Wireless sensor networks are now-a-days used in various applications to facilitate monitoring and actuation tasks, e.g., for smart grids and industrial automation. Some of these applications require guarantees or at least assurances on reliability. Such applications expect predictable throughput and delay, which are hard to maintain in environments with changing radio conditions. QoS (Quality of Service)-aware MAC protocols capable of handling such environments are well explored. They require however protocol changes and are therefore difficult to deploy.