Network layer (NWK) in ZigBee protocol stack extends IEEE 802.15.4 functionality in terms of possible node interconnections, data transmission and network management and provides mechanisms for efficient data exchange on the level of entire network. There is a parent-child relationship where routers and end devices enter the existing network by associating themselves with a node already present in the network. After association the parent-child relationship is established between two nodes. The child is the node that has entered the network while the parent is the node that has provided network access. Only coordinators and routers can act as parent nodes. A child node can have only one parent node at a time but is able to change parent nodes. ZigBee network hierarchy can be visualized as a tree with the coordinator being on top and end devices being tree leaves but a tree hierarchy does not necessarily mean that a child can directly communicate only to its parent.