Abstract: A method is disclosed for determining delays in a network (10) carrying messages, such as an SS7 signalling network or a TCP/IP network. The method involves monitoring the network (10) at a predetermined location to detect first messages (REQ) and second messages (ACK) of respective predetermined categories (X, Y), the first and second messages each producing a respective time-dependent loading of the network and the second messages being caused by the first messages whereby the loadings vary in a similar manner with time but with the loading produced by the second messages being offset from that produced by the first messages. Load measures (L.sub.X,L.sub.Y) are then derived for the first and second messages (REQ, ACK), each load measure being derived by counting the number of messages of the category concerned that are detected in a predetermined time interval. These load measures (L.sub.X,L.sub.

Abstract: A message interceptor is interposed in a link of a communications signalling network such as an SS7 network. The interceptor separately maintains the link level protocols on the two portions of the link with which it interfaces. Message data is transferred across between the link portions by a transfer circuit of the interceptor. In the course of this transfer, the nature of the data is checked by a selective action control circuit against selection criteria held in a store. If a selection criteria is met, the control circuit acts to suppress or modify the data.