In a trunk transmission network for transmitting information signals between nodes (A-C) via paths, flexible path operation is achieved by setting up paths between source nodes and destination nodes after pre-classifying paths into a higher service class (Class A) in which any loss of information occurring in that path is made good, and a lower service class (Class B) which permits loss of information to occur in the path, and by arranging for each node, when it acts as a source node, to recognise the service class of the information signal it is sending to a destination node, and to select a path corresponding to that service class.