Abstract: A message switching network is disclosed which consists of end units between which messages flow. The end units are coupled together directly (via LANs) or via common switching nodes through level 0 links; the nodes are coupled together via level 1 links; and the nodes are grouped into areas which are coupled together via level 2 links. A message entering a node has its destination area code compared with the node's area code, and an area/port table or an end unit/port table is used to look up the output port which is coupled to the end unit, next node in the area, or next area to which that message is to be delivered. Logic circuitry determines the incoming and outgoing levels, and a transition between levels is logged together with certain details of the message. Messages are thus monitored--i.e., their passage is recorded--when they cross levels in the hierarchy.