Cluster scheduling respecting collision avoidance is crucial issue in large scale IEEE 802.15.4/ZigBee with cluster-tree topology. The problem becomes more difficult to solve when time-constrained data flows with opposite directions are considered with the objective of  minimizing  the energy consumption of  the nodes. Our research center deals not only with designing energy-efficient cluster scheduling algorithms, but also designing new joining mechanism, that defines the form of the cluster-tree topology, guided by the scheduling problem. The new joining mechanism will represent a revolution in this area and can substitute the non efficient Distributed Address Assignment Mechanism (DAAM) suggested by ZigBee.