A packet of data, which contains a timestamp value and which is to be transmitted through an ATM network, is encapsulated in a convergence sublayer protocol data unit (92) having a header (96) and a payload (94). The CS-PDU is then segmented into successive segments (106, 108, 110 and 112) with one segment (106) containing the CS-PDU header (96). The segments (106, 108, 110 and 112) are inserted in respective ATM cells (114, 116, 118 and 120) and transmitted through the ATM network. At each node in the ATM network, any variable delays imposed on the cell (114) that contains CS-PDU header (96) are added to a time reference correction field (100) in the CS-PDU header (96). At a destination node, the accumulated delay value in the TRC field (100) of the CS-PDU header (96) can be added to the timestamp value of the packet to compensate for the variable delays imposed by the ATM network.