A network node (21), which is placed within a core network, receives a message from a transmission source (30) placed outside the core network. The message includes an indicator indicating whether or not the message is addressed to a group of one or more MTC devices attached to the core network. The network node (21) determines to authorize the transmission source (30), when the indicator indicates that the message is addressed to the group. Further, the message includes an ID for identifying whether or not the message is addressed to the group. The MTC device determines to discard the message, when the ID does not coincide with an ID allocated for the MTC device itself. Furthermore, the MTC device communicates with the transmission source (30) by use of a pair of group keys shared therewith.