A polymer electrolyte fuel cells system wherein a reacted gas that has been passed through a cell stack is enabled to be contacted with an unreacted gas to be passed through the cell stack, thereby carrying out a temperaturehumidity exchange for the purpose of preventing the drying of a solid polymer electrolyte membrane. Multicast messages may be efficiently routed by creating separate multicast groups for upstream traffic from a source spoke to the hub and downstream traffic from the hub to spokes. A parameter relating to at least one apparatus is indicative of the availability andor the reliability of the apparatus expressed as a function of time.