The system definition describes how the different units in the system communicate. System level operation includes whether the receiver transmits a confirmation back to the transmitting side, if the transmitter re-sends the data in the case of a failure or if there are any networking aspects such as relays or forwarding.  If there are multiple nodes reporting to a master, then the master can poll each node in turn or the nodes can report only when they have new data. There are many options for system operation. Simple operation is quick and inexpensive to implement, but not robust in the presence of interference. However, if the user is opening a garage door it does not need to be robust, just keep sending packets until the door opens. The application determines the level of complexity that is required.