A packet transmission system comprising a network of one or more nodes (1) each comprising a number of inputs connected via packet switching means to a number of outputs each of which has storage means (11) associated with it in which to store a queue of packets to be outputted; characterized in that each node (1) is provided with timer means (14) to measure the time each packet spends in a queue of said storage means (11); and stamping means (12) to add to a time stamp field (TS) of each packet before it is outputted from the node, the time the packet has spent in a queue as measured by the timer means (14).