In this article we elaborate on reality considerations when deploying wireless sensor networks with strict end-to-end delay requirement. Our improvements are particularly important for a design and implementation of strict real-time systems while at the same time we decrease overall power consumption. Firstly, we design and implement application tailored TDMA-FDMA medium access protocol with guaranteed end-to-end delay. Secondly, we integrate in the implementation of the protocol the bootstrapping mechanism and thirdly, the time synchronization mechanism. Next, we show that by combining medium access protocol, bootstrapping, and time synchronization mechanisms within the link-layer, we can limit on average clock drift in the network to 0.5 micro seconds, as well as achieve 81 % energy efficiency while keeping collision probability at its minimum of 1 %. Finally, we conclude with challenges and lessons learned in real-world deployment of TDMA/FDMA based link-layer with guaranteed end-to-end delay in wireless sensor networks.