanisms impact overall path reliability. In general, our analysis shows that the combination of link-level re-transmissions and multi-path routing is a viable solution in small-scale WSNs. However, due to the increased length of paths between sensor nodes and sinks in large-scale WSNs, it becomes costly to sustain the overall reliability at an acceptable level. Therefore, the future RDT protocols should focus on minimizing the path lengths using hierarchical structures in large-scale WSNs. It is also necessary to couple RDT protocols with routing protocols that can take link reliability measures into account.